Wikipedia article: Speed of light

Question: how fast is the speed of light in a vacuum
Short answer: 299,792,458 metres per second ( approximately 300,000 km / s ( 186,000 mi / s ) )
Answer: The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 7008299792458000000 ♠ 299,792,458 metres per second ( approximately 300,000 km / s ( 186,000 mi / s ) ). It is exact because by international agreement a metre is defined to be the length of the path travelled by light in vacuum during a time interval of 1 / 299792458 second. According to special relativity, c is the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel. Though this speed is most commonly associated with light, it is in fact the speed at which all massless particles and changes of the associated fields travel in vacuum ( including electromagnetic radiation and gravitational waves ). Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. In the special and general theories of relativity, c interrelates space and time, and also appears in the famous equation of mass -- energy equivalence E = mc.

Question: who discovered that light is faster than sound
Short answer: Abū Rayhān al - Bīrūnī agreed
Answer: Empedocles ( c. 490 -- 430 BC ) was the first to propose a theory of light and claimed that light has a finite speed. He maintained that light was something in motion, and therefore must take some time to travel. Aristotle argued, to the contrary, that `` light is due to the presence of something, but it is not a movement ''. Euclid and Ptolemy advanced Empedocles ' emission theory of vision, where light is emitted from the eye, thus enabling sight. Based on that theory, Heron of Alexandria argued that the speed of light must be infinite because distant objects such as stars appear immediately upon opening the eyes. Early Islamic philosophers initially agreed with the Aristotelian view that light had no speed of travel. In 1021, Alhazen ( Ibn al - Haytham ) published the Book of Optics, in which he presented a series of arguments dismissing the emission theory of vision in favour of the now accepted intromission theory, in which light moves from an object into the eye. This led Alhazen to propose that light must have a finite speed, and that the speed of light is variable, decreasing in denser bodies. He argued that light is substantial matter, the propagation of which requires time, even if this is hidden from our senses. Also in the 11th century, Abū Rayhān al - Bīrūnī agreed that light has a finite speed, and observed that the speed of light is much faster than the speed of sound.

Question: what is the speed in miles per second of a beam of light traveling at
Short answer: 186 000
Answer: Speed of light Sunlight takes about 8 minutes 17 seconds to travel the average distance from the surface of the Sun to the Earth. Exact values metres per second 7008299792458000000 ♠ 299 792 458 Planck length per Planck time ( i.e., Planck units ) Approximate values ( to three significant digits ) kilometres per hour 7003108000000000000 ♠ 1080 million ( 7009108000000000000 ♠ 1.08 × 10 ) miles per second 7005186000000000000 ♠ 186 000 miles per hour 671 million ( 7008671000000000000 ♠ 6.71 × 10 ) astronomical units per day 173 parsecs per year 0.307 Approximate light signal travel times Distance Time one foot 1.0 ns one metre 3.3 ns from geostationary orbit to Earth 119 ms the length of Earth 's equator 134 ms from Moon to Earth 1.3 s from Sun to Earth ( 1 AU ) 8.3 min one light year 1.0 year one parsec 3.26 years from nearest star to Sun ( 1.3 pc ) 4.2 years from the nearest galaxy ( the Canis Major Dwarf Galaxy ) to Earth 7011788940000000000 ♠ 25 000 years across the Milky Way 7012315576000000000 ♠ 100 000 years from the Andromeda Galaxy to Earth 2.5 million years from Earth to the edge of the observable universe 46.5 billion years

Question: what is the speed of light in km
Short answer: 300,000 km / s (
Answer: The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 7008299792458000000 ♠ 299,792,458 metres per second ( approximately 7008300000000000000 ♠ 3.00 × 10 m / s, or 300,000 km / s ( 186,000 mi / s ) ). It is exact because the unit of length, the metre, is defined from this constant and the international standard for time. According to special relativity, c is the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel. Though this speed is most commonly associated with light, it is in fact the speed at which all massless particles and changes of the associated fields travel in vacuum ( including electromagnetic radiation and gravitational waves ). Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. In the theory of relativity, c interrelates space and time, and also appears in the famous equation of mass -- energy equivalence E = mc.

Question: how fast does light travel in a vaccum
Answer: The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 7008299792458000000 ♠ 299,792,458 metres per second ( approximately 300,000 km / s ( 186,000 mi / s ) ). It is exact because by international agreement a metre is defined to be the length of the path travelled by light in vacuum during a time interval of 1 / 299792458 second. According to special relativity, c is the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel. Though this speed is most commonly associated with light, it is in fact the speed at which all massless particles and changes of the associated fields travel in vacuum ( including electromagnetic radiation and gravitational waves ). Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. In the special and general theories of relativity, c interrelates space and time, and also appears in the famous equation of mass -- energy equivalence E = mc.

Question: what is the value of speed of light in vaccum
Short answer: 299,792,458 metres per second (
Answer: The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 7008299792458000000 ♠ 299,792,458 metres per second ( approximately 300,000 km / s ( 186,000 mi / s ) ). It is exact because by international agreement a metre is defined to be the length of the path travelled by light in vacuum during a time interval of 1 / 299792458 second. According to special relativity, c is the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel. Though this speed is most commonly associated with light, it is in fact the speed at which all massless particles and changes of the associated fields travel in vacuum ( including electromagnetic radiation and gravitational waves ). Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. In the special and general theories of relativity, c interrelates space and time, and also appears in the famous equation of mass -- energy equivalence E = mc.

Question: who discovered the speed of light in a vacuum
Answer: In 1972, using the laser interferometer method and the new definitions, a group at the US National Bureau of Standards in Boulder, Colorado determined the speed of light in vacuum to be c = 7008299792456200000 ♠ 299 792 456. 2 ± 1.1 m / s. This was 100 times less uncertain than the previously accepted value. The remaining uncertainty was mainly related to the definition of the metre. As similar experiments found comparable results for c, the 15th General Conference on Weights and Measures in 1975 recommended using the value 7008299792458000000 ♠ 299 792 458 m / s for the speed of light.

Question: velocity of light is minimum in which medium
Answer: The phase velocity is important in determining how a light wave travels through a material or from one material to another. It is often represented in terms of a refractive index. The refractive index of a material is defined as the ratio of c to the phase velocity v in the material: larger indices of refraction indicate lower speeds. The refractive index of a material may depend on the light 's frequency, intensity, polarization, or direction of propagation; in many cases, though, it can be treated as a material - dependent constant. The refractive index of air is approximately 1.0003. Denser media, such as water, glass, and diamond, have refractive indexes of around 1.3, 1.5 and 2.4, respectively, for visible light. In exotic materials like Bose -- Einstein condensates near absolute zero, the effective speed of light may be only a few metres per second. However, this represents absorption and re-radiation delay between atoms, as do all slower - than - c speeds in material substances. As an extreme example of light `` slowing '' in matter, two independent teams of physicists claimed to bring light to a `` complete standstill '' by passing it through a Bose -- Einstein condensate of the element rubidium, one team at Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other at the Harvard -- Smithsonian Center for Astrophysics, also in Cambridge. However, the popular description of light being `` stopped '' in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse. During the time it had `` stopped, '' it had ceased to be light. This type of behaviour is generally microscopically true of all transparent media which `` slow '' the speed of light.

Question: who discovered that light travels faster than sound
Short answer: Abū Rayhān al - Bīrūnī agreed
Answer: Early Islamic philosophers initially agreed with the Aristotelian view that light had no speed of travel. In 1021, Alhazen ( Ibn al - Haytham ) published the Book of Optics, in which he presented a series of arguments dismissing the emission theory of vision in favour of the now accepted intromission theory, in which light moves from an object into the eye. This led Alhazen to propose that light must have a finite speed, and that the speed of light is variable, decreasing in denser bodies. He argued that light is substantial matter, the propagation of which requires time, even if this is hidden from our senses. Also in the 11th century, Abū Rayhān al - Bīrūnī agreed that light has a finite speed, and observed that the speed of light is much faster than the speed of sound.