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 :**

S.No. | Quantity | S.I. units | Symbol |

1. | Plane angel | radian | rad |

2. | Solid angel | steradian | sr |