Question: What part of Rotterdam is the China Town located?

Answer: Kruiskade

In the Netherlands, Rotterdam has the highest percentage of foreigners from non-industrialised nations. They form a large part of Rotterdam's multi ethnic and multicultural diversity. 47.7% of the population are of non Dutch origins or have at least one parent born outside the country. There are 80,000 Muslims, constituting 13% of the population. The mayor of Rotterdam, Ahmed Aboutaleb, is of Moroccan descent and is a practicing Muslim. The city is home to the largest Dutch Antillean community. <hl> The city also has its own China Town at the (West-) Kruiskade , close to the central railway station. <hl>

In the Netherlands, Rotterdam has the highest percentage of foreigners from non-industrialised nations. They form a large part of Rotterdam's multi ethnic and multicultural diversity. 47.7% of the population are of non Dutch origins or have at least one parent born outside the country. There are 80,000 Muslims, constituting 13% of the population. The mayor of Rotterdam, Ahmed Aboutaleb, is of Moroccan descent and is a practicing Muslim. The city is home to the largest Dutch Antillean community. The city also has its own China Town at the (West-) <hl> Kruiskade <hl>, close to the central railway station.

The city also has its own China Town at the (West-) <hl> Kruiskade <hl> , close to the central railway station.


Question: What festivals does Rotterdam celebrate?

Rotterdam features some urban architecture projects, nightlife, and many summer festivals celebrating the city's multicultural population and identity, such as the Caribbean-inspired "Summer Carnival", the Dance Parade, Rotterdam 666, the Metropolis pop festival and the World Port days. In the years 2005–2011 the city struggled with venues for popmusic.[citation needed]  Many of the venues suffered severe financial problems. This resulted in the disappearance of the major music venues Nighttown and WATT and smaller stages such as Waterfront, Exit, and Heidegger. Currently the city has a few venues for pop music like Rotown, Poortgebouw. The venue WORM focuses on experimental music and related cutting edge subcultural music.
There are also the International Film Festival in January, the Poetry International Festival in June, the North Sea Jazz Festival in July, the Valery Gergiev Festival in September, September in Rotterdam and the World of the Witte de With. In June 1970, The Holland Pop Festival (which featured Jefferson Airplane, The Byrds, Canned Heat, It's a Beautiful Day, and Santana) was held and filmed at the Stamping Grounds in Rotterdam.

Answer: "Summer Carnival", the Dance Parade, Rotterdam 666, the Metropolis pop festival and the World Port days

<hl> Rotterdam features some urban architecture projects, nightlife, and many summer festivals celebrating the city's multicultural population and identity, such as the Caribbean-inspired "Summer Carnival", the Dance Parade, Rotterdam 666, the Metropolis pop festival and the World Port days . <hl> In the years 2005–2011 the city struggled with venues for popmusic.[citation needed] Many of the venues suffered severe financial problems. This resulted in the disappearance of the major music venues Nighttown and WATT and smaller stages such as Waterfront, Exit, and Heidegger. Currently the city has a few venues for pop music like Rotown, Poortgebouw. The venue WORM focuses on experimental music and related cutting edge subcultural music. There are also the International Film Festival in January, the Poetry International Festival in June, the North Sea Jazz Festival in July, the Valery Gergiev Festival in September, September in Rotterdam and the World of the Witte de With. In June 1970, The Holland Pop Festival (which featured Jefferson Airplane, The Byrds, Canned Heat, It's a Beautiful Day, and Santana) was held and filmed at the Stamping Grounds in Rotterdam.

Rotterdam features some urban architecture projects, nightlife, and many summer festivals celebrating the city's multicultural population and identity, such as the Caribbean-inspired <hl> "Summer Carnival", the Dance Parade, Rotterdam 666, the Metropolis pop festival and the World Port days <hl>. In the years 2005–2011 the city struggled with venues for popmusic.[citation needed] Many of the venues suffered severe financial problems. This resulted in the disappearance of the major music venues Nighttown and WATT and smaller stages such as Waterfront, Exit, and Heidegger. Currently the city has a few venues for pop music like Rotown, Poortgebouw. The venue WORM focuses on experimental music and related cutting edge subcultural music. There are also the International Film Festival in January, the Poetry International Festival in June, the North Sea Jazz Festival in July, the Valery Gergiev Festival in September, September in Rotterdam and the World of the Witte de With. In June 1970, The Holland Pop Festival (which featured Jefferson Airplane, The Byrds, Canned Heat, It's a Beautiful Day, and Santana) was held and filmed at the Stamping Grounds in Rotterdam.

Rotterdam features some urban architecture projects, nightlife, and many summer festivals celebrating the city's multicultural population and identity, such as the Caribbean-inspired <hl> "Summer Carnival", the Dance Parade, Rotterdam 666, the Metropolis pop festival and the World Port days <hl> .


Question: What music venues went out of business?

Answer: Nighttown and WATT and smaller stages such as Waterfront, Exit, and Heidegger

Rotterdam features some urban architecture projects, nightlife, and many summer festivals celebrating the city's multicultural population and identity, such as the Caribbean-inspired "Summer Carnival", the Dance Parade, Rotterdam 666, the Metropolis pop festival and the World Port days. In the years 2005–2011 the city struggled with venues for popmusic.[citation needed] Many of the venues suffered severe financial problems. <hl> This resulted in the disappearance of the major music venues Nighttown and WATT and smaller stages such as Waterfront, Exit, and Heidegger . <hl> Currently the city has a few venues for pop music like Rotown, Poortgebouw. The venue WORM focuses on experimental music and related cutting edge subcultural music. There are also the International Film Festival in January, the Poetry International Festival in June, the North Sea Jazz Festival in July, the Valery Gergiev Festival in September, September in Rotterdam and the World of the Witte de With. In June 1970, The Holland Pop Festival (which featured Jefferson Airplane, The Byrds, Canned Heat, It's a Beautiful Day, and Santana) was held and filmed at the Stamping Grounds in Rotterdam.

Rotterdam features some urban architecture projects, nightlife, and many summer festivals celebrating the city's multicultural population and identity, such as the Caribbean-inspired "Summer Carnival", the Dance Parade, Rotterdam 666, the Metropolis pop festival and the World Port days. In the years 2005–2011 the city struggled with venues for popmusic.[citation needed] Many of the venues suffered severe financial problems. This resulted in the disappearance of the major music venues <hl> Nighttown and WATT and smaller stages such as Waterfront, Exit, and Heidegger <hl>. Currently the city has a few venues for pop music like Rotown, Poortgebouw. The venue WORM focuses on experimental music and related cutting edge subcultural music. There are also the International Film Festival in January, the Poetry International Festival in June, the North Sea Jazz Festival in July, the Valery Gergiev Festival in September, September in Rotterdam and the World of the Witte de With. In June 1970, The Holland Pop Festival (which featured Jefferson Airplane, The Byrds, Canned Heat, It's a Beautiful Day, and Santana) was held and filmed at the Stamping Grounds in Rotterdam.

This resulted in the disappearance of the major music venues <hl> Nighttown and WATT and smaller stages such as Waterfront, Exit, and Heidegger <hl> .


Question: What years did Rotterdam have trouble filling pop music venues?

Answer: 2005–2011

Rotterdam features some urban architecture projects, nightlife, and many summer festivals celebrating the city's multicultural population and identity, such as the Caribbean-inspired "Summer Carnival", the Dance Parade, Rotterdam 666, the Metropolis pop festival and the World Port days. <hl> In the years 2005–2011 the city struggled with venues for popmusic.[citation needed] Many of the venues suffered severe financial problems. <hl> This resulted in the disappearance of the major music venues Nighttown and WATT and smaller stages such as Waterfront, Exit, and Heidegger. Currently the city has a few venues for pop music like Rotown, Poortgebouw. The venue WORM focuses on experimental music and related cutting edge subcultural music. There are also the International Film Festival in January, the Poetry International Festival in June, the North Sea Jazz Festival in July, the Valery Gergiev Festival in September, September in Rotterdam and the World of the Witte de With. In June 1970, The Holland Pop Festival (which featured Jefferson Airplane, The Byrds, Canned Heat, It's a Beautiful Day, and Santana) was held and filmed at the Stamping Grounds in Rotterdam.

Rotterdam features some urban architecture projects, nightlife, and many summer festivals celebrating the city's multicultural population and identity, such as the Caribbean-inspired "Summer Carnival", the Dance Parade, Rotterdam 666, the Metropolis pop festival and the World Port days. In the years <hl> 2005–2011 <hl> the city struggled with venues for popmusic.[citation needed] Many of the venues suffered severe financial problems. This resulted in the disappearance of the major music venues Nighttown and WATT and smaller stages such as Waterfront, Exit, and Heidegger. Currently the city has a few venues for pop music like Rotown, Poortgebouw. The venue WORM focuses on experimental music and related cutting edge subcultural music. There are also the International Film Festival in January, the Poetry International Festival in June, the North Sea Jazz Festival in July, the Valery Gergiev Festival in September, September in Rotterdam and the World of the Witte de With. In June 1970, The Holland Pop Festival (which featured Jefferson Airplane, The Byrds, Canned Heat, It's a Beautiful Day, and Santana) was held and filmed at the Stamping Grounds in Rotterdam.


Question: What festival is held in January?

Answer: International Film Festival

Rotterdam features some urban architecture projects, nightlife, and many summer festivals celebrating the city's multicultural population and identity, such as the Caribbean-inspired "Summer Carnival", the Dance Parade, Rotterdam 666, the Metropolis pop festival and the World Port days. In the years 2005–2011 the city struggled with venues for popmusic.[citation needed] Many of the venues suffered severe financial problems. This resulted in the disappearance of the major music venues Nighttown and WATT and smaller stages such as Waterfront, Exit, and Heidegger. Currently the city has a few venues for pop music like Rotown, Poortgebouw. The venue WORM focuses on experimental music and related cutting edge subcultural music. <hl> There are also the International Film Festival in January, the Poetry International Festival in June, the North Sea Jazz Festival in July, the Valery Gergiev Festival in September, September in Rotterdam and the World of the Witte de With. <hl> In June 1970, The Holland Pop Festival (which featured Jefferson Airplane, The Byrds, Canned Heat, It's a Beautiful Day, and Santana) was held and filmed at the Stamping Grounds in Rotterdam.

Rotterdam features some urban architecture projects, nightlife, and many summer festivals celebrating the city's multicultural population and identity, such as the Caribbean-inspired "Summer Carnival", the Dance Parade, Rotterdam 666, the Metropolis pop festival and the World Port days. In the years 2005–2011 the city struggled with venues for popmusic.[citation needed] Many of the venues suffered severe financial problems. This resulted in the disappearance of the major music venues Nighttown and WATT and smaller stages such as Waterfront, Exit, and Heidegger. Currently the city has a few venues for pop music like Rotown, Poortgebouw. The venue WORM focuses on experimental music and related cutting edge subcultural music. There are also the <hl> International Film Festival <hl> in January, the Poetry International Festival in June, the North Sea Jazz Festival in July, the Valery Gergiev Festival in September, September in Rotterdam and the World of the Witte de With. In June 1970, The Holland Pop Festival (which featured Jefferson Airplane, The Byrds, Canned Heat, It's a Beautiful Day, and Santana) was held and filmed at the Stamping Grounds in Rotterdam.


Question: What causes Chromatic aberration?

Chromatic aberration is caused by the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light. Since, from the formulae above, f is dependent upon n, it follows that light of different wavelengths is focused to different positions. Chromatic aberration of a lens is seen as fringes of colour around the image. It can be minimised by using an achromatic doublet (or achromat) in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The use of achromats was an important step in the development of the optical microscope. An apochromat is a lens or lens system with even better chromatic aberration correction, combined with improved spherical aberration correction. Apochromats are much more expensive than achromats.

Answer: the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light.

<hl> Chromatic aberration is caused by the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light. Since, from the formulae above, f is dependent upon n, it follows that light of different wavelengths is focused to different positions. <hl> Chromatic aberration of a lens is seen as fringes of colour around the image. It can be minimised by using an achromatic doublet (or achromat) in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The use of achromats was an important step in the development of the optical microscope. An apochromat is a lens or lens system with even better chromatic aberration correction, combined with improved spherical aberration correction. Apochromats are much more expensive than achromats.

Chromatic aberration is caused by <hl> the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light. <hl> Since, from the formulae above, f is dependent upon n, it follows that light of different wavelengths is focused to different positions. Chromatic aberration of a lens is seen as fringes of colour around the image. It can be minimised by using an achromatic doublet (or achromat) in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The use of achromats was an important step in the development of the optical microscope. An apochromat is a lens or lens system with even better chromatic aberration correction, combined with improved spherical aberration correction. Apochromats are much more expensive than achromats.


Question: What does chromatic aberration look like?

Answer: fringes of colour around the image

Chromatic aberration is caused by the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light. Since, from the formulae above, f is dependent upon n, it follows that light of different wavelengths is focused to different positions. <hl> Chromatic aberration of a lens is seen as fringes of colour around the image . <hl> It can be minimised by using an achromatic doublet (or achromat) in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The use of achromats was an important step in the development of the optical microscope. An apochromat is a lens or lens system with even better chromatic aberration correction, combined with improved spherical aberration correction. Apochromats are much more expensive than achromats.

Chromatic aberration is caused by the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light. Since, from the formulae above, f is dependent upon n, it follows that light of different wavelengths is focused to different positions. Chromatic aberration of a lens is seen as <hl> fringes of colour around the image <hl>. It can be minimised by using an achromatic doublet (or achromat) in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The use of achromats was an important step in the development of the optical microscope. An apochromat is a lens or lens system with even better chromatic aberration correction, combined with improved spherical aberration correction. Apochromats are much more expensive than achromats.

Chromatic aberration of a lens is seen as <hl> fringes of colour around the image <hl> .


Question: What minimises chromatic aberration?

Answer: an achromatic doublet (or achromat)

Chromatic aberration is caused by the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light. Since, from the formulae above, f is dependent upon n, it follows that light of different wavelengths is focused to different positions. Chromatic aberration of a lens is seen as fringes of colour around the image. <hl> It can be minimised by using an achromatic doublet (or achromat) in which two materials with differing dispersion are bonded together to form a single lens. <hl> This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The use of achromats was an important step in the development of the optical microscope. An apochromat is a lens or lens system with even better chromatic aberration correction, combined with improved spherical aberration correction. Apochromats are much more expensive than achromats.

Chromatic aberration is caused by the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light. Since, from the formulae above, f is dependent upon n, it follows that light of different wavelengths is focused to different positions. Chromatic aberration of a lens is seen as fringes of colour around the image. It can be minimised by using <hl> an achromatic doublet (or achromat) <hl> in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The use of achromats was an important step in the development of the optical microscope. An apochromat is a lens or lens system with even better chromatic aberration correction, combined with improved spherical aberration correction. Apochromats are much more expensive than achromats.


Question: What uses achromats in its design?

Answer: optical microscope

Chromatic aberration is caused by the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light. Since, from the formulae above, f is dependent upon n, it follows that light of different wavelengths is focused to different positions. Chromatic aberration of a lens is seen as fringes of colour around the image. It can be minimised by using an achromatic doublet (or achromat) in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. <hl> The use of achromats was an important step in the development of the optical microscope . <hl> An apochromat is a lens or lens system with even better chromatic aberration correction, combined with improved spherical aberration correction. Apochromats are much more expensive than achromats.

Chromatic aberration is caused by the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light. Since, from the formulae above, f is dependent upon n, it follows that light of different wavelengths is focused to different positions. Chromatic aberration of a lens is seen as fringes of colour around the image. It can be minimised by using an achromatic doublet (or achromat) in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The use of achromats was an important step in the development of the <hl> optical microscope <hl>. An apochromat is a lens or lens system with even better chromatic aberration correction, combined with improved spherical aberration correction. Apochromats are much more expensive than achromats.

The use of achromats was an important step in the development of the <hl> optical microscope <hl> .


Question: What works better than an achromat?

Answer: apochromat

Chromatic aberration is caused by the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light. Since, from the formulae above, f is dependent upon n, it follows that light of different wavelengths is focused to different positions. Chromatic aberration of a lens is seen as fringes of colour around the image. It can be minimised by using an achromatic doublet (or achromat) in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The use of achromats was an important step in the development of the optical microscope. <hl> An apochromat is a lens or lens system with even better chromatic aberration correction, combined with improved spherical aberration correction. <hl> Apochromats are much more expensive than achromats.

Chromatic aberration is caused by the dispersion of the lens material—the variation of its refractive index, n, with the wavelength of light. Since, from the formulae above, f is dependent upon n, it follows that light of different wavelengths is focused to different positions. Chromatic aberration of a lens is seen as fringes of colour around the image. It can be minimised by using an achromatic doublet (or achromat) in which two materials with differing dispersion are bonded together to form a single lens. This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. The use of achromats was an important step in the development of the optical microscope. An <hl> apochromat <hl> is a lens or lens system with even better chromatic aberration correction, combined with improved spherical aberration correction. Apochromats are much more expensive than achromats.


Question: When are the FFL and BFL infinite?

If the separation distance is equal to the sum of the focal lengths (d = f1+f2), the FFL and BFL are infinite. This corresponds to a pair of lenses that transform a parallel (collimated) beam into another collimated beam. This type of system is called an afocal system, since it produces no net convergence or divergence of the beam. Two lenses at this separation form the simplest type of optical telescope. Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam. The magnification of such a telescope is given by

Answer: If the separation distance is equal to the sum of the focal lengths

<hl> If the separation distance is equal to the sum of the focal lengths (d = f1+f2), the FFL and BFL are infinite. <hl> This corresponds to a pair of lenses that transform a parallel (collimated) beam into another collimated beam. This type of system is called an afocal system, since it produces no net convergence or divergence of the beam. Two lenses at this separation form the simplest type of optical telescope. Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam. The magnification of such a telescope is given by

<hl> If the separation distance is equal to the sum of the focal lengths <hl> (d = f1+f2), the FFL and BFL are infinite. This corresponds to a pair of lenses that transform a parallel (collimated) beam into another collimated beam. This type of system is called an afocal system, since it produces no net convergence or divergence of the beam. Two lenses at this separation form the simplest type of optical telescope. Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam. The magnification of such a telescope is given by


Question: What creates a collimated beam?

Answer: afocal system

If the separation distance is equal to the sum of the focal lengths (d = f1+f2), the FFL and BFL are infinite. This corresponds to a pair of lenses that transform a parallel (collimated) beam into another collimated beam. <hl> This type of system is called an afocal system , since it produces no net convergence or divergence of the beam. <hl> Two lenses at this separation form the simplest type of optical telescope. Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam. The magnification of such a telescope is given by

If the separation distance is equal to the sum of the focal lengths (d = f1+f2), the FFL and BFL are infinite. This corresponds to a pair of lenses that transform a parallel (collimated) beam into another collimated beam. This type of system is called an <hl> afocal system <hl>, since it produces no net convergence or divergence of the beam. Two lenses at this separation form the simplest type of optical telescope. Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam. The magnification of such a telescope is given by

This type of system is called an <hl> afocal system <hl> , since it produces no net convergence or divergence of the beam.


Question: What can two afocal lense form?

Answer: the simplest type of optical telescope

If the separation distance is equal to the sum of the focal lengths (d = f1+f2), the FFL and BFL are infinite. This corresponds to a pair of lenses that transform a parallel (collimated) beam into another collimated beam. This type of system is called an afocal system, since it produces no net convergence or divergence of the beam. <hl> Two lenses at this separation form the simplest type of optical telescope . <hl> Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam. The magnification of such a telescope is given by

If the separation distance is equal to the sum of the focal lengths (d = f1+f2), the FFL and BFL are infinite. This corresponds to a pair of lenses that transform a parallel (collimated) beam into another collimated beam. This type of system is called an afocal system, since it produces no net convergence or divergence of the beam. Two lenses at this separation form <hl> the simplest type of optical telescope <hl>. Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam. The magnification of such a telescope is given by

Two lenses at this separation form <hl> the simplest type of optical telescope <hl> .


Question: What are used to transform beams?

Answer: afocal system

If the separation distance is equal to the sum of the focal lengths (d = f1+f2), the FFL and BFL are infinite. This corresponds to a pair of lenses that transform a parallel (collimated) beam into another collimated beam. <hl> This type of system is called an afocal system , since it produces no net convergence or divergence of the beam. <hl> Two lenses at this separation form the simplest type of optical telescope. Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam. The magnification of such a telescope is given by

If the separation distance is equal to the sum of the focal lengths (d = f1+f2), the FFL and BFL are infinite. This corresponds to a pair of lenses that transform a parallel (collimated) beam into another collimated beam. This type of system is called an <hl> afocal system <hl>, since it produces no net convergence or divergence of the beam. Two lenses at this separation form the simplest type of optical telescope. Although the system does not alter the divergence of a collimated beam, it does alter the width of the beam. The magnification of such a telescope is given by

This type of system is called an <hl> afocal system <hl> , since it produces no net convergence or divergence of the beam.


Question: What helps correct visual defects?

Lenses are used as prosthetics for the correction of visual impairments such as myopia, hyperopia, presbyopia, and astigmatism. (See corrective lens, contact lens, eyeglasses.) Most lenses used for other purposes have strict axial symmetry; eyeglass lenses are only approximately symmetric. They are usually shaped to fit in a roughly oval, not circular, frame; the optical centres are placed over the eyeballs; their curvature may not be axially symmetric to correct for astigmatism. Sunglasses' lenses are designed to attenuate light; sunglass lenses that also correct visual impairments can be custom made.

Answer: Lenses

<hl> Lenses are used as prosthetics for the correction of visual impairments such as myopia, hyperopia, presbyopia, and astigmatism. <hl> (See corrective lens, contact lens, eyeglasses.) Most lenses used for other purposes have strict axial symmetry; eyeglass lenses are only approximately symmetric. They are usually shaped to fit in a roughly oval, not circular, frame; the optical centres are placed over the eyeballs; their curvature may not be axially symmetric to correct for astigmatism. Sunglasses' lenses are designed to attenuate light; sunglass lenses that also correct visual impairments can be custom made.

<hl> Lenses <hl> are used as prosthetics for the correction of visual impairments such as myopia, hyperopia, presbyopia, and astigmatism. (See corrective lens, contact lens, eyeglasses.) Most lenses used for other purposes have strict axial symmetry; eyeglass lenses are only approximately symmetric. They are usually shaped to fit in a roughly oval, not circular, frame; the optical centres are placed over the eyeballs; their curvature may not be axially symmetric to correct for astigmatism. Sunglasses' lenses are designed to attenuate light; sunglass lenses that also correct visual impairments can be custom made.


Question: What shapes are typical for glasses?

Answer: They are usually shaped to fit in a roughly oval, not circular, frame;

Lenses are used as prosthetics for the correction of visual impairments such as myopia, hyperopia, presbyopia, and astigmatism. (See corrective lens, contact lens, eyeglasses.) Most lenses used for other purposes have strict axial symmetry; eyeglass lenses are only approximately symmetric. <hl> They are usually shaped to fit in a roughly oval, not circular, frame; the optical centres are placed over the eyeballs; their curvature may not be axially symmetric to correct for astigmatism. <hl> Sunglasses' lenses are designed to attenuate light; sunglass lenses that also correct visual impairments can be custom made.

Lenses are used as prosthetics for the correction of visual impairments such as myopia, hyperopia, presbyopia, and astigmatism. (See corrective lens, contact lens, eyeglasses.) Most lenses used for other purposes have strict axial symmetry; eyeglass lenses are only approximately symmetric. <hl> They are usually shaped to fit in a roughly oval, not circular, frame; <hl> the optical centres are placed over the eyeballs; their curvature may not be axially symmetric to correct for astigmatism. Sunglasses' lenses are designed to attenuate light; sunglass lenses that also correct visual impairments can be custom made.


Question: What issues do eyeglasses correct?

Answer: myopia, hyperopia, presbyopia, and astigmatism

<hl> Lenses are used as prosthetics for the correction of visual impairments such as myopia, hyperopia, presbyopia, and astigmatism . <hl> (See corrective lens, contact lens, eyeglasses.) Most lenses used for other purposes have strict axial symmetry; eyeglass lenses are only approximately symmetric. They are usually shaped to fit in a roughly oval, not circular, frame; the optical centres are placed over the eyeballs; their curvature may not be axially symmetric to correct for astigmatism. Sunglasses' lenses are designed to attenuate light; sunglass lenses that also correct visual impairments can be custom made.

Lenses are used as prosthetics for the correction of visual impairments such as <hl> myopia, hyperopia, presbyopia, and astigmatism <hl>. (See corrective lens, contact lens, eyeglasses.) Most lenses used for other purposes have strict axial symmetry; eyeglass lenses are only approximately symmetric. They are usually shaped to fit in a roughly oval, not circular, frame; the optical centres are placed over the eyeballs; their curvature may not be axially symmetric to correct for astigmatism. Sunglasses' lenses are designed to attenuate light; sunglass lenses that also correct visual impairments can be custom made.

Lenses are used as prosthetics for the correction of visual impairments such as <hl> myopia, hyperopia, presbyopia, and astigmatism <hl> .


Question: What attenuates light?

Answer: Sunglasses

Lenses are used as prosthetics for the correction of visual impairments such as myopia, hyperopia, presbyopia, and astigmatism. (See corrective lens, contact lens, eyeglasses.) Most lenses used for other purposes have strict axial symmetry; eyeglass lenses are only approximately symmetric. They are usually shaped to fit in a roughly oval, not circular, frame; the optical centres are placed over the eyeballs; their curvature may not be axially symmetric to correct for astigmatism. <hl> Sunglasses ' lenses are designed to attenuate light; sunglass lenses that also correct visual impairments can be custom made. <hl>

Lenses are used as prosthetics for the correction of visual impairments such as myopia, hyperopia, presbyopia, and astigmatism. (See corrective lens, contact lens, eyeglasses.) Most lenses used for other purposes have strict axial symmetry; eyeglass lenses are only approximately symmetric. They are usually shaped to fit in a roughly oval, not circular, frame; the optical centres are placed over the eyeballs; their curvature may not be axially symmetric to correct for astigmatism. <hl> Sunglasses <hl>' lenses are designed to attenuate light; sunglass lenses that also correct visual impairments can be custom made.

<hl> Sunglasses <hl> ' lenses are designed to attenuate light; sunglass lenses that also correct visual impairments can be custom made.


Question: What is required to make a catadioptric system?

Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors. Some of these instruments produce a virtual image when applied to the human eye; others produce a real image that can be captured on photographic film or an optical sensor, or can be viewed on a screen. In these devices lenses are sometimes paired up with curved mirrors to make a catadioptric system where the lens's spherical aberration corrects the opposite aberration in the mirror (such as Schmidt and meniscus correctors).

Answer: lenses are sometimes paired up with curved mirrors

Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors. Some of these instruments produce a virtual image when applied to the human eye; others produce a real image that can be captured on photographic film or an optical sensor, or can be viewed on a screen. <hl> In these devices lenses are sometimes paired up with curved mirrors to make a catadioptric system where the lens's spherical aberration corrects the opposite aberration in the mirror (such as Schmidt and meniscus correctors). <hl>

Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors. Some of these instruments produce a virtual image when applied to the human eye; others produce a real image that can be captured on photographic film or an optical sensor, or can be viewed on a screen. In these devices <hl> lenses are sometimes paired up with curved mirrors <hl> to make a catadioptric system where the lens's spherical aberration corrects the opposite aberration in the mirror (such as Schmidt and meniscus correctors).


Question: What time of image does the human eye see?

Answer: virtual

Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors. <hl> Some of these instruments produce a virtual image when applied to the human eye; others produce a real image that can be captured on photographic film or an optical sensor, or can be viewed on a screen. <hl> In these devices lenses are sometimes paired up with curved mirrors to make a catadioptric system where the lens's spherical aberration corrects the opposite aberration in the mirror (such as Schmidt and meniscus correctors).

Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors. Some of these instruments produce a <hl> virtual <hl> image when applied to the human eye; others produce a real image that can be captured on photographic film or an optical sensor, or can be viewed on a screen. In these devices lenses are sometimes paired up with curved mirrors to make a catadioptric system where the lens's spherical aberration corrects the opposite aberration in the mirror (such as Schmidt and meniscus correctors).


Question: What is used for real image recording?

Answer: photographic film or an optical sensor

Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors. <hl> Some of these instruments produce a virtual image when applied to the human eye; others produce a real image that can be captured on photographic film or an optical sensor , or can be viewed on a screen. <hl> In these devices lenses are sometimes paired up with curved mirrors to make a catadioptric system where the lens's spherical aberration corrects the opposite aberration in the mirror (such as Schmidt and meniscus correctors).

Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors. Some of these instruments produce a virtual image when applied to the human eye; others produce a real image that can be captured on <hl> photographic film or an optical sensor <hl>, or can be viewed on a screen. In these devices lenses are sometimes paired up with curved mirrors to make a catadioptric system where the lens's spherical aberration corrects the opposite aberration in the mirror (such as Schmidt and meniscus correctors).

Some of these instruments produce a virtual image when applied to the human eye; others produce a real image that can be captured on <hl> photographic film or an optical sensor <hl> , or can be viewed on a screen.


Question: What correctors are considered catadioptric systems?

Answer: Schmidt and meniscus

Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors. Some of these instruments produce a virtual image when applied to the human eye; others produce a real image that can be captured on photographic film or an optical sensor, or can be viewed on a screen. <hl> In these devices lenses are sometimes paired up with curved mirrors to make a catadioptric system where the lens's spherical aberration corrects the opposite aberration in the mirror (such as Schmidt and meniscus correctors). <hl>

Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors. Some of these instruments produce a virtual image when applied to the human eye; others produce a real image that can be captured on photographic film or an optical sensor, or can be viewed on a screen. In these devices lenses are sometimes paired up with curved mirrors to make a catadioptric system where the lens's spherical aberration corrects the opposite aberration in the mirror (such as <hl> Schmidt and meniscus <hl> correctors).


Question: How long have burning glasses been used?

Convex lenses produce an image of an object at infinity at their focus; if the sun is imaged, much of the visible and infrared light incident on the lens is concentrated into the small image. A large lens creates enough intensity to burn a flammable object at the focal point. Since ignition can be achieved even with a poorly made lens, lenses have been used as burning-glasses for at least 2400 years. A modern application is the use of relatively large lenses to concentrate solar energy on relatively small photovoltaic cells, harvesting more energy without the need to use larger and more expensive cells.

Answer: at least 2400 years

Convex lenses produce an image of an object at infinity at their focus; if the sun is imaged, much of the visible and infrared light incident on the lens is concentrated into the small image. A large lens creates enough intensity to burn a flammable object at the focal point. <hl> Since ignition can be achieved even with a poorly made lens, lenses have been used as burning-glasses for at least 2400 years . <hl> A modern application is the use of relatively large lenses to concentrate solar energy on relatively small photovoltaic cells, harvesting more energy without the need to use larger and more expensive cells.

Convex lenses produce an image of an object at infinity at their focus; if the sun is imaged, much of the visible and infrared light incident on the lens is concentrated into the small image. A large lens creates enough intensity to burn a flammable object at the focal point. Since ignition can be achieved even with a poorly made lens, lenses have been used as burning-glasses for <hl> at least 2400 years <hl>. A modern application is the use of relatively large lenses to concentrate solar energy on relatively small photovoltaic cells, harvesting more energy without the need to use larger and more expensive cells.

Since ignition can be achieved even with a poorly made lens, lenses have been used as burning-glasses for <hl> at least 2400 years <hl> .


Question: What is used to burn an object at its focal point?

Answer: Convex lenses

<hl> Convex lenses produce an image of an object at infinity at their focus; if the sun is imaged, much of the visible and infrared light incident on the lens is concentrated into the small image. <hl> A large lens creates enough intensity to burn a flammable object at the focal point. Since ignition can be achieved even with a poorly made lens, lenses have been used as burning-glasses for at least 2400 years. A modern application is the use of relatively large lenses to concentrate solar energy on relatively small photovoltaic cells, harvesting more energy without the need to use larger and more expensive cells.

<hl> Convex lenses <hl> produce an image of an object at infinity at their focus; if the sun is imaged, much of the visible and infrared light incident on the lens is concentrated into the small image. A large lens creates enough intensity to burn a flammable object at the focal point. Since ignition can be achieved even with a poorly made lens, lenses have been used as burning-glasses for at least 2400 years. A modern application is the use of relatively large lenses to concentrate solar energy on relatively small photovoltaic cells, harvesting more energy without the need to use larger and more expensive cells.


Question: What are convex lenses used to increase the efficiency of?

Answer: photovoltaic cells

Convex lenses produce an image of an object at infinity at their focus; if the sun is imaged, much of the visible and infrared light incident on the lens is concentrated into the small image. A large lens creates enough intensity to burn a flammable object at the focal point. Since ignition can be achieved even with a poorly made lens, lenses have been used as burning-glasses for at least 2400 years. <hl> A modern application is the use of relatively large lenses to concentrate solar energy on relatively small photovoltaic cells , harvesting more energy without the need to use larger and more expensive cells. <hl>

Convex lenses produce an image of an object at infinity at their focus; if the sun is imaged, much of the visible and infrared light incident on the lens is concentrated into the small image. A large lens creates enough intensity to burn a flammable object at the focal point. Since ignition can be achieved even with a poorly made lens, lenses have been used as burning-glasses for at least 2400 years. A modern application is the use of relatively large lenses to concentrate solar energy on relatively small <hl> photovoltaic cells <hl>, harvesting more energy without the need to use larger and more expensive cells.

A modern application is the use of relatively large lenses to concentrate solar energy on relatively small <hl> photovoltaic cells <hl> , harvesting more energy without the need to use larger and more expensive cells.


Question: Do convex lenses need to be well made to burn materials?

Answer: ignition can be achieved even with a poorly made lens

Convex lenses produce an image of an object at infinity at their focus; if the sun is imaged, much of the visible and infrared light incident on the lens is concentrated into the small image. A large lens creates enough intensity to burn a flammable object at the focal point. <hl> Since ignition can be achieved even with a poorly made lens , lenses have been used as burning-glasses for at least 2400 years. <hl> A modern application is the use of relatively large lenses to concentrate solar energy on relatively small photovoltaic cells, harvesting more energy without the need to use larger and more expensive cells.

Convex lenses produce an image of an object at infinity at their focus; if the sun is imaged, much of the visible and infrared light incident on the lens is concentrated into the small image. A large lens creates enough intensity to burn a flammable object at the focal point. Since <hl> ignition can be achieved even with a poorly made lens <hl>, lenses have been used as burning-glasses for at least 2400 years. A modern application is the use of relatively large lenses to concentrate solar energy on relatively small photovoltaic cells, harvesting more energy without the need to use larger and more expensive cells.

Since <hl> ignition can be achieved even with a poorly made lens <hl> , lenses have been used as burning-glasses for at least 2400 years.


Question: What is a lens?

A lens is a transmissive optical device that affects the focus of a light beam through refraction. A simple lens consists of a single piece of material, while a compound lens consists of several simple lenses (elements), usually along a common axis. Lenses are made from transparent materials such as glass, ground and polished to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly refract radiation other than visible light are also called lenses, such as microwave lenses or acoustic lenses.

Answer: transmissive optical device that affects the focus of a light beam through refraction

<hl> A lens is a transmissive optical device that affects the focus of a light beam through refraction . <hl> A simple lens consists of a single piece of material, while a compound lens consists of several simple lenses (elements), usually along a common axis. Lenses are made from transparent materials such as glass, ground and polished to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly refract radiation other than visible light are also called lenses, such as microwave lenses or acoustic lenses.

A lens is a <hl> transmissive optical device that affects the focus of a light beam through refraction <hl>. A simple lens consists of a single piece of material, while a compound lens consists of several simple lenses (elements), usually along a common axis. Lenses are made from transparent materials such as glass, ground and polished to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly refract radiation other than visible light are also called lenses, such as microwave lenses or acoustic lenses.

A lens is a <hl> transmissive optical device that affects the focus of a light beam through refraction <hl> .


Question: What does a simple lens consist of?

Answer: single piece of material

A lens is a transmissive optical device that affects the focus of a light beam through refraction. <hl> A simple lens consists of a single piece of material , while a compound lens consists of several simple lenses (elements), usually along a common axis. <hl> Lenses are made from transparent materials such as glass, ground and polished to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly refract radiation other than visible light are also called lenses, such as microwave lenses or acoustic lenses.

A lens is a transmissive optical device that affects the focus of a light beam through refraction. A simple lens consists of a <hl> single piece of material <hl>, while a compound lens consists of several simple lenses (elements), usually along a common axis. Lenses are made from transparent materials such as glass, ground and polished to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly refract radiation other than visible light are also called lenses, such as microwave lenses or acoustic lenses.

A simple lens consists of a <hl> single piece of material <hl> , while a compound lens consists of several simple lenses (elements), usually along a common axis.


Question: What does a compound lens consist of?

Answer: several simple lenses (elements)

A lens is a transmissive optical device that affects the focus of a light beam through refraction. <hl> A simple lens consists of a single piece of material, while a compound lens consists of several simple lenses (elements) , usually along a common axis. <hl> Lenses are made from transparent materials such as glass, ground and polished to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly refract radiation other than visible light are also called lenses, such as microwave lenses or acoustic lenses.

A lens is a transmissive optical device that affects the focus of a light beam through refraction. A simple lens consists of a single piece of material, while a compound lens consists of <hl> several simple lenses (elements) <hl>, usually along a common axis. Lenses are made from transparent materials such as glass, ground and polished to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly refract radiation other than visible light are also called lenses, such as microwave lenses or acoustic lenses.

A simple lens consists of a single piece of material, while a compound lens consists of <hl> several simple lenses (elements) <hl> , usually along a common axis.


Question: What are lenses made out of?

Answer: transparent materials such as glass, ground and polished to a desired shape

A lens is a transmissive optical device that affects the focus of a light beam through refraction. A simple lens consists of a single piece of material, while a compound lens consists of several simple lenses (elements), usually along a common axis. <hl> Lenses are made from transparent materials such as glass, ground and polished to a desired shape . <hl> A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly refract radiation other than visible light are also called lenses, such as microwave lenses or acoustic lenses.

A lens is a transmissive optical device that affects the focus of a light beam through refraction. A simple lens consists of a single piece of material, while a compound lens consists of several simple lenses (elements), usually along a common axis. Lenses are made from <hl> transparent materials such as glass, ground and polished to a desired shape <hl>. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly refract radiation other than visible light are also called lenses, such as microwave lenses or acoustic lenses.

Lenses are made from <hl> transparent materials such as glass, ground and polished to a desired shape <hl> .


Question: When were lenses created?

The earliest written records of lenses date to Ancient Greece, with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire). Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. Such lenses were used by artisans for fine work, and for authenticating seal impressions. The writings of Pliny the Elder (23–79) show that burning-glasses were known to the Roman Empire, and mentions what is arguably the earliest written reference to a corrective lens: Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness, though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water.

Answer: earliest written records of lenses date to Ancient Greece

<hl> The earliest written records of lenses date to Ancient Greece , with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire). <hl> Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. Such lenses were used by artisans for fine work, and for authenticating seal impressions. The writings of Pliny the Elder (23–79) show that burning-glasses were known to the Roman Empire, and mentions what is arguably the earliest written reference to a corrective lens: Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness, though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water.

The <hl> earliest written records of lenses date to Ancient Greece <hl>, with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire). Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. Such lenses were used by artisans for fine work, and for authenticating seal impressions. The writings of Pliny the Elder (23–79) show that burning-glasses were known to the Roman Empire, and mentions what is arguably the earliest written reference to a corrective lens: Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness, though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water.

The <hl> earliest written records of lenses date to Ancient Greece <hl> , with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire).


Question: Where were lenses used for in the past?

Answer: used by artisans for fine work, and for authenticating seal impressions

The earliest written records of lenses date to Ancient Greece, with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire). Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. <hl> Such lenses were used by artisans for fine work, and for authenticating seal impressions . <hl> The writings of Pliny the Elder (23–79) show that burning-glasses were known to the Roman Empire, and mentions what is arguably the earliest written reference to a corrective lens: Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness, though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water.

The earliest written records of lenses date to Ancient Greece, with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire). Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. Such lenses were <hl> used by artisans for fine work, and for authenticating seal impressions <hl>. The writings of Pliny the Elder (23–79) show that burning-glasses were known to the Roman Empire, and mentions what is arguably the earliest written reference to a corrective lens: Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness, though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water.

Such lenses were <hl> used by artisans for fine work, and for authenticating seal impressions <hl> .


Question: When were lenses first mentioned?

Answer: Aristophanes' play The Clouds (424 BC)

<hl> The earliest written records of lenses date to Ancient Greece, with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire). <hl> Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. Such lenses were used by artisans for fine work, and for authenticating seal impressions. The writings of Pliny the Elder (23–79) show that burning-glasses were known to the Roman Empire, and mentions what is arguably the earliest written reference to a corrective lens: Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness, though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water.

The earliest written records of lenses date to Ancient Greece, with <hl> Aristophanes' play The Clouds (424 BC) <hl> mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire). Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. Such lenses were used by artisans for fine work, and for authenticating seal impressions. The writings of Pliny the Elder (23–79) show that burning-glasses were known to the Roman Empire, and mentions what is arguably the earliest written reference to a corrective lens: Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness, though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water.


Question: When were corrective lenses first mentioned?

Answer: writings of Pliny the Elder (23–79)

The earliest written records of lenses date to Ancient Greece, with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire). Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. Such lenses were used by artisans for fine work, and for authenticating seal impressions. <hl> The writings of Pliny the Elder (23–79) show that burning-glasses were known to the Roman Empire, and mentions what is arguably the earliest written reference to a corrective lens: <hl> Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness, though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water.

The earliest written records of lenses date to Ancient Greece, with Aristophanes' play The Clouds (424 BC) mentioning a burning-glass (a biconvex lens used to focus the sun's rays to produce fire). Some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. Such lenses were used by artisans for fine work, and for authenticating seal impressions. The <hl> writings of Pliny the Elder (23–79) <hl> show that burning-glasses were known to the Roman Empire, and mentions what is arguably the earliest written reference to a corrective lens: Nero was said to watch the gladiatorial games using an emerald (presumably concave to correct for nearsightedness, though the reference is vague). Both Pliny and Seneca the Younger (3 BC–65) described the magnifying effect of a glass globe filled with water.


Question: When did use lenses become common?

Lenses came into widespread use in Europe with the invention of spectacles, probably in Italy in the 1280s. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the thirteenth century, and later in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands.

Answer: Italy in the 1280s

<hl> Lenses came into widespread use in Europe with the invention of spectacles, probably in Italy in the 1280s . <hl> This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the thirteenth century, and later in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands.

Lenses came into widespread use in Europe with the invention of spectacles, probably in <hl> Italy in the 1280s <hl>. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the thirteenth century, and later in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands.

Lenses came into widespread use in Europe with the invention of spectacles, probably in <hl> Italy in the 1280s <hl> .


Question: When did the optical industry start?

Answer: Venice and Florence in the thirteenth century

Lenses came into widespread use in Europe with the invention of spectacles, probably in Italy in the 1280s. <hl> This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the thirteenth century , and later in the spectacle-making centres in both the Netherlands and Germany. <hl> Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands.

Lenses came into widespread use in Europe with the invention of spectacles, probably in Italy in the 1280s. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in <hl> Venice and Florence in the thirteenth century <hl>, and later in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands.

This was the start of the optical industry of grinding and polishing lenses for spectacles, first in <hl> Venice and Florence in the thirteenth century <hl> , and later in the spectacle-making centres in both the Netherlands and Germany.


Question: What were lenses used for?

Answer: correction of vision

Lenses came into widespread use in Europe with the invention of spectacles, probably in Italy in the 1280s. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the thirteenth century, and later in the spectacle-making centres in both the Netherlands and Germany. <hl> Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). <hl> The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands.

Lenses came into widespread use in Europe with the invention of spectacles, probably in Italy in the 1280s. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the thirteenth century, and later in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the <hl> correction of vision <hl> based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands.


Question: When was the compound optical microscope invented?

Answer: around 1595

Lenses came into widespread use in Europe with the invention of spectacles, probably in Italy in the 1280s. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the thirteenth century, and later in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). <hl> The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595 , and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands. <hl>

Lenses came into widespread use in Europe with the invention of spectacles, probably in Italy in the 1280s. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the thirteenth century, and later in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). The practical development and experimentation with lenses led to the invention of the compound optical microscope <hl> around 1595 <hl>, and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands.

The practical development and experimentation with lenses led to the invention of the compound optical microscope <hl> around 1595 <hl> , and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands.


Question: When was the refracting telescope invented?

Answer: 1608

Lenses came into widespread use in Europe with the invention of spectacles, probably in Italy in the 1280s. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the thirteenth century, and later in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). <hl> The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in 1608 , both of which appeared in the spectacle-making centres in the Netherlands. <hl>

Lenses came into widespread use in Europe with the invention of spectacles, probably in Italy in the 1280s. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the thirteenth century, and later in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses (probably without the knowledge of the rudimentary optical theory of the day). The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in <hl> 1608 <hl>, both of which appeared in the spectacle-making centres in the Netherlands.

The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in <hl> 1608 <hl> , both of which appeared in the spectacle-making centres in the Netherlands.


Question: Who invented the compound achromatic lens?

With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces. Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. This led to the invention of the compound achromatic lens by Chester Moore Hall in England in 1733, an invention also claimed by fellow Englishman John Dollond in a 1758 patent.

Answer: Chester Moore Hall

With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces. Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. <hl> This led to the invention of the compound achromatic lens by Chester Moore Hall in England in 1733, an invention also claimed by fellow Englishman John Dollond in a 1758 patent. <hl>

With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces. Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. This led to the invention of the compound achromatic lens by <hl> Chester Moore Hall <hl> in England in 1733, an invention also claimed by fellow Englishman John Dollond in a 1758 patent.


Question: When was the compound achromatic lens created?

Answer: 1733

With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces. Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. <hl> This led to the invention of the compound achromatic lens by Chester Moore Hall in England in 1733 , an invention also claimed by fellow Englishman John Dollond in a 1758 patent. <hl>

With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces. Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. This led to the invention of the compound achromatic lens by Chester Moore Hall in England in <hl> 1733 <hl>, an invention also claimed by fellow Englishman John Dollond in a 1758 patent.

This led to the invention of the compound achromatic lens by Chester Moore Hall in England in <hl> 1733 <hl> , an invention also claimed by fellow Englishman John Dollond in a 1758 patent.


Question: Where was the compound achromatic lens created?

Answer: England

With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces. Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. <hl> This led to the invention of the compound achromatic lens by Chester Moore Hall in England in 1733, an invention also claimed by fellow Englishman John Dollond in a 1758 patent. <hl>

With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces. Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. This led to the invention of the compound achromatic lens by Chester Moore Hall in <hl> England <hl> in 1733, an invention also claimed by fellow Englishman John Dollond in a 1758 patent.


Question: Who else claims to have invented the compound achromatic lens?

Answer: Englishman John Dollond

With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces. Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. <hl> This led to the invention of the compound achromatic lens by Chester Moore Hall in England in 1733, an invention also claimed by fellow Englishman John Dollond in a 1758 patent. <hl>

With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces. Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. This led to the invention of the compound achromatic lens by Chester Moore Hall in England in 1733, an invention also claimed by fellow <hl> Englishman John Dollond <hl> in a 1758 patent.


Question: What is the most common shape of lenses?

Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres. Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.

Answer: spherical

<hl> Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres. <hl> Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.

Most lenses are <hl> spherical <hl> lenses: their two surfaces are parts of the surfaces of spheres. Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.


Question: What are spherical lenses?

Answer: their two surfaces are parts of the surfaces of spheres

<hl> Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres . <hl> Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.

Most lenses are spherical lenses: <hl> their two surfaces are parts of the surfaces of spheres <hl>. Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.

Most lenses are spherical lenses: <hl> their two surfaces are parts of the surfaces of spheres <hl> .


Question: What are the different shapes of lenses?

Answer: convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat)

Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres. <hl> Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat) . <hl> The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.

Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres. Each surface can be <hl> convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat) <hl>. The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.

Each surface can be <hl> convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat) <hl> .


Question: What is the axis of the lens?

Answer: line joining the centres of the spheres making up the lens surfaces

Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres. Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). <hl> The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens. <hl> Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.

Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres. Each surface can be convex (bulging outwards from the lens), concave (depressed into the lens), or planar (flat). The <hl> line joining the centres of the spheres making up the lens surfaces <hl> is called the axis of the lens. Typically the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may be cut or ground after manufacturing to give them a different shape or size. The lens axis may then not pass through the physical centre of the lens.


Question: What kind of lens can project a real image even if it's negative?

Using a positive lens of focal length f, a virtual image results when S1 < f, the lens thus being used a magnifying glass (rather than if S1 >> f as for a camera). Using a negative lens (f < 0) with a real object (S1 > 0) can only produce a virtual image (S2 < 0), according to the above formula. It is also possible for the object distance S1 to be negative, in which case the lens sees a so-called virtual object. This happens when the lens is inserted into a converging beam (being focused by a previous lens) before the location of its real image. In that case even a negative lens can project a real image, as is done by a Barlow lens.

Answer: Barlow lens

a Barlow lens .

Using a positive lens of focal length f, a virtual image results when S1 < f, the lens thus being used a magnifying glass (rather than if S1 >> f as for a camera). Using a negative lens (f < 0) with a real object (S1 > 0) can only produce a virtual image (S2 < 0), according to the above formula. It is also possible for the object distance S1 to be negative, in which case the lens sees a so-called virtual object. This happens when the lens is inserted into a converging beam (being focused by a previous lens) before the location of its real image. In that case even a negative lens can project a real image, as is done by <hl> a Barlow lens . <hl>

Using a positive lens of focal length f, a virtual image results when S1 < f, the lens thus being used a magnifying glass (rather than if S1 >> f as for a camera). Using a negative lens (f < 0) with a real object (S1 > 0) can only produce a virtual image (S2 < 0), according to the above formula. It is also possible for the object distance S1 to be negative, in which case the lens sees a so-called virtual object. This happens when the lens is inserted into a converging beam (being focused by a previous lens) before the location of its real image. In that case even a negative lens can project a real image, as is done by a <hl> Barlow lens <hl>.


Question: If the object distance is negative then what kind of object does the lens see?

Answer: a so-called virtual object

Using a positive lens of focal length f, a virtual image results when S1 < f, the lens thus being used a magnifying glass (rather than if S1 >> f as for a camera). Using a negative lens (f < 0) with a real object (S1 > 0) can only produce a virtual image (S2 < 0), according to the above formula. <hl> It is also possible for the object distance S1 to be negative, in which case the lens sees a so-called virtual object . <hl> This happens when the lens is inserted into a converging beam (being focused by a previous lens) before the location of its real image. In that case even a negative lens can project a real image, as is done by a Barlow lens.

Using a positive lens of focal length f, a virtual image results when S1 < f, the lens thus being used a magnifying glass (rather than if S1 >> f as for a camera). Using a negative lens (f < 0) with a real object (S1 > 0) can only produce a virtual image (S2 < 0), according to the above formula. It is also possible for the object distance S1 to be negative, in which case the lens sees <hl> a so-called virtual object <hl>. This happens when the lens is inserted into a converging beam (being focused by a previous lens) before the location of its real image. In that case even a negative lens can project a real image, as is done by a Barlow lens.

It is also possible for the object distance S1 to be negative, in which case the lens sees <hl> a so-called virtual object <hl> .


Question: A virtual object can be seen when a lens is inserted into a what?

Answer: converging beam

Using a positive lens of focal length f, a virtual image results when S1 < f, the lens thus being used a magnifying glass (rather than if S1 >> f as for a camera). Using a negative lens (f < 0) with a real object (S1 > 0) can only produce a virtual image (S2 < 0), according to the above formula. It is also possible for the object distance S1 to be negative, in which case the lens sees a so-called virtual object. <hl> This happens when the lens is inserted into a converging beam (being focused by a previous lens) before the location of its real image. <hl> In that case even a negative lens can project a real image, as is done by a Barlow lens.

Using a positive lens of focal length f, a virtual image results when S1 < f, the lens thus being used a magnifying glass (rather than if S1 >> f as for a camera). Using a negative lens (f < 0) with a real object (S1 > 0) can only produce a virtual image (S2 < 0), according to the above formula. It is also possible for the object distance S1 to be negative, in which case the lens sees a so-called virtual object. This happens when the lens is inserted into a <hl> converging beam <hl> (being focused by a previous lens) before the location of its real image. In that case even a negative lens can project a real image, as is done by a Barlow lens.


Question: In this context, what does f stand for?

Answer: focal length

<hl> Using a positive lens of focal length f, a virtual image results when S1 < f, the lens thus being used a magnifying glass (rather than if S1 >> f as for a camera) <hl> . Using a negative lens (f < 0) with a real object (S1 > 0) can only produce a virtual image (S2 < 0), according to the above formula. It is also possible for the object distance S1 to be negative, in which case the lens sees a so-called virtual object. This happens when the lens is inserted into a converging beam (being focused by a previous lens) before the location of its real image. In that case even a negative lens can project a real image, as is done by a Barlow lens.

Using a positive lens of <hl> focal length <hl> f, a virtual image results when S1 < f, the lens thus being used a magnifying glass (rather than if S1 >> f as for a camera). Using a negative lens (f < 0) with a real object (S1 > 0) can only produce a virtual image (S2 < 0), according to the above formula. It is also possible for the object distance S1 to be negative, in which case the lens sees a so-called virtual object. This happens when the lens is inserted into a converging beam (being focused by a previous lens) before the location of its real image. In that case even a negative lens can project a real image, as is done by a Barlow lens.


Question: For visual telescopes or binoculars, what kind of magnification is more important?

Linear magnification M is not always the most useful measure of magnifying power. For instance, when characterizing a visual telescope or binoculars that produce only a virtual image, one would be more concerned with the angular magnification—which expresses how much larger a distant object appears through the telescope compared to the naked eye. In the case of a camera one would quote the plate scale, which compares the apparent (angular) size of a distant object to the size of the real image produced at the focus. The plate scale is the reciprocal of the focal length of the camera lens; lenses are categorized as long-focus lenses or wide-angle lenses according to their focal lengths.

Answer: angular

Linear magnification M is not always the most useful measure of magnifying power. <hl> For instance, when characterizing a visual telescope or binoculars that produce only a virtual image, one would be more concerned with the angular magnification—which expresses how much larger a distant object appears through the telescope compared to the naked eye. <hl> In the case of a camera one would quote the plate scale, which compares the apparent (angular) size of a distant object to the size of the real image produced at the focus. The plate scale is the reciprocal of the focal length of the camera lens; lenses are categorized as long-focus lenses or wide-angle lenses according to their focal lengths.

Linear magnification M is not always the most useful measure of magnifying power. For instance, when characterizing a visual telescope or binoculars that produce only a virtual image, one would be more concerned with the <hl> angular <hl> magnification—which expresses how much larger a distant object appears through the telescope compared to the naked eye. In the case of a camera one would quote the plate scale, which compares the apparent (angular) size of a distant object to the size of the real image produced at the focus. The plate scale is the reciprocal of the focal length of the camera lens; lenses are categorized as long-focus lenses or wide-angle lenses according to their focal lengths.


Question: What kind of magnification shows how much larger an object is seen in contrast to how it's seen by the naked eye?

Answer: angular magnification

Linear magnification M is not always the most useful measure of magnifying power. <hl> For instance, when characterizing a visual telescope or binoculars that produce only a virtual image, one would be more concerned with the angular magnification —which expresses how much larger a distant object appears through the telescope compared to the naked eye. <hl> In the case of a camera one would quote the plate scale, which compares the apparent (angular) size of a distant object to the size of the real image produced at the focus. The plate scale is the reciprocal of the focal length of the camera lens; lenses are categorized as long-focus lenses or wide-angle lenses according to their focal lengths.

Linear magnification M is not always the most useful measure of magnifying power. For instance, when characterizing a visual telescope or binoculars that produce only a virtual image, one would be more concerned with the <hl> angular magnification <hl>—which expresses how much larger a distant object appears through the telescope compared to the naked eye. In the case of a camera one would quote the plate scale, which compares the apparent (angular) size of a distant object to the size of the real image produced at the focus. The plate scale is the reciprocal of the focal length of the camera lens; lenses are categorized as long-focus lenses or wide-angle lenses according to their focal lengths.

For instance, when characterizing a visual telescope or binoculars that produce only a virtual image, one would be more concerned with the <hl> angular magnification <hl> —which expresses how much larger a distant object appears through the telescope compared to the naked eye.


Question: Lens are either long-focus lenses or wide-angle lenses depending on their what?

Answer: focal lengths

Linear magnification M is not always the most useful measure of magnifying power. For instance, when characterizing a visual telescope or binoculars that produce only a virtual image, one would be more concerned with the angular magnification—which expresses how much larger a distant object appears through the telescope compared to the naked eye. In the case of a camera one would quote the plate scale, which compares the apparent (angular) size of a distant object to the size of the real image produced at the focus. <hl> The plate scale is the reciprocal of the focal length of the camera lens; lenses are categorized as long-focus lenses or wide-angle lenses according to their focal lengths . <hl>

Linear magnification M is not always the most useful measure of magnifying power. For instance, when characterizing a visual telescope or binoculars that produce only a virtual image, one would be more concerned with the angular magnification—which expresses how much larger a distant object appears through the telescope compared to the naked eye. In the case of a camera one would quote the plate scale, which compares the apparent (angular) size of a distant object to the size of the real image produced at the focus. The plate scale is the reciprocal of the focal length of the camera lens; lenses are categorized as long-focus lenses or wide-angle lenses according to their <hl> focal lengths <hl>.

The plate scale is the reciprocal of the focal length of the camera lens; lenses are categorized as long-focus lenses or wide-angle lenses according to their <hl> focal lengths <hl> .


Question: What is the term for the reciprocal focal length of the lens?

Answer: plate scale

Linear magnification M is not always the most useful measure of magnifying power. For instance, when characterizing a visual telescope or binoculars that produce only a virtual image, one would be more concerned with the angular magnification—which expresses how much larger a distant object appears through the telescope compared to the naked eye. <hl> In the case of a camera one would quote the plate scale , which compares the apparent (angular) size of a distant object to the size of the real image produced at the focus. <hl> The plate scale is the reciprocal of the focal length of the camera lens; lenses are categorized as long-focus lenses or wide-angle lenses according to their focal lengths.

Linear magnification M is not always the most useful measure of magnifying power. For instance, when characterizing a visual telescope or binoculars that produce only a virtual image, one would be more concerned with the angular magnification—which expresses how much larger a distant object appears through the telescope compared to the naked eye. In the case of a camera one would quote the <hl> plate scale <hl>, which compares the apparent (angular) size of a distant object to the size of the real image produced at the focus. The plate scale is the reciprocal of the focal length of the camera lens; lenses are categorized as long-focus lenses or wide-angle lenses according to their focal lengths.

In the case of a camera one would quote the <hl> plate scale <hl> , which compares the apparent (angular) size of a distant object to the size of the real image produced at the focus.