(a) How does the angle of deviation produced by a prism change with increase in the angle of incidence. Draw a curve showing the variation in the angle of deviation with the angle of incidence at a prism surface.

(b) Using the curve in part (a) above, how would you infer that for a given prism, the angle of minimum deviation 𝛿_min is unique for light of a given wavelength.

(a) It is experimentally observed that as the angle of incidence increases, the angle of deviation first decreases, reaches to a minimum value for a certain angle of incidence and then on further increasing the angle of incidence, the angle of deviation begins to increase. The below curve shows the variation in the angle of deviation with the angle of incidence at a prism surface:

(b) For a given prism and given colour of light, 𝛿_min (angle of minimum deviation) is unique since only one horizontal line can be drawn parallel to i – axis at the lowest point of i – 𝛿 curve i.e only for one value of angle of incidence i, the refracted ray inside the prism is parallel to its base.