What is a prism? Draw a ray diagram to show the refraction of a light ray through a prism.

A prism is a transparent medium bounded by five plane surfaces with a triangular cross section.

In symbol form it is represented by the triangle ABC :

The refraction of a light ray through a prism is shown below.

The light ray PQ of a single colour is incident obliquely on the surface AB of the prism. As the ray PQ moves from air (rarer) to glass (denser), it bends towards normal and travels as refracted ray QR. The refracted ray QR at the face AC of the prism again undergoes refraction (bends away from the normal) as it moves from glass (denser) to air (rarer) and emerges out as ray RS (emergent ray) from the prism.