1) Let us consider that APQR represents outline of the prism where it rests on its triangular base.
2) Let us assume that a light ray is incident on the plane surface PQ of a prism at M. This ray is called 'incident ray'. Draw a perpendicular to PQ at M. It becomes a normal to that surface.
3) The angle between incident ray and normal is called "angle of incidence" (i).
4) The ray is refracted at M. It moves through prism and meets the other plane surface at N and finally comes out of the prism. The ray which comes out of the surface PR at N is called "emergent ray".
5) Draw a perpendicular to PR at point N. The angle between the emergent ray and normal is called angle of emergence
6) The angle between -the plane surfaces PQ and PR is called the angle of the prism or refracting "angle of prism" (A).
7) The angle between the incident ray and emergent ray formed by producing theme backwards is called "angle of deviation" (D).