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 sin t integrating over the curve C yields d l sin e 2 raglan (2 ) Solid angle subtended by y a surtace(see Fig3.3 a small element of area da subtends a small solid angle at p do cos Ada r1. da integrating over a finite area s yields cos oda .2, Steradian