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.

Consider 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.

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

If we consider light passing from a vacuum (or air) to a medium the ratio of the velocities is called the absolute refractive index of the medium. The velocity of light in a vacuum (or air) is c.

In the diagram above

so the relative refractive index of light passing from medium 1 to medium 2 is given by

It has been shown, using Huygens’ principle, that

therefore, we have

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