MadSci Network: Physics |
The speed of light in a vacuum is the universal constant c, which is 2.998 * 10^8 meters/second. Now, in any medium (e.g. air, water, or glass, etc.), light will travel slower than this. This is because light is essentially an electromagnetic wave, and the transmission of this wave depends on the electrical characteristics and density of the medium it is passing through. The 'index of refraction' (N) of a medium gives the average velocity (v) of light in it by the relation: v = c/N where c is the speed of light in vacuum, above. N is equal to 1 for a vacuum, and will be greater than 1 for any other medium. Below are the approximate indices of refraction of a few materials for visible light: Medium N ------ ------ air ~1.02 ice 1.309 water 1.333 glass 1.50 quartz 1.644 diamond 2.417 You can look for other indices in books like the CRC Handbook of Physics, or other sources. Keep in mind that these numbers are approximate, and that the index of refraction of a material can change with temperature, pressure, and even the wavelength of the light (which is how prisms work!).
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