Intensity Formula

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)