In optics a prism is a transparent optical element with flat polished surfaces that refract light. At least two of the flat surfaces must have an angle between them. The exact angles between the surfaces depend on the application.

A dispersive prism can be used to break light up into its constituent spectral colors (the colors of the rainbow it can help you find the pot of gold!). Furthermore prisms can be used to reflect light or to split light into components with different polarizations.