Box with seven equilateral prisms

A prism is a triangular, transparent piece of material, usually glass. When a narrow light beam falls on a prism, a band of colours will appear behind the prism. In this way Isaac Newton discovered that white light is composed of several colours. This set of colours is called the colour spectrum. The phenomenon occurs because every colour is refracted in the prism in a different way. The violet light, for example, is refracted more strongly than the red light. Prisms are often used to analyse light. As from the middle of the nineteenth century, research into electromagnetic spectra (spectrography) increased enormously. Such a spectrum can be made visible by means of a prism. By studying such a spectrum thoroughly, it became possible to gain more knowledge about the composition of gases and stars.