A crown glass.prism of angle 5 is to be combined with a flint glass prism in such a way that the mean ray passes undeviated. Find (a) the angle of the flint glass prism needed and (b) the angular dispersion produced by the combination when white light goes through it. Refractive indices for red, yellow and violet light are 1.514, 1517 and 1.523 respectively for crown glass and 1.613, 1.620 and 1.632 for flint glass.