Question

Light with an energy flux of $18W/c{m}^2$ falls on a non-reflecting surface at normal incidence. The pressure exerted on the surface is:

• A. $2N/m^2$
• B. $2\times {10}^{-4}N/m^2$
• C. $6N/m^2$
• D. $6\times {10}^{-4}N/m^2$

Answer 1

The correct option is D $6\times {10}^{-4}N/m^2$

Pressure exerted on the surface$=P=\frac{F}{A}$$=\frac{\frac{dp}{dt}}{A}$

Momentum $p=\frac{E}{c}$

where E is the energy of incident light, c is the speed of light.

Thus $P=\frac{1}{c}\left(\frac{1}{A}\frac{dE}{dt}\right)$$=\frac{Flux}{c}$$=\frac{18\times {10}^4}{3\times {10}^8}=6\times {10}^{-4}N/m^2$