What Is a Prism (in Optics)?

An optical prism is typically a triangular block of glass or crystal. When white light enters one face of the prism, it slows down and bends (refracts) at the air-glass boundary. Different wavelengths bend by different amounts — violet light bends most, red least — because the refractive index of glass varies with wavelength (dispersion). This separates white light into a continuous spectrum of colours: red, orange, yellow, green, blue, indigo, and violet. Isaac Newton famously demonstrated this in 1666 and proved that white light is a mixture of all colours, not a pure substance as previously believed.