Prentice position
Encyclopedia
The Prentice position is an orientation of a prism, used in optics
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

 and ophthalmology
Ophthalmology
Ophthalmology is the branch of medicine that deals with the anatomy, physiology and diseases of the eye. An ophthalmologist is a specialist in medical and surgical eye problems...

. In this position, the prism is oriented such that light enters it at an angle of 90° to the first surface, so that the beam does not refract
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

at that surface. All the deviation caused by the prism takes place at the exit surface.

Plastic prisms and prism bars are calibrated according to the angle of minimum deviation, whereas glass prisms are calibrated according to Prentice's rule.

Classically, the Prentice position was used only for glass prisms. With plastic prisms, the prism is held in the frontal position.
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