A prism is a clear solid used to manipulate the path of a light ray, either by bending it, splitting it, or polarizing it. Generally, the light ray is reflected one or more times within the body of the prism, an effect known as total internal reflection.

Prisms can be found in a wide variety of shapes and sizes and can possess a number of sides. Any prism whose lateral faces are perpendicular to the base is called a right prism; other prisms are called oblique prisms. Most are made of glass or quartz and are at least partially transparent, allowing light to pass through the body.

One of the best known prismatic effects is the rainbow. When a rainbow occurs, thousands of tiny drops of water suspended in the air act as prisms, splitting sunlight into its color spectrum. In addition to separating colors, prisms, such as those used in binoculars and telescopes, help to bend and direct light, as well as to correct color aberrations.

One particular prism, the Nicol prism, is used to convert simple light into polarized light. Invented by the Scottish scientist William Nicol, this device is actually composed of two prisms cemented together with Canadian balsam; the completed system has the effect of polarizing light along a certain plane.