The speed of light (c) in a vacuum, is exactly299,792,458 meters per second (ms-1), which rounds up very nicely to 300,000,000 ms-1, which scientists write as 3 x 108 ms-1.

In more dense media the speed is slower,

e.g. # Air, only slightly less than c, speed is 0.9997 of c. # water 0.75 of c. # fused quartz 0.686 of c. # crown glass* 0.658 of c. # dense flint glass* 0.60 of c. # diamond, approx 0.41 of c. CALCULATIONS The speed of light, using a very close approximation for calculation purposes, is taken as 3.0 x 108 m/s (metres per second). i.e. 300,000,000 m/s The greater the refractive index of the medium, the slower the speed of light in that medium/material. The speed of light in a vacuum [c] divided by the velocity of light in the material [v] equals the refractive index [n] of the material. Examples 1. calculate the refractive index of space: c/c equals 1 2. water (the speed of light in water is 225,056,264 m/s): c/ 225056264 equals 1.333 Inversely, if we know the refractive index of a material, we can calculate the velocity of light through that material, i.e. the speed of light [c] divided by the refractive index [n] equals the velocity of light [v] in that material Using the example of water, refractive index 1.333 :- 300,000,000 [c] divided by 1.333 [n] equals 225,056,264 [n] i.e. the velocity of light in water is 225,056,264 m/s. This is 225,056,264/300,000,000 ths the speed of light, or 0.75 of c. SOME REFRACTIVE INDICES * vacuum1.00 * air 1.0003 * water 1.333 * fused quartz 1.4585 * plexiglass 1.51 * crown glass* 1.52 * diamond 2.417 * gallium phosphide 3.50 *Crown glass is a type of glass used in lenses, and has a lower refractive index than flint glass which is also used in lenses. For more information, see Related Links below this box

