A NUMERICAL EXAMPLE on wave optics

A numerical example from Halliday and Resnick ( solved in my class on 18/9 2009) Two light rays travel through two media having R.I. n1 and n2 . What multiple of λ gives their phase difference when they emerge if (a) n1 =1.50, n2 =1.60 and L= 8.5 μm. (b) n1 =1.62 , n2 =1.72 and L = 8.5 μm. (c) n1 = 1.59, n2 =1.79, L =3.25 μm (d) suppose that in each of these three situations waves arrive at a common point ( with same amplitude ) after emerging. Rank the situations according to the brightness the waves produce at the common point. .