How do lenses work? | What are the different types of lens? Tweet by Chris Woodford . Last updated: May 25, 2016. Microscopes let us peer inside invisible worlds our eyes could never see, telescopes take us far beyond the Earth to the stars and planets of the night sky, movie projectors throw enormous images onto screens, and lighthouses cast reassuring beams of light far across the ocean. Amazing curves of glass or plastic called lenses make all these things possible. Let's take a closer look at what they are and how they work! Photo: Lenses in the headlamps of this car focus beams of light down onto the road so you can see where you're going. Some car headlights use Fresnel lenses to make powerful beams, just like lighthouses! What are lenses? A lens is a transparent piece of glass or plastic with at least one curved surface. It gets its name from the Latin word for "lentil" (a type of pulse used in cooking), but don't let that confuse you. There's no real reason for this other than that the most common kind of lens (called a convex lens) looks very much like a lentil! Photo: Lentils gave lenses their name.

Convex lenses bulge out in the middle like lentils, while concave lenses "cave in" in the middle and bulge out at the edges. How do lenses work? A lens works by refraction: it bends light rays as they pass through it so they change direction. (You can read a full explanation of why this happens in our article on light .) That means the rays seem to come from a point that's closer or further away from where they actually originate—and that's what makes objects seen through a lens seem either bigger or smaller than they really are. Types of lenses There are two main types of lenses, known as convex (or converging) and concave (or diverging). Convex lenses In a convex lens (sometimes called a positive lens), the glass (or plastic) surfaces bulge outwards in the center giving the classic lentil-like shape. A convex lens is also called a converging lens because it makes parallel light rays passing through it bend inward and meet (converge) at a spot just beyond the lens known as the focal point. Photo: A convex lens makes light rays converge (come together) at the focal point or focus.

The distance from the center of the lens to the focal point is the focal length of the lens. Convex lenses are used in things like telescopes and binoculars to bring distant light rays to a focus in your eyes. Concave lenses A concave lens is exactly the opposite with the outer surfaces curving inward, so it makes parallel light rays curve outward or diverge. That's why concave lenses are sometimes called diverging lenses. (One easy way to remember the difference between concave and convex lenses is to think of concave lenses caving inwards.) Photo: A concave lens makes light rays diverge (spread out). Concave lenses are used in things like TV projectors to make light rays spread out into the distance. In a flashlight, it's easier to do this job with a mirror , which usually weighs much less than a lens and is cheaper to manufacture as well. Compound lenses It's possible to make lenses that behave in more complex ways by combining convex and concave lenses. A lens that uses two or more simpler lenses in this way is called a compound lens. How do you measure the power of a lens?

If you've ever looked through binoculars , a telescope , or a magnifying glass, you'll know that some lenses magnify (or reduce) the apparent size of an object much more than others. There's a simple measurement that tells you how powerful a lens is and it's known as the focal length. The focal length of a lens is the distance from the center of the lens to the point at which it focuses light rays. The shorter the focal length, the more powerful the lens. (It's easy to see why: an ordinary piece of glass would be like a lens of infinite focal length and wouldn't bring light rays to a focus at all. On the other hand, an infinitely powerful lens would bring lays rays to a focus in an infinitely short distance, with zero focal length. A real lens is somewhere between these two extremes.) You'll find focal lengths written either in ordinary units of length (such as centimeters, millimeters, or inches) or in special optical units called diopters.

The diopter measurement of a lens is the reciprocal of the focal length in meters (one divided by the focal length), so 1 diopter = 1 m, 2 diopters = 0.5 m, 3 diopters = 0.33 meters, and so on. Eyeglass prescriptions from opticians typically show the strength of the corrective lenses you need in diopters. The focal length isn't the only important feature of a lens. Bigger lenses gather more light than smaller ones, so they make a brighter image. This is particularly important if you're choosing a lens for a camera , because the amount of light the lens gathers will determine what the image looks like. Camera lenses are usually rated with a measurement called the f-number, which is the focal length divided by the diameter. Generally speaking, lenses with a small f-number make brighter images. Lenses with a higher f-number have a bigger depth of focus: essentially, more of the object you're photographing and its surroundings are in focus at the same time. (If you want to know more, take a look at Louis Bloomfield's explanation of lens size .) How are lenses made?

Until plastics became common in the 20th century, virtually all lenses were made by grinding solid pieces of glass into different shapes. Convex lenses were made by using a concave-shaped grinding tool (and vice-versa), and then the roughly shaped lens was polished to make its final shape. The ordinary glass we use in windows and crockery isn't good enough to use for lenses, because it contains air bubbles and other imperfections. These cause light rays to divert from their correct path, making a fuzzy image or one that makes different colors of light behave in different ways (problems that optical scientists refer to as aberrations). Instead, lenses are made using a more refined material known as optical glass. For eyeglasses, many people now prefer plastic lenses because they're much lighter and safer than optical glass. Plastic lenses can be molded to shape, instead of being ground, so they can be made in huge quantities far more cheaply than glass lenses. Although ordinary plastic scratches easily, it can be coated with a thin layer of a protective material such as diamond-like carbon (DLC) to reduce the risk of damage. Photo: This magnifying glass uses a single convex lens made from plastic. Make a water lens!

Do this in your kitchen or bathroom to avoid making a mess. Take an old newspaper or magazine no-one wants anymore. Lay a small piece of cellophane, cling film, or clear plastic on top of the newspaper. You don't need much—maybe a piece half the size of a paperback book cover. Using an eye-dropper, pipette, syringe, teaspoon, or even the tip of your pinkie, place a single, small drop of water on top of the cling film. Look at the newsprint and you should be able to see that the water drop (which has a curved upper edge and a flat lower edge) magnifies the words. Well done, you just made a lens! What happens if you make the water drop bigger or smaller? What if you lift the plastic away from the paper and move your lens nearer or further from the print? What other cunning things can you do to change the way your lens works? Like all great scientists, take the chance to play around and experiment. Photo: I made this water lens by cutting a small piece of plastic from a grocery bag and laying it over a newspaper. I dropped the water on, very slowly and carefully, using a teaspoon. What are lenses used for?

Lenses are everywhere in the world around us—in everything from car headlamps and flashlights to the LED lights used in electronic instrument panels. Photo: You might not have noticed, but the tiny LEDs (light-emitting diodes) used in instrument panels have tiny plastic lenses built into them to magnify the light they produce. The lens is the curved plastic on the left (the top of the LED that shines toward you.) Our eyes contain probably the most amazing lenses of all. Think what happens when you look at the world around you. One minute you're staring at the ground in front of your feet. Seconds later, you hear an airplane screaming past, turn your head, and watch it fly by. Do this trick with a pair of binoculars and you'll find it takes you quite a while to adjust the focus from near-sight (looking at the ground) to far (watching the plane). Try it with the naked eye and you won't even notice what you're doing. That's because your eyes have flexible lenses, controlled by tiny muscles, that can bulge in and out, changing shape instantly to focus on anything from the prints on your finger to the surface of the Moon. How amazing is that?

We all have lenses in our eyes, but many of us balance extra ones on the end of our noses to correct long and short sight: more glass and plastic lenses are used for eyeglasses and contact lenses than for any other purpose. There are all kinds of eyeglass lenses, including light-sensitive photochromic ones that darken in sunlight and double-up as sunglasses. You'll also find lenses in binoculars (which use two or three lenses in each of the cylinders serving your eyes) and telescopes , though not all microscopes use them. Ordinary (optical) microscopes use a series of glass lenses to magnify tiny objects, while super-powerful electron microscopes use electromagnets to bend electron beams that help us see in even more detail. Movie projectors and projection televisions use lenses to convert small movie pictures into giant images that lots of people can view at once. Cameras work the opposite way, catching light rays from a distance and bringing them to focus on chemically treated plastic film or light-sensitive electronic chips called CCDs .

You can even find lenses built into magazine and book covers to make images change as you shift your head from side to side; this cunning trick is called lenticular printing —but it really just means "printing with built-in lenses." Photos: Lighthouses don't use huge and heavy lenses: instead, they rely on Fresnel lenses (ones with a stepped surface pattern of concentric rings) and prisms, like the one in this exhibit at Think Tank, the science museum in Birmingham, England. Read how they work in our article on Fresnel lenses . What type of lens bends parallel rays inwards: convex or concave? | febb 1 reply on “What type of lens bends parallel rays inwards: convex or concave?” says: Lens & Ray Diagrams - MrCorfe.com Lens & Ray Diagrams There are two main types of lens: Shape Converging Diverging The thickness of the lens affects how much it bends light and therefore the power of the lens. This can be measured using the focal length (f) of the lens. For a convex lens this is the distance from the centre of the lens to the point at which parallel rays of light converge (meet).

The shorter the distance the more powerful the lens, the light has been refracted more. For a concave lens this is the distance from the centre of the lens to a where rays appear to have come from when parallel rays entered the lens. However is is quite rare that we will be dealing with parallel rays as they are from a distance source. The lenses will will bring the rays of light to a focus at points other than the focal distance. To understand this we need to be able to construct ray diagrams. To draw a ray diagram, we draw 3 rays front the top of the object: Horizontally to the axis of the lens and the through the far focal point Through the centre of the lens which is no refracted ie straight through Through the near focal point to the axis of the lens and then horizontally out the otherside Where the rays converge (meet/cross) or appear to have come from converging is were the image is formed. Convex Lens You can move and change the size of the object (black arrow) using your mouse. See what it does to the rays and where the image (blue arrow) is formed.

The green lines represent where the rays appear to have come from Thin Lens demonstration by Fu-Kwun Hwang The nature of the image produced depends on where the object is relative to the focal length: If the image is larger than the object, we say 'the image has been magnified'. If it is smaller then we say 'the image has been diminished'. It the image is formed on the other side of the axis to the object, then we say 'the image is inverted'. If the images is formed on the same side then we say 'the image is upright'. If the rays that form the image actually cross then we say 'the image is real'. This type of image can be projected on to a screen. If the image is formed where rays appeared to come from then we say 'the image is virtual'. In order for the images to be seen, it takes the extra lens in the eye to converge the rays. Use either the applet above, or construct your own ray diagrams on paper , to complete the table below: Position of object Q & A: Light rays though lenses | Department of Physics | University of Illinois at Urbana-Champaign University of Illinois at Urbana-Champaign Q & A: Light rays though lenses Learn more physics!

Explain how parell light rays react, when traveling though an convex or concave lens. - Isaac Tan (age 11) Johns Hill Magnet School, Decatur, IL- USA A: Isaac, The only thing you have to know to understand how all lenses work is that light, when going from air into glass, will bend away from the surface of the glass. This is shown in the picture below: The light (red line) starts in the air and hits the glass in the bottom half of the picture. As it enters the glass, it bends away from the surface as shown. (It is actually more correct to say that it bends toward the dashed line, which is perpendicular to the surface, but don't worry about it). When light travels from glass into air, the exact opposite thing happens: It will bend toward the surface, like in the next picture: If we put these pictures together, we can understand both convex and concave lenses. A convex lens can take parallel light rays and bend them together, as shown below. Light comes in from the left, hits the glass and is bent inward (away from the surface).

When it gets to the other side of the lens it is bent again on the way out, this time toward the surface, changing its direction even more. A concave lens can take parallel light rays and bend them apart, as shown in the final picture. Again, each light ray is bent away from the surface in the way into the glass and toward the surface on the way back out. Mats Greeneville, TN A: Hi Bess- I'm putting up a quick answer so you'll have something to help right away in class. We may improve it as we find better illustrations, etc. I've marked your question as a follow-up to an old one that has some pictures of how light rays bend as they pass through lenses. On the second question, I think the key thing is to use lots of pictures. Draw a little figure, with rays of light coming off say the head and the foot. Trace how those rays would go through the lens. As the rays reach an eyeball, they each come from some direction.Tracing them backward to where they seem to come from shows that the concave spreading makes them look like they're coming from points closer together than they really were. Maybe that's what you've already done.

This site has some useful sketches: . As I said, we'll look for some better figures, but your own sketches probably would work well enough. Mike W. Definition of Convex lens | define Convex lens - physics dictionary Online - Convex Lens Definition of Convex Lens A lens which is thinner at the edges and thicker at the centre is known as convex lens. Generally lenses are made up of glass or transparent plastic and the same can be used for transmitting, refracting or for focusing the light. More About Convex Lens A convex lens is supposed to be made up of number of prisms with the base of each prism towards the centre of the lens. When rays parallel to the principal axis pass through a convex lens, they are bent inwards. The point at which they converge is known as principle focus. . The distance from the principle focus to the centre of the lens is known as focal length. This type of lens bends the light inwards. The following are the types of convex lenses: Double convex Concavo-convex Example: Magnifying glass. When an object is placed closer to a convex lens than the principle focus, the rays never converge and magnified image is formed on the same side of the object.

Thick, bulging convex lenses have the shortest focal lengths, which are commonly used as powerful magnifying glasses. Translate : Lenses and Mirrors TITLE: Concave and Convex Mirrors and Lenses Presented to grade level 5 GOALS Concave and Convex Mirrors and Lenses is designed to teach fifth grade level students about light relection and refraction as shown in curved mirrors and lenses. SPECIFIC OBJECTIVES Light rays are reflected off of opaque surfaces and if these surfaces are very shiny as mirrors are, they will produce an image. A concave mirror bends in towards the center like the inside of a bowl and a convex mirror bends outward to the edges. Light rays are reflected inwards toward a focal point by concave mirrors. A concave mirror can produce images that are right side up and very large or upside down. Light rays are reflected outwards by a convex mirror. Images produced by a convex mirror will always appear right side up and smaller than the actual object. Light rays are refracted as they pass through translucent objects such as lenses. A concave lens is thicker at the edges than it is at the center, and a convex lens is thicker in the middle than it is at its edges.

A concave lens refracts light rays spreading them outward producing an image that is upright and smaller than the actual object. A convex lens refracts light rays inward. If a convex lens is held close to a person�s eyes, they will see an image that is upright and larger than the actual object. If the convex lens is held further away from a person�s eyes, they will see an image upside down. A refracting telescope uses two convex lenses to magnify images in the sky. A reflecting telescope uses a concave mirror, a plane mirror, and a convex lens to do the same job. A microscope, like a reflecting telescope, uses a concave mirror, a plane mirror, and a convex lens. However, they are used to magnify very small images on slides not in the sky. BACKGROUND Concave mirrors bend inward like a cave. These mirrors reflect light rays inward towards a focal point. Images will appear upside down from far away, but very close to the mirror, they will appear right side up and very large. In contrast, concave lenses reflect light rays outwards. The image through a concave lens will always be right side up. Convex mirrors curve outwards and reflect light rays away from the center.

Images will appear right side up and smaller than the actual object. Convex lenses, on the other hand, refract light rays in toward a central point. The image seen through a convex lens is upside down when the lens is held away from your face, but the image is magnified and right side up when the lens is held closer to your eye. SOLs COVERED SOL 5.9 The students will describe the effects of lenses and mirrors on the path of light. Descriptive statement: Experiences should include the manipulation of lenses and mirrors and examples of their uses in flashlights, eyeglasses, cameras, and telescopes. DEMONSTRATIONS AND EXHIBITS Materials needed: tennis ball, cereal bowl, shiny spoon, concave-lensed eyeglasses, magnifying glass, two convex lenses, plane mirror, concave mirror, microscope; also possible access to a reflecting telescope. 1. Light rays are reflected from mirrors. Concave mirrors: With a tennis ball and a cereal bowl, show how light rays will reflect back to a central point. It is helpful to draw the reflected rays on an overhead.

Using a concave mirror, show students how they will appear upside down from far away and right side up very close to the mirror. You can use a shiny spoon as a replacement for a mirror. Convex mirrors: With a cereal bowl and a tennis bowl, bounce the tennis ball off of the back side of the bowl to show how light rays will reflect outwards off of a convex mirror. It is helpful to draw the rays reflecting on an overhead. Using the back side of a spoon, show students that their image will always be right side up and smaller than normal. Compare this image with the front of the spoon. 2. Light rays are refracted as they travel through lenses. Concave lenses: Using concave-lensed eyeglasses allow students to look through the lens to observe the upright image. Draw parallels to the convex mirror which reflects light rays away from the center. Use an image to show how concave lenses cure nearsightedness when an eyeball is too long. Convex lenses: Use a magnifying glass to demonstrate how images can be either right side up or upside down. Draw parallels to the concave mirror which reflects rays toward a central point.

Make an overhead to explain how convex lenses can be used to correct farsightedness when an eyeball is too short. 3. Telescopes and microscopes use different combinations of mirrors and lenses to magnify objects that are hard to see. (See Resources for local public access to both reflecting and refracting telescopes.) A reflecting telescope uses two convex lenses. Use two successive convex lenses to show the students how they can magnify an image. If you have access to a reflecting telescope, allowing them to look through one would be helpful. A refracting telescope uses a concave mirror, a plane mirror, and a convex lens to magnify images. Have the students try to arrange these three components to magnify an image. A microscope uses the same arrangement of lenses and mirrors to magnify slides. Show students a cell under a microscope. (See interactive exercise .) INTERACTIVE EXERCISES Materials needed: microscope, clean microscope slide and cover, sample specimen (onion layer, a typed letter, eyelash, etc.) 1. Prepare a slide for the microscope. Let the students see the item without magnification, and prepare a dry or wet mount (as appropriate for the sample).

They can then observe the specimen under the microscope and compare the images. Is it inverted? Why or why not? The Anatomy of a Lens Refraction and the Ray Model of Light - Lesson 5 - Image Formation by Lenses The Anatomy of a Lens The Anatomy of a Lens The Mathematics of Lenses If a piece of glass or other transparent material takes on the appropriate shape, it is possible that parallel incident rays would either converge to a point or appear to be diverging from a point. A piece of glass that has such a shape is referred to as a lens. A lens is merely a carefully ground or molded piece of transparent material that refracts light rays in such as way as to form an image. Lenses can be thought of as a series of tiny refracting prisms, each of which refracts light to produce their own image. When these prisms act together, they produce a bright image focused at a point. Types of Lenses There are a variety of types of lenses. Lenses differ from one another in terms of their shape and the materials from which they are made. Our focus will be upon lenses that are symmetrical across their horizontal axis - known as the principal axis.

In this unit, we will categorize lenses as converging lenses and diverging lenses. A converging lens is a lens that converges rays of light that are traveling parallel to its principal axis. Converging lenses can be identified by their shape; they are relatively thick across their middle and thin at their upper and lower edges. A diverging lens is a lens that diverges rays of light that are traveling parallel to its principal axis. Diverging lenses can also be identified by their shape; they are relatively thin across their middle and thick at their upper and lower edges. A double convex lens is symmetrical across both its horizontal and vertical axis. Each of the lens' two faces can be thought of as originally being part of a sphere. The fact that a double convex lens is thicker across its middle is an indicator that it will converge rays of light that travel parallel to its principal axis. A double convex lens is a converging lens. A double concave lens is also symmetrical across both its horizontal and vertical axis. The two faces of a double concave lens can be thought of as originally being part of a sphere.

The fact that a double concave lens is thinner across its middle is an indicator that it will diverge rays of light that travel parallel to its principal axis. A double concave lens is a diverging lens. These two types of lenses - a double convex and a double concave lens will be the only types of lenses that will be discussed in this unit of The Physics Classroom Tutorial . The Language of Lenses As we begin to discuss the refraction of light rays and the formation of images by these two types of lenses, we will need to use a variety of terms. Many of these terms should be familiar to you because they have already been discussed during Unit 13 . If you are uncertain of the meaning of the terms, spend some time reviewing them so that their meaning is firmly internalized in your mind. They will be essential as we proceed through Lesson 5. These terms describe the various parts of a lens and include such words as Principal axis Convex and Concave Lenses in the Ophthalmic Industry - United Lens Convex and Concave Lenses in the Ophthalmic Industry With 75 percent of Americans using some type of corrective lens, the ophthalmic industry plays an important role in our world today.

This percentage translates to roughly 225 million people with 64 percent of this group wearing glasses and 11 percent using contact lenses. Historically, these glasses lenses were made using glass and grinding it into various shapes. Plastics became a more popular material for lenses in the 20th century. Instead of grinding the plastic, the material is molded to the desired shape. This method can be done in mass quantities and for much less money than its glass counterpart. Though plastic is much lighter to wear in a pair of glasses, it also scratches more easily. So, protective coatings have been introduced to reduce possible damage. Regardless of if it is plastic or glass, the theory remains the same on how a lens helps improve a person’s vision. Refraction is what makes a lens work. As rays pass through the lens, the light rays bend and change direction. This gives the illusion that the rays come from a point closer or farther from actuality, and, therefore, making objects seen through the lens seem smaller or larger than they are in reality. To achieve either of these results, there are two basic lens types—convex and concave. Convex lenses can also be referred to as positive lenses or converging.

With a surface curving outward in the center, the lens bends parallel light rays to meet, or converge, at a point just past the lens, which is the focal point. Farsighted people use glasses with convex lenses, and you’ll also find this type in telescopes and binoculars. Concave, or diverging, lenses have the exact opposite effect. The outer surface curves inward, making parallel light rays diverge. Nearsighted people wear glasses with concave lenses. TV projectors also spread light rays using this type of lens. With medical and scientific advancements, it is also possible to have a combination of these two lenses in a pair of glasses. Named the compound lens, it offers a more complex design for multiple purposes. For example, some people require a lens to see small words in a book, as well as far away street signs, which means part of the lens is concave and part is convex. More products use lens technology than the glasses we see used every day. Many medical fields require the use of microscopes for research and tools using lenses to check and correct a patient’s’ vision imperfections or eye disorders.

United Lens Company has supported the ophthalmic industry for 100 years using a special molding process for a wide range of glasses types. Our experience helps us easily work on custom projects and continue to innovate for only the best products. We know how important it can be to see properly, as it’s how most of us obtain and share information with one another. That’s why our standards are at their strictest in safety, quality and accuracy before products leave our facility. For more information on lenses or the ophthalmic industry, contact our team. LENSES LENSES FIND OUT MORE A lens is a piece of transparent plastic or glass that can make things seem to change size. It works by bending light rays so they appear to come from a slightly different place. Some lenses make things look nearer and bigger. Others make things look smaller and further away. Without their spectacle lenses, many people would be unable to see clearly, read books, or drive safely. CONTACT LENS A contact lens is a tiny piece of plastic or glass that rests on the front of the eyeball. It bends light rays before they enter the eye in ways that help the wearer to see more clearly.

CONCAVE AND CONVEX LENSES The two main types of lens are called concave and convex. A concave lens is thin in the middle and thick at the edges, so it seems to “cave” inwards. It makes light rays bend outwards, or diverge. A convex lens works in the opposite way. It is thicker in the middle and thinner at the edges. Light rays passing through a convex lens bend inwards, or converge. FRESNEL LENS A lighthouse must send a long beam of light far out to sea. To do that a very large and heavy lens would normally be needed. Instead, lighthouses use a specially shaped Fresnel lens. It has steps like a staircase, each of which helps to bend the light into a single, powerful beam. A Fresnel lens can be made from glass or lightweight plastic. SLIDE PROJECTOR By shining a powerful beam of light through a transparent photographic slide, a projector can make a much larger image on a wall. The little image on the slide is shined through a concave lens, which spreads the light rays outwards. The further away the projector is from the wall, the bigger the image becomes.

MAGNIFYING GLASS To magnify means to increase in apparent size. If you look closely at a magnifying glass, you can see that it is a large convex lens, thicker in the centre than at the edges. When you hold a magnifying glass over an object, it makes light rays from the object seem to come from a nearer point, causing it to look bigger than it really is. Concave spherical lens | Article about concave spherical lens by The Free Dictionary Concave spherical lens | Article about concave spherical lens by The Free Dictionary Related to concave spherical lens: concave spherical mirror lens, device for forming an image image, in optics, likeness or counterpart of an object produced when rays of light coming from that object are reflected from a mirror or are refracted by a lens. An image of an object is also formed when this light passes through a very small opening like that of a pinhole ..... Click the link for more information. of an object by the refraction refraction, in physics, deflection of a wave on passing obliquely from one transparent medium into a second medium in which its speed is different, as the passage of a light ray from air into glass. ..... Click the link for more information.

of light. In its simplest form it is a disk of transparent substance, commonly glass, with its two surfaces curved or with one surface plane and the other curved. Lenses are used singly or in groups in such instruments as cameras camera, lightproof box or container, usually fitted with a lens, which gathers incoming light and concentrates it so that it can be directed toward the film (in an optical camera) or the imaging device (in a digital camera) contained within. ..... Click the link for more information. , projectors, microscopes microscope, optical instrument used to increase the apparent size of an object. Simple Microscopes A magnifying glass, an ordinary double convex lens having a short focal length, is a simple microscope. The reading lens and hand lens are instruments of this type. ..... Click the link for more information. , telescopes telescope, traditionally, a system of lenses, mirrors, or both, used to gather light from a distant object and form an image of it. Traditional optical telescopes, which are the subject of this article, also are used to magnify objects on earth and in astronomy; other types of ..... Click the link for more information.

, binoculars binocular, small optical instrument consisting of two similar telescopes mounted on a single frame so that separate images enter each of the viewer's eyes. As with a single telescope, distant objects appear magnified, but the binocular has the additional advantage that it ..... Click the link for more information. , opera glasses, and eyeglasses eyeglasses or spectacles, instrument or device for aiding and correcting defective sight. Eyeglasses usually consist of a pair of lenses mounted in a frame to hold them in position before the eyes. ..... Click the link for more information. . The lens of the eye eye, organ of vision and light perception. In humans the eye is of the camera type, with an iris diaphragm and variable focusing, or accommodation. Other types of eye are the simple eye, found in many invertebrates, and the compound eye, found in insects and many other ..... Click the link for more information. is known as a crystalline lens. Classification of Lenses All rays of light passing through a lens are refracted (bent) except those that pass directly through a point called the optical center. Lenses are classified according to the way in which they bend the rays of light entering them.

Parallel rays of light passing through converging lenses are bent toward one another; these lenses are thicker at the center than at the edges. Examples are the double convex lens (both surfaces curved outward as in the simple magnifying glass), the plano-convex (one flat and one convex surface), and the concavo-convex (one surface concave, the other convex). Diverging lenses bend parallel rays away from one another; they are thicker at the edges than at the center. Examples are the double concave lens (both surfaces curved inward), the plano-concave (one surface flat, the other concave), and the convexo-concave (one surface convex, the other concave). Design and Production of Lenses Generally each curved surface of a lens is made as a portion of a spherical surface. The center of the sphere is called the center of curvature of the surface; every point on the surface is equidistant to it, this distance being the radius of curvature. The line joining the two centers of curvature also passes through the optical center of the lens and is called the principal axis. Any other line through the optical center at an angle to the principal axis is called a secondary axis.

In converging lenses all rays entering parallel to the principal axis are bent toward a point on the principal axis called the principal focus. The distance from the principal focus to the optical center of the lens is the focal length of the lens. It varies with different lenses, according to the curvature of the surfaces and index of refraction of the lens material. Conjugate points are two points on opposite sides of a lens in such position that rays from one, after passing through the lens, will converge at the other. Light rays are not always brought to a focus at one point; this condition of inexact focus is known as aberration and may be of two types: spherical, resulting from the shape of the lens, and chromatic, resulting from the fact that different colors are refracted by different amounts (see aberration aberration, in optics, condition that causes a blurring and loss of clearness in the images produced by lenses or mirrors. Of the many types of aberration, the two most significant to the lens maker are spherical and chromatic. ..... Click the link for more information. , in optics).

Lenses have long been made of glass; a piece roughly approximating the desired size and shape of the lens is cut from a glass block and then ground and polished to the correct curvature. Great skill and accuracy are required in this process and also in mounting the lenses so that the principal axes of all the lenses fall on the same line. A number of transparent plastics that permit the lenses to be cast in a mold are used as substitutes for glass. Formation of Images The image formed by a diverging lens is always virtual (cannot be projected on a screen as can a real image), erect (upright), and smaller than the object and is located on the same side of the lens as the object. The image formed by a converging lens depends on the position of the object relative to the focal length of the lens and the center of curvature. If the object is beyond the center of curvature, the image is real, inverted, and smaller than the object. As the object is brought toward the lens, the size of the image grows, becoming as large as the object when the object is at the center of curvature and larger than the object as the object is brought closer.

When the object is one focal length away from the lens, however, no image at all is formed; and when the object moves closer than this distance, the image becomes virtual, erect, and larger than the object, as when one uses a magnifying glass. Lens (läNs), city (1990 pop. 35,278), Pas-de-Calais dept., N France. During the 19th and 20th cent. it was one of the most important coal centers in N France, but its mines are now closed. Much of the city's manufacturing has been replaced by service-oriented industries. The victory there (1648) of the French under Louis II de Condé was the last important battle of the Thirty Years War Thirty Years War, 1618–48, general European war fought mainly in Germany. General Character of the War There were many territorial, dynastic, and religious issues that figured in the outbreak and conduct of the war. ..... Click the link for more information. . Lens was occupied and devastated by the Germans in both world wars. A branch of the Louvre Louvre , foremost French museum of art, located in Paris.

The building was a royal fortress and palace built by Philip II in the late 12th cent. In 1546 Pierre Lescot was commissioned by Francis I to erect a new building on the site of the Louvre. ..... Click the link for more information. museum opened here in 2012. Lens (optics) A curved piece of ground and polished or molded material, usually glass, used for the refraction of light. Its two surfaces have the same axis. Usually this is an axis of rotation symmetry for both surfaces; however, one or both of the surfaces can be toric, cylindrical, or a general surface with double symmetry (see illustration). The intersection points of the symmetry axis with the two surfaces are called the front and back vertices and their separation is called the thickness of the lens. There are three lens types, namely, compound, single, and cemented. A group of lenses used together is a lens system. Such systems may be divided into four classes: telescopes, oculars (eyepieces), photographic objectives, and enlarging lenses.

Common lenses Lens types A compound lens is a combination of two or more lenses in which the second surface of one lens has the same radius as the first surface of the following lens and the two lenses are cemented together. Compound lenses are used instead of single lenses for color correction, or to introduce a surface which has no effect on the aperture rays but large effects on the principal rays, or vice versa. Sometimes the term compound lens is applied to any optical system consisting of more than one element, even when they are not in contact. The diameter of a simple lens is called the linear aperture, and the ratio of this aperture to the focal length is called the relative aperture. This latter quantity is more often specified by its reciprocal, called the f-number. Thus, if the focal length is 50 mm and the linear aperture 25 mm, the relative aperture is 0.5 and the f-number is f/2. See Focal length A compound lens made of two or more simple thin lenses cemented together is called a cemented lens. Lens systems A lens system consisting of two systems combined so that the back focal point of the first (the objective) coincides with the front focal point of the second (the ocular) is called a telescope.

Parallel entering rays leave the system as parallel rays. The magnification is equal to the ratio of the focal length of the first system to that of the second. See Telescope A photographic objective images a distant object onto a photographic plate or film. The amount of light reaching the light-sensitive layer depends on the aperture of the optical system, which is equivalent to the ratio of the lens diameter to the focal length. The larger the aperture (the smaller the f-number), the less adequate may be the scene luminance required to expose the film. Therefore, if pictures of objects in dim light are desired, the f-number must be small. On the other hand, for a lens of given focal length, the depth of field is inversely proportional to the aperture. In general, photographic objectives with large fields have small apertures; those with large apertures have small fields. The basic type of enlarger lens is a holosymmetric system consisting of two systems of which one is symmetrical with the first system except that all the data are multiplied by the enlarging factor m. When the object is in the focus of the first system, the combination is free from all lateral errors even before correction.

A magnifier in optics is a lens that enables an object to be viewed so that it appears larger than its natural size. The magnifying power is usually given as equal to one-quarter of the power of the lens expressed in diopters. See Diopter , Magnification Lenses: (a) converging; (b) diverging lens A specially shaped piece of transparent material, such as glass, quartz, or plastic, bounded by two surfaces of a regular (usually spherical) but not necessarily identical curvature. A ray of light passing through a lens is bent as a result of refraction . Spherical lenses may be either converging or diverging in their action on a light beam, depending on their shape (see illustration). See also aberration ; coating of lenses ; eyepiece ; focal length ; objective . Lens (geological), a mode of occurrence of rock and ores in the form of a lens that tapers toward the edges. The dimensions of a lens vary, ranging from several meters in length and several centimeters in thickness to 1 km and more in length and several dozen meters in thickness. Lens a transparent body limited by two light-refracting surfaces; one of the main elements of an optical system.

In the most useful lenses, both surfaces have a common axis of symmetry; among these, the most useful and easiest to produce are those with spherical surfaces. Less common are lenses with two mutually perpendicular planes of symmetry and with surfaces that are either cylindrical or toroidal. Such lenses are used in the eyeglasses prescribed for astigmatism of the eye and anamorphic attachments. Optical glass and organic glass are the most commonly used materials for lenses. Special lenses for use in the ultraviolet region of the spectrum are made from crystals of quartz, fluorite, and lithium fluoride. Lenses for use in the infrared region of the spectrum are produced from special glasses, silicon, germanium, fluorite, lithium fluoride, and cesium iodide. Description of the optical properties of axially symmetrical lenses usually involves the rays that are incident to the lens at a small angle with respect to the axis. These rays constitute what is called the parallaxial pencil of rays.

The effect of a lens on these rays is determined by the position of its cardinal points (that is, of the principal points H and H’), at which the principal planes of the lens intersect the axis, and by the position of its front and back focuses F and F’ (see Figure 1). The segments HF = f and H’ F’ = f’ are called the lens’ focal lengths; when the media adjoining the lens have the same indexes of refraction, f is always equal to f’. The points O of intersection of the surfaces of the lens with the axis are called its vertexes. The distance between the vertexes is called the thickness. Figure 1 The geometric values that characterize lenses and systems of lenses are considered positive if the directions of corresponding segments coincide with the direction of the light rays. In Figure 1, the rays pass through the lens from left to right and segment H’ F’ is oriented the same way; for this reason, f’ > 0 and f < 0. Refraction at the surfaces of the lens alters the directions of the incident rays. If the lens transforms a parallel beam into a convergent beam, it is called a converging lens.

A parallel beam becomes a divergent one after passing through a diverging lens. Rays that were parallel to the lens axis before refraction intersect at the principal focus F’ of a converging lens, for which f’ is always positive. The principal focus F’ of a diverging lens is the point of intersection not of the rays themselves but of their imaginary continuation in the direction opposite the direction of propagation of the light, so that f’ < 0. The externally distinguishing feature of thin converging and diverging lenses is that the edges are less thick than the center in the case of the former and thicker in the case of the latter. The measure of the refracting power of a lens is its lens power Φ, which is a quantity inversely proportional to the focal length (Φ = 1/f’) and measured in diopters (m-1). For converging lenses, Φ > 0, so that the lenses are called positive. Diverging lenses, for which Φ < 0, are called negative. Afocal lenses, for which Φ = 0, are also used; the focal length in these cases equals infinity.

These lenses have neither a convergent nor a divergent effect on light rays but generate aberrations and are used in mirror-lens and lens objectives as aberration compensators. All of the parameters determining the optical properties of a lens bounded by spherical surfaces may be expressed in terms of the radii of curvature r1 and r2 of the surfaces, the lens thickness along the axis d, and the index of refraction n of the lens’ materials. For example, the lens power and the focal length of a lens are given by the relationship The radii r1 and r2 are considered positive if the direction from the vertex of the lens to the center of the corresponding surface coincides with the direction of the rays (r1 > 0, r2 < 0 in Figure 1). Equation (1) is valid only when applied to paraxial rays. The shape of a lens may differ given the same material and lens power. Figure 2 shows several ienses with the same optical power and different forms. The first three lenses are positive; the last three are negative. A lens is called thin if its thickness d is small in comparison to r1 and r2.

A sufficiently accurate expression of the optical power of such a lens is obtained when the second term in equation (1) is omitted. Figure 2 The position of the principal planes with respect to the vertexes may also be determined from the values of r1, r2, n, and d. The distance between the principal planes is relatively independent of the form and power of a lens and is approximately equal to [d(n— 1)]/n. In the case of a thin lens this distance is small; it may be assumed for practical purposes that the principal planes coincide. When the positions of the cardinal points are known, the position of the optical image of a point produced by the lens (see Figure 1) is determined by the equations x · x′ = f · f′ = f′2 (2) l′/l = -f/x = -x′/f′ = V where V is the linear magnification of the lens, l and l’ are the distances from the point and its image to the axis (positive, if they are located above the axis), x is the distance from the front focus to the point, and x’ is the distance from the back focus to the image.

If t and t’ are the distances from the principal points to the planes of the object and image, respectively, then (since x = t - f, x’ = t’ - f’) f′/t′ + f/t = 1

Question: What type of lens bends parallel rays inwards: convex or concave?
Answer: Convex