9th
Q.9. Express the relation for solid angle.

Solid angle:

The angle subtended at the center of the sphere by the surface of the sphere is called solid angle.This is a three dimensional angle. Suppose area of the surface which makes a cone on the center (O) of the sphere is 'S' and the radius of sphere is r,then solid angle is defined as and represented as .The unit of solid angle is Steradian.

follow the figure below.