Basic Optical Principles
Prism

Geometrically, a prism may be said to look like a triangle. The parts of a prism include its apex and base as illustrated above in figure C. Since the sides of a prism are not parallel, rays of light entering it are usually at some angle other than parallel to the normal.

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Light is always bent toward the base of a prism.

A prism has the ability to displace images. That is, when a ray of light which has been refracted toward the base of the prism enters the eye, it is perceived as having been traveling in a straight line, even though in reality it has been bent. Therefore, the image of the original object appears to be in a position other than the position from which the light had originally emanated. This is known as image displacement. Figure D above illustrates this principle where the letter �O� indicates the original position of the object and the letter �I� is the location of the displaced image. If the original object is one meter from the prism, and the image is displaced a distance of one centimeter, that prism is said to have the power of one prism diopter. This basic definition of a prism diopter may be expressed mathematically as: 1 cm/1m = 1.