Please refer to the figure above.
ABCABC is a cross-section of a triangular block of transparent material. Usually it is in the form of an isosceles triangle. Triangular block is called Prism.
A ray of light PQPQ is incident on one of its faces at point EE, making an angle PEN∠PEN, also called the angle of incidence angle i∠i, at the point of incidence. N N' being the normal at the point of incidence.
As the ray of light enters the transparent material, it moves from air, a rarer medium, to a denser medium. Refractive index mu of the material of the block being more than that of air. The ray of light is refracted and bends towards the normal.
The ray of light moves in a straight path through the block and impinges the other face of the block at F. As it emerges out of the block, it moves from a denser to a rarer medium, and therefore, bends away from the normal MM' at point F. Once out of the block it proceeds again along a straight line path RS as the emergent ray.
In the ray diagram angle of emergence and the angle of deviation are also depicted.
