Dove prism
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A dove prism
A dove prism is a type of reflective prism which is used to invert an image. Dove prisms are shaped from a truncated right-angle prism. A beam of light entering one of the sloped faces of the prism undergoes total internal reflection from the inside of the longest (bottom) face and emerges from the opposite sloped face. Images passing through the prism are flipped, and because only one reflection takes place, the image's handedness is changed to the opposite sense.
Dove prisms have an interesting property that when they are rotated along their longitudinal axis, the transmitted image rotates at twice the rate of the prism. This property means they can rotate a beam of light by an arbitrary angle, making them useful in beam rotators, which have applications in fields such as astronomy.