Prism (optics)

Prism (optics), block of glass or other transparent material that has the same cross-section—usually a triangle—along its length. The two commonest types of prism have cross-sections that are 60° and 45° triangles. Prisms have various effects on light passing through them.

When a ray of white light is directed on to a 60° prism, its different-coloured components are refracted, or bent, to a different extent on passing through each surface, so that a coloured band of light called a spectrum is produced. This is known as dispersion, and is caused by the fact that the different colours of light have different wavelengths, and are slowed down to different extents when they pass through the glass—red light being slowed least, and violet light most. The 17th-century English physicist Isaac Newton was the first to conclude, from experiments with prisms, that ordinary sunlight is a mixture of all the different colours.

When a ray of light is directed at a suitable angle in to a prism, it strikes the face of the prism internally at an angle greater than the critical angle (see Optics; Geometrical Optics), and is therefore totally internally reflected. The prism then acts as a highly efficient mirror, and this arrangement is used in many optical instruments, such as periscopes and binoculars.