Two coherent plane light waves propagating with a small divergence angle $\psi \ll 1$ fall almost no

Question

Two coherent plane light waves propagating with a small divergence angle $\psi \ll 1$ fall almost normally on a screen. If the wavelength of the light is $\lambda$ and the amplitudes of the waves are equal, what is the distance $\Delta x$ between the neighbouring interference maxima on the screen?

Professor PC Thomas Classes · Answer

$\begin{gathered}\psi \ll 1 \Rightarrow \psi=\frac{d}{D} \ 	ext { Fringe width }=\frac{\lambda D}{d}=\frac{\lambda}{\psi}\end{gathered}$