The back surface of a glass slab (refractive index $n$ and thickness $t$) is polished to act as a mirror,as shown in the figure. $A$ laser beam is incident on it,which is partially reflected and partially refracted at the air-glass interface. The refracted part is fully reflected at the mirror surface. $A$ pattern of discrete spots of light is observed on a screen placed perpendicular to the glass surface. The spacing between the spots on the screen is:

• A
  $\frac{2 t \cos \theta}{\sqrt{n^{2}-\sin ^{2} \theta}}$
• B
  $\frac{2 t \sin \theta}{\sqrt{n^{2}-\sin ^{2} \theta}}$
• C
  $\frac{2 t \tan \theta}{\sqrt{n^{2}-\sin ^{2} \theta}}$
• D
  $\frac{2 t \sin \theta}{\sqrt{1-\frac{\sin ^{2} \theta}{n^{2}}}}$