The thickness of a doubly refracting plate, capable of producing a path difference l/4 between

The thickness of a doubly refracting plate, capable of producing a path difference l/4 between extraordinary and ordinary waves, is (l = 6000 Å; µ₀ =1.53; µ_e = 1.54):
(a) 1.0 x 10^-3 cm
(b) 1.5 x 10 ^-3 cm
(c) 3.0 x 10^-3 cm
(d) 6.0 x 10^-3 cm