Light travels fastest in vacuum, followed by air, water, glass, and diamond. This is because the speed of light in a medium is inversely proportional to the refractive index of the medium. The refractive index of a medium is a measure of how much light bends when it passes through the medium. The refractive index of vacuum is 1, the
refractive index of air is 1.0003, the refractive index of water is 1.33, the refractive index of glass is 1.52, and the refractive index of diamond is 2.42.When light passes from one medium to another, it bends. This is called refraction. The amount of refraction depends on the difference in the refractive indices of the two media. The greater the difference in the refractive indices, the greater the amount of refraction.
When light passes from vacuum to air, there is very little refraction. This is because the refractive indices of vacuum and air are very close to each other. When light passes from air to water, there is more refraction. This is because the refractive index of water is greater than the refractive index of air. When light passes from water to glass, there is even more refraction. This is because the refractive index of glass is greater than the refractive index of water. When light passes from glass to diamond, there is the most refraction. This is because the refractive index of diamond is the greatest of all the media mentioned.