Question

A light beam of intensity $20\, W/cm^2$ is incident normally on a perfectly reflecting surface of sides $25 \,cm \times 15 \,cm$. The momentum imparted to the surface by the light per second is

• A. $2\times10^{-5} kg \,ms^{-1}$
• B. $1\times10^{-5} kg\, ms^{-1}$
• C. $5\times10^{-5} kg \,ms^{-1}$
• D. $1.2\times10^{-5} kg\, ms^{-1}$

Answer 1

Answer: (C)

Intensity $I = E / A$
where $E$ is energy of radiation and $A$ is incident area
$\Rightarrow E = IA$
Momentum of radiation is given by
$P =2 E / c =2 IA / c$
Where $c$ is speed of light.
$P=\frac{2 \times 20 \times 10^{4} \frac{ W }{ m ^{2}} \times 375 \times 10^{-4} m ^{2}}{3 \times 10^{8} m / s }$
$=5 \times 10^{-5} kg\,ms ^{-1}$