A diverging beam of light from a point source S having devergence 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 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)`
