Prism

Description

Light is refracted, i.e. its path changes, when it passes through a solid, transparent material with a triangular cross-section, such as a prism. The beam of light passes from the air to the prism (and vice versa), causing it to be refracted and resolved into the colours of the iris.  The prism separates the colours at different angles, depending on their wavelength, and thus, a rainbow is formed!