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

• A
  $Zero$
• B
  $\alpha$
• C
  ${\sin ^{ - 1}}(1/n)$
• D
  $2\,{\sin ^{ - 1}}(1/n)$