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

• A

  $\left(\mathrm{\alpha }-\mathrm{\beta }\right)$

• B

  $2\left(\mathrm{\alpha }-\mathrm{\beta }\right)$

• C

  $\frac{\left(\mathrm{\alpha }-\mathrm{\beta }\right)}{2}$

• D

  $\mathrm{\beta }-\mathrm{\alpha }$