SOLUTION

When the normal (perpendicular) lines are drawn from the boundary of a portion on the surface of a sphere, such that they meet at its center and form a cone, then the angle enclosed by the conical surface is called solid angle.

Mathematically, the solid angle is the ratio of an area of the base of the conical surface (portion on the spherical surface of the sphere) to the square of the radius of the sphere. The unit of solid angle is the steradian

Ω = 4πR² ⁄ R² = 4π (sr)

Note:- 

The Radian is the SI unit for plane angles.

Candela is the SI unit of luminous Intensity.