Question

Define solid angle? and also its formula,SI unit,its use.

Answer 1

In geometry, a solid angle (symbol: Ω) is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large the object appears to an observer looking from that point. In the International System of Units (SI), a solid angle is expressed in a dimensionless unitcalled a steradian (symbol: sr).

A small object nearby may subtend the same solid angle as a larger object farther away. For example, although the Moon is much smaller than the Sun, it is also much closer to Earth. Indeed, as viewed from any point on Earth, both objects have approximately the same solid angle as well as apparent size. This is evident during a solar eclipse.

Answer 2

Solid angle is defined as ratio of area subtended with respect to square of radius.
It is measured in steradian(Ω)

          Ω=area ÷ r^{2}
where r is radius.
it is use to measure 3d figures