A ray of light falls on the surface of a spherical glass paper weight making an angle  $\alpha$ with the normal and is refracted in the glass medium at an angle  $\beta .$ The angle of deviation of the emergent ray from the direction of incident ray is

• A

  $(\alpha -\beta )$

• B

  $2(\alpha -\beta )$

• C

  $\frac{1}{2}(\alpha -\beta )$

• D

  $(\beta -\alpha )$