Double Plate Interference

Figure 1

Assume that the normally incident light bounces off the bottom of the top glass and the top of the bottom glass. In this case there is a phase change between the two cases and the conditions of constructive and destructive interference flip.

So constructive interference occurs when 2 d = n λ / 2    n = 1 , 3 , 5 … 2 d = n λ / 2    n = 1 , 3 , 5 … 2 x α = n λ / 2 2 x α = n λ / 2 or the position of the maxima are x m a x = n λ 4 α   n = 1 , 3 , 5 … x m a x = n λ 4 α   n = 1 , 3 , 5 …

Destructive interference will occur at 2 d = n λ   n = 1 , 2 , 3 , … 2 d = n λ   n = 1 , 2 , 3 , … x m i n = n λ 2 α   n = 1 , 2 , 3 … x m i n = n λ 2 α   n = 1 , 2 , 3 …