A uniaxial birefringent crystal is cut to form a parallel plate with its optic axis parallel to the front face. For such a retarder plate of refractive indices n0 and ne , with thickness d , the expression for the relative phase difference, Δφ , for the wavelength λ is given by:
(a) |n0 - ne|/λ
(b) π|n0 - ne|d/λ
(c) π|n0 - ne|d/2λ
(d) 2π|n0 - ne|d/λ