Example: What is the intensity of light incident normal to a circular surface of radius 5 cm from a 100 W source of light?
Solution:
r = 5 cm = 5 × 10–2 m, P = 100 W, [I]=?
[I = frac{P}{{pi {r^2}}} = frac{{100}}{{pi times {{(5 times {{10}^{–2}})}^2}}} = 1.27 times {10^4}W{m^{–2}}]
Question: The intensity of a light source with power P on a circular surface of radius r placed at a certain distance is [I]. If at the same distance from another source, if the intensity on a circular surface of double the radius, is 4[I], then what is the power of the second source?
Options:
(a) 4 P
(b) 16 P
(c) 2 P
(d) P / 4
Answer: (b)