3.1 Solid angles (1)Angle subtended by a curve(see Fig 3. 1 a small segment of curve dl subtends a small angle at P dl d sin e do integrating over the curve C yields d l sin e a=JC g radlan. (2 )Solid angle subtended by a surface(see Fig3. 3) a small element of area da subtends a small solid angle at P cos 8da r1. da integrating over a finite area S yields cos eda 2 steradian