solid angle SI unit steradian. An area ds forms a solid angle dW at a point P (see fig s5). The solid angle dW is given by,

dW = (ds cos q )/r² (s7)

where r is the distance between the point P and center of ds, at O. Angle q is the angle between the normal to ds and r.

The solid angle that completely surrounds a point is 4p steradians.