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en.wikipedia.org/wiki/Speed_of_light

The speed at which light propagates through transparent materials, such as glass or air, is less than c; similarly, the speed of electromagnetic waves in wire cables is slower than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for  ...

www.space.com/15830-light-speed.html

Mar 6, 2018 ... The speed of light in a vacuum is 186282 miles per second (299792 kilometers per second), and in theory nothing can travel faster than light.

www.quora.com/Is-the-speed-of-light-same-in-air-or-vacuum

In a vacuum, the speed of light remains constant assuming its path does not get interrupted by another medium. This speed, known as C, is equal to - 299792458 m / s Now when light encounters a medium, its phase velocity is altered and it ends up...

www.rpi.edu/dept/phys/Dept2/APPhys1/optics/optics/node4.html

Have you heard these statements before? They are often quoted as results of Einstein's theory of relativity. Unfortunately, these statements are somewhat misleading. Let's add a few words to them to clarify. "Nothing can travel faster than the speed of light in a vacuum." "Light in a vacuum always travels at the same speed.

math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html

Light is slowed down in transparent media such as air, water and glass. The ratio by which it is slowed is called the refractive index of the medium and is usually greater than one.* This was discovered by Jean Foucault in 1850. When people talk about "the speed of light" in a general context, they usually mean the speed of ...

www.popsci.com/scientists-have-found-way-slow-down-speed-light-it-travels-through-air

Jan 23, 2015 ... Two photons, or particles of light approach a finish line used to determine if light can travel at different speeds through the air. Illustration courtesy University of Glasgow. Light passes through air at about 299,000,000 meters per second, an accepted constant that hasn't been challengedâ€”until now.

www.education.com/science-fair/article/refraction-fast-light-travel-air

How fast does light travel, and does it travel faster in water or air? The fastest thing in the whole universe is the speed of light in a vacuum (like outer space!), clocking in at a great 2.99 x 108 m/s. Light travels in waves, and we call this traveling propagation. Propagation of waves has both a speed and a direction, called the ...