The Complete Handbook of Prisms

Chapter 1: Understanding Prisms

Light does not travel in a straight line. This is the first practical fact to embrace when working with prisms. When a beam of light passes from one transparent medium into another, its path bends. This phenomenon, called refraction, is the foundation on which every prism, from the tiny cube in a binocular to the massive optics in a research spectrometer, is built. Before you can spec a prism for a project, you must understand that a prism is not a passive piece of glass. It is a precise, passive optical computer that uses geometry and physics to control the path and character of light. Understanding this is the difference between buying a part and engineering a solution.

The Nature of Refraction and Dispersion

Refraction occurs because light changes speed as it moves between media with different optical densities. In a vacuum, light travels at its maximum speed. When it enters glass, it slows down. This change in speed causes the light's path to bend at the interface between the two materials. The amount of bending is governed by a material property known as the index of refraction, commonly denoted as 'n'. For air, the index is approximately 1.00. For typical optical glasses, it ranges from about 1.5 to over 1.9. The higher the index, the slower light travels in that material and the more it bends.

This bending is not a small effect. Consider a standard right-angle prism made of N-BK7 glass, a common optical