Dispersion

The index of refraction of a material is not a constant but usually is dependent on the frequency and wavelength. This results in (slightly) different deflection angles for different wavelengths when light passes from one medium another. This effect is called dispersion.

The most commonly known examples of dispersion are found in prisms and rainbows. In the figure above, we schematically show the refraction and reflection of sunlight on water droplets, giving rise to rainbows. The average angle between incoming and reflected light is 41^$\circ$. However, the smaller the wavelength, the bigger the refraction angle; red is deflected less than blue. This gives rise to rainbows.

Another example for dispersion is the refraction of light by a prism, as shown below:

 

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