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what is a refractive index?

A refractive index is a numerical value that indicates the way in which light will travel through a particular substance in relation to how it travels through a vacuum. In general, the index is indicated as a decimal value, but it is based on the speed of light in a vacuum divided by the speed of light traveling through a particular medium, such as glass or water. These other mediums tend to reduce the speed of light, and alter its direction, and so the resulting number has a value greater than one. The refractive index for a given material can be different for different spectrums of light; therefore an index will usually indicate the type of light energy used as well.

The refractive index indicates how much the velocity of light is reduced when traveling through a particular medium. As light passes from a vacuum into clear glass, for example, it still passes through the glass, but the greater density of the glass alters the way in which the light is traveling through it. The light is slowed down as it passes through the glass, and its direction is altered slightly while going through the glass; this is true of any type of clear material. This effect can often be observed by placing a straight stick into a body of water. As the light traveling from the stick to a person’s eye travels between the water and the air, it is refracted and the stick appears bent in the water.


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