When light enters a solid material from air, the speed at which the light travels decreases. If the light enters at an off-normal angle, the direction of the light changes. The light is said to be refracted and the refracted angle is given by Snell's law:
n1 sin theta1 = n2 sin theta2
Reflection and refraction of an incident light ray at a surface
A prism is a transparent optic that is shaped to bend light. Since the refractive index of a material varies with wavelength, prisms are useful for dispersing different wavelengths of light.
Dispersion of white light by a prism:
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Copyright © 1996 by Brian M. Tissue