Two coherent plane light waves propagating with a divergence angle `psi lt lt 1` fall almost normally on a screen. The amplitudes neighbouring maxima on the screen is equal to `Delta x = lambda//psi`, where `lambda` is the wavelength.

Explanation

We can think of the two coherent plane waves as emitted from two copherent point source very far away. Then
`Delta x = (l lambda)/(d) = (lambda)/(d//l)`
But `(d)/(l) = Psi`(if `Psi lt 1)`
so `Delta x = (lambda)/(Psi)`.