Question

Electromagnetic radiation with an energy flux of $10{\text{W cm}}^{-2}$ falls on a reflecting surface at normal incidence. The surface has an area of $20{\text{cm}}^2$, what is average force exerted on surface during a $30$ minute time span ?

• A. $1.2\times {10}^{–6}\text{N}$
• B. $2.4\times {10}^{–6}\text{N}$
• C. $1.33\times {10}^{–6}\text{N}$
• D. $2.66\times {10}^{–6}\text{N}$

Answer 1

The correct option is C $1.33\times {10}^{–6}\text{N}$

For reflecting surface, the force exerted by radiation is given by
$F=P_{rad}.A$
Where $P_{rad}=\frac{2I}{c}$
$\Rightarrow F=\frac{2IA}{c}=\frac{2\times 10\times 20}{3\times {10}^8}=1.33\times {10}^{–6}\text{N}$