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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.
Also called the index of refraction, the refractive index of a material is indicated in relationship to a vacuum. When people generally refer to the speed of light, about 3x108 meters per second (m/s), what they are usually referring to is the speed of light through a vacuum. This speed is a constant and is the standard against which other speeds are compared when dealing with light and determining the refractive index for a particular material.
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.
To determine the refractive index for a particular medium, someone determines the velocity of light traveling through that medium and then divides the velocity of light in a vacuum by the velocity in the medium. This value is indicated as a decimal and this number is referred to as the refractive index for that material. The index for a particular medium can be slightly different for different light frequencies, and so the type of light used is usually indicated, often a yellow light. Water has a refractive index of about 1.33, meaning that light in a vacuum travels about 1.33 times faster than light in water, while diamonds have an index of about 2.4, meaning light travels twice as fast in a vacuum as it does through a diamond.