i. Variation of angle of incidence i with angle of deviation δ is as shown in figure.
ii. It shows that, with increasing values of i, the angle of deviation δ decreases initially to a certain minimum (δm) value and then increases.
iii. The curve shown in the figure is not a symmetric parabola, but the slope in the part towards right is less.
iv. Except at δ = δm there are two values of i for any given δ. From principle of reversibility of light, we can conclude that if one of these values is i, the other must be e and vice versa.
Thus at δ = δm, we have i = e.