A palne light wave with wavelength `lambda = 0.70 mu m` falls normally on the base of a biprism made of glass `(n = 1.520)` with refracting angle `theta 5.0^(@)`. Behind the biprism (Fig.) there is a plane-parallel plate, with the space between them filled up with benzence `(n' = 1.500)`. Find the width of a fringe on the screen `Sc` placed behind this system.

Explanation

It will be assumed that the space between the biprism and the glass plate filled with benzene consitiutes complementary prism as shown.
Then the two prism being oppositely placed, the net deviation produced by them is
`del = (n -1) theta- (n' - 1) theta = (n - n') theta`
Hence as in the pervious problem
`d = 2 a del = 2a theta(n - n')`
So `Deltax = ((a + b) lambda)/(2a theta(n - n'))`
For plane incident wave we let `a rarr oo`
so `Deltax = (lambda)/(2 theta(n - n')) = 0.2 mm`.