Mirrors

Prisms can be roughly divided into three general categories: reflecting prisms, polarising prisms, and refracting or dispersion prisms. The former are useful for redirecting light beams by total internal reflection while the latter can be employed to bend and separate light into its component colours.

Mirrors are commonly utilised to fold the light beam through an optical system. Prisms can also serve an identical function, except that the reflecting internal surfaces of prisms behave as rigidly mounted mirrors with each face having a permanent orientation with respect to all others. This feature is attractive to designers, because once a prism has been constructed, it will retain orientational parameters that do not deviate and require no further adjustment in the final assembly, except for positioning the prism unit itself. Depending upon the entrance angle for a light beam, prisms can refract light or allow it to enter undeviated and undergo total internal reflection.