Question

A bulb of power 660 W radiates uniformly in all directions. The pressure exerted by the radiation on the surface at a distance of 5 m is

• A. \(5 \times 10^{-5}\) Pa
• B. \(2 \times 10^{-5}\) Pa
• C. \(7 \times 10^{-5}\) Pa
• D. \(\frac{3}{\pi} \times 10^{-4}\) Pa

Hint:

Radiation pressure is given by \(\frac{I}{c}\)
Intensity \(I\) from a point source is \(\frac{P}{4\pi r^2}\)

Answer 1

The Correct Option is D

Radiation pressure at distance r: \[ P = \frac{I}{c}, \quad \text{where } I = \frac{P}{4\pi r^2} \] \[ I = \frac{660}{4\pi (5)^2} = \frac{660}{100\pi} = \frac{6.6}{\pi} \Rightarrow P = \frac{6.6}{\pi \cdot 3 \times 10^8} \approx \frac{3}{\pi} \times 10^{-4} \text{ Pa} \]