Question

The pressure exerted by an electromagnetic wave of intensity $I\left(watt/m^2\right)$ on a non-reflecting surface is : [c is the velocity of light]

• A. $Ic$
• B. ${Ic}^2$
• C. $I/c$
• D. $I/c^2$

Answer 1

Answer: (C)

$I/c$

Momentum of a photon
$=\frac{h}{\lambda }=\frac{h}{c/v}=\frac{hv}{c}=\frac{E}{c}$
Momentum over unit area
$=\frac{E}{Ac}=\frac{I}{c}[I=\frac{E}{A}forwave]$
Since surface is non reflecting, final momentum of photon = 0, change in momentum $=\frac{I}{c}$
So, force per unit area $=\frac{I}{c}$
Pressure of radiation $=\frac{I}{c}$