A diverging beam of light from a point source S having divergence angle $α$, falls symmetrically on a slab as shown. The angle of incidence of the two extreme rays are equal. If the thickness of the glass slab is t and the refractive index n then the divergence angle of the emergent beam is

$α$

When a ray of light gets reflected through a glass slab, the immergent ray is parallel to the incident ray is parallel to the incident ray. Therefore angle between the two emergent rays, that is to ray the divergence angle of the emergent ray is equal to the angle between the incident rays which is equal to $α$.