Tutorial 22: Dispersion I

In the previous simulations on refraction we assumed that all wavelengths bend by the same amount. This is not true; the index of refraction changes slightly for different wavelengths. So if we start with several different wavelengths (different colors) we expect there may be some situations in which the colors will separate. If the sides of the medium are parallel, each color unbends by the same amount that it bent going into the medium so all the colors are again going in the same direction. However if the sides are not parallel, such as a prism or lens, there will be a separation of color. This is in fact how a prism and water droplents separate colors and why good camera lenses (which compensate for this effect by using compound lenses) are expensive.

The change of wave speed as a function of wavelength is called dispersion and occurs for all types of waves. For example, longer wavelength surface waves on the ocean travel faster than shorter wavelength waves. There is not much dispersion for sound waves in air but acoustic waves in solids do experience significant dispersion. The simulation below is for visible light passing through a prism. You can choose the color and see what the index is for that wavelength. A different manifestation of dispersion is shown in the next simulation.