RADIAN AND STERADIAN:
Radian: The radian is the measure of a plane angle. One radian is defined as a plane angle with its vertex at the centre of a circle of radius r which is subtended by an arc whose length is equal to r.
Steradian: It is the measure of a solid angle. It is defined as the solid angle with its vertex at the centre for a sphere of radius r which is subtended by a spherical surface area equal to the area of a square, whose side length is r.
Statement: The solid angle subtended at a point by a small surface is defined as the ratio of radial component of the area of surface to the square of mean distance between surface and that point.
The solid angle subtended by the surface of a sphere at its centre is always 4П steradian.
In fact solid angle subtended at an interior point of any surface is 4П steradian.
Reference: This article is referred from my authored book “concepts of electromagnetic field theory” having ISBN 978-81-272-5245-8. In case of any doubt in this article or any other EMFT or physics related article, kindly post in the comment section.