[UPSC CSE 1989] Polarisation and Modern Optics (Q477, 20M)

A beam of lineal polarised light is changed into circularly polarised light by passing it through a slice of double-refracting crystal 0.0030 cm thick. Calculate to difference in the two refractive indices of the crystal, assuming this to be minimum thickness that will produce the result, and that the wavelength is 6 \times 10^{-5}\text{ cm}.