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Refraction of Light
When light goes from one medium to another its speed can change.
The change in speed can result in a change in the direction of motion of the light.

The change in direction is called refraction.

Optical Density
Given two transparent media, A and B, if light travels more slowly in medium B than in medium A, we say that medium B is more (optically) dense than medium A.

Refractive Index
The diagram below represents a narrow beam of light passing across a boundary between two different media.
No arrows have been drawn on the light because it could be going either way; the path would be the same.

The refractive index for light passing from medium 1 to medium 2 (written 1n2) is defined as

Since the situation is reversible, it is clear that

If medium 1 is a vacuum (or air) then the ratio of the two velocities is called the absolute refractive index of medium 2.

For example:
The velocity of light in a vacuum, c is about 108ms-1 and in water is about 2.2108ms-1 so the absolute refractive index of water is

Using the letters on the diagram above we can write
Therefore, we have

It has been shown, using Huygens' principle that

which therefore gives

usually written as

This is a useful equation when calculating the trajectory of light when it passes through a series of different media.
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