Question

Calculate the solid angle subtended by the periphery of an area of 1 cm² at a point situated symmetrically at a distance of 5 cm from the area.

Answer 1

Solid angle, Ω = `"Area"/("Radial distance")^2`

= `(1  cm^2)/(5  cm)^2`

= `1/25`

= 4 × 10^–2 steradian  ......(∵ Area = 1 cm², distance = 5 cm)

Important point: Please keep in mind that a solid angle is for 3-D figures like spheres, cones etc and a plane angle is for plane objects or 2-D figures like circles, arcs etc.