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A prism is an optical devce that will refract light. Typically it is in the shape of a triangular prism. The angles in the prism are determined by the practical use in which the prism is being used. The material of prisms cane be a number of transparent materials however, they are typically glass, plastic or fluorite. 

Prisms work through the principle that light changes speed from one material to another. The change of speed is due to refraction. The angle at which the lights path is bend is referred to as the refractive index of the two materials. The refractive index varies with the wavelength or color of the light used; this is known as dispersion.

Prisms are sometimes used for the internal reflection at the surface rather than just for dispersion. If light inside the prism hits one of the surfaces at a sufficiently steep angle, the total internal reflection occurs. This is when all the light is reflected. A prism is a useful substitute for a mirror in this instance.