Question

A perfectly reflecting mirror has an area of $1 cm ^{2} .$ Light energy is allowed to fall on it for $1 h$ at the rate of $10 W$ $cm ^{-2} .$ The force that acts on the mirror is:

• A. $3.35 \times 10^{-8} N$
• B. $6.7 \times 10^{-8} N$
• C. $1.34 \times 10^{-7} N $
• D. $2.4 \times 10^{-4} N$

Answer 1

Answer: (B)

Let, $E=$ energy falling on the surface per second
$=10 J$
Momentum of photons $p=\frac{h}{\lambda}=\frac{h}{(c / v)}=\frac{h v}{c}=\frac{E}{c}$
On reflection, change in momentum per second
$=2 p=\frac{2 E}{c}=\frac{2 \times 10}{3 \times 10^{8}}$
$=6.7 \times 10^{-8} N$