Question

Define solid angle. How do you define one steradian on the basis of the surface area of a sphere that subtends a given solid angle.

Answer 1

Angle made by a surface on some point is called solid angle. This is three dimensional analogous of two dimensional planar angle. Its unit is steradian. Solid angle is

where A is surface area which is making solid angle at a point is r distance from A. Now 1 steradian is solid angle made by unit area surface at a point which is unit distance from surface.

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