The incident ray I when enters the prism it gets refracted, bends and form ∠r with the normal. Angle of refraction is smaller than the angle of incidence. The incident ray bends towards the normal, as it passes from rarer medium to denser medium. When this refracted ray passes from denser medium to rarer medium it bends away from the normal. This emergent ray has bent out at an angle to the direction of incident ray. This angle is called the angle of deviation ∠D.
