The refractive index indicates how light travels through a substance compared to a vacuum. It is determined by dividing the speed of light in a vacuum by the speed of light in the substance. The index can vary for different types of light and materials.
An index of refraction is a numerical value that indicates how light will travel through a particular substance in relation to how it travels in a vacuum. In general, the index is given 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 means 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 spectra of light; therefore an index will usually also indicate the type of light energy used.
Also called refractive index, the refractive index of a material is indicated in relation to vacuum. When people generally refer to the speed of light, about 3×108 meters per second (m/s), what they are usually referring to is the speed of light in a vacuum. This speed is a constant and is the standard against which other speeds are compared when dealing with light and determining the value of refraction for a particular material.
The refractive index indicates how much the speed of light slows down when traveling through a particular medium. When light passes from a vacuum into clear glass, for example, it still passes through the glass, but the higher density of the glass alters how light passes through it. Light is slowed as it passes through the glass and its direction is changed slightly as it passes through the glass; this is true for any type of transparent material. This effect can often be observed by inserting an upright stick into a body of water. When light traveling from the stick to a person’s eye travels between water and air, it is refracted and the stick appears bent in the water.
To determine the refractive index for a particular medium, someone determines the speed of light traveling through that medium and then divides the speed of light in a vacuum by the speed in the medium. This value is shown as a decimal and this number is shown as the refraction value for that material. The index for a particular medium may be slightly different for different light frequencies, so usually the type of light used, often a yellow light, is indicated. 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. times faster in a vacuum than a diamond.
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