Prisms

Excerpt

A prism is a transparent, triangular refracting surface with an apex and a base. The two nonparallel surfaces intersect at an angle called the apex, and the surface opposite to the apex forms the bottom of the prism. The light rays refracted through the prism bend towards the base. The amount of deviation of the path of refracted light from the incident light depends on the power of the prism measured in "prism diopters."

Charles Prentice was the first to introduce the term prism diopters to describe the intensity of prism. One prism diopter represents the deviation of light by 1 centimeter and perpendicular to the initial direction of the light ray on a plane placed 1 meter away from the prism. The power of the prism in prism diopters is represented by the symbol D. Thus, a prism of 2 prism diopters would deviate a light ray by 2 centimeters, perpendicular to the direction of the initial light ray, measured 100 cm beyond the prism.

Another unit of measurement of prism power is centrad. This is less frequently used as compared to prism diopters. Centrad unit is represented by the symbol Ñ. One centrad represents the deviation of light by 1 centimeter and perpendicular to the initial direction of the light ray on an arc of a circle 1 meter away from the prism. Further, the deflection of the light ray after passing through the prism also depends on the refractive index of the material and the position in which the prism is held. It is essential to understand that the light ray passing through the prism deviates towards its base, but the image appears to be displaced towards the apex. Thus, the eye being tested will deviate towards the apex of the prism.