| As 
                          the dove prism is rotated about its own long axis, the 
                          orientation of its image rotates at twice the angular 
                          displacement. Thus, an image can be rotated through 
                          180¡ã by rotating the dove prism through only 90¡ã . Nevertheless, 
                          the dove prism is simply one section of a right-angle 
                          prism  A 
                          collimated beam of light directed into the prism through 
                          one inclined face will be refracted toward the base, 
                          where total internal reflection inverts the beam and 
                          directs it out through the second inclined face  Because 
                          the two inclined faces are symmetrically angled with 
                          respect to the base, the output beam travels the same 
                          trajectory as the input beam; there is no deviation 
                          or displacement of the beam. Engineers 
                          use dove prisms to invert an image or to provide continuous 
                          control of the orientation of an inverted image. Limitations 
                          are related to its size (it must be rather long compared 
                          to its aperture) and to its aberrational effects upon 
                          beams that are converging or diverging. Collimated beams 
                          are preferred because they do not experience aberration 
                          as they pass through the prism. |