Question

Photons of wavelength $6620A^0$ are incident normally on a perfectly reflecting screen. Calculate the number of photons paper per second falling on the screen as the total power of photons such that the exerted force is 1N.

• A. $5\times {10}^{26}$
• B. $5\times {10}^{25}$
• C. $1.5\times {10}^8$
• D. None of these

Answer 1

The correct option is A $5\times {10}^{26}$

Momentum, $p=\frac{h}{\lambda }$ reflected normally

$\Delta p=\frac{2h}{\lambda }$

Force $=$ rate of charge of momentum

$1=\frac{\Delta p}{t}$ for each photon $(t=1sec)$

let $n$ be no. of photon

$1=\frac{n\times 2(6.63\times {10}^{-34})}{6620\times {10}^{-10}}$

$n=\frac{6620\times {10}^{-10}}{2\times 6.63\times {10}^{-34}}$

$\approx 5\times {10}^{26}$ photons.