A monochromatic light wave is incident normally on a glass slab of...

Dashed line $MN$ and $PQ$ are incident wave front.

Just before entering the slab $PQ$ is maxima.

${P}_{1}$ and ${Q}_{1}$ are the points on other face of slab.

So, ${P}_{1}$ will be at a lead of ${n}_{2}d$ w.r.t. $P$

and $Q '$ will be at lead of ${n}_{1}d$ w.r.t. $Q$

As the medium on the right side is homogeneous, so to have same path lead as ${P}_{1}, Q '$ must travel to ${Q}_{1}$, such that ${n}_{2}d = {n}_{1}d + hsin\theta$.

$sin\theta = \dfrac{\left({n}_{2} - {n}_{1}\right)d}{h} ⇒ \theta = \sin^{- 1}\left[\dfrac{\left({n}_{2} - {n}_{1}\right)d}{h}\right]$