1. Radian (rad): The radian is a unit of plane angle. It may be defined as the angle between
two radii subtended at the centre of a circle by an arc equal in length to the radius.
2. Steradian (sr): The steradian is the unit of solid angle. It may be defined as the solid
angle, which, having its vertex in the centre of a sphere, cuts off an area of the surface of
sphere equal to that of a square having sides of length equal to the radius of the sphere.
The two supplementary units are :