Question

Find the force exerted by the light beam on the surface.

• A. $6.66\times {10}^{8}N$
• B. $6.66\times {10}^{-8}N$
• C. $3.33\times {10}^{8}N$
• D. $3.33\times {10}^{-8}N$

Answer 1

The correct option is D $3.33\times {10}^{-8}N$

Power of the beam $P=10W$
Wavelength $\lambda =500nm=500\times {10}^{-9}m$
Energy of photon $E=\frac{hc}{\lambda }$
Number of photons absorbed per second $N=\frac{P}{E}=\frac{P\lambda }{hc}$
$\therefore$ $N=\frac{10\times 500\times {10}^{-9}}{6.62\times {10}^{-34}\times 3\times {10}^8}=2.52\times {10}^{19}$
Force exerted by the light beam $F=\frac{Nh}{\lambda }$
$\therefore$ $F=\frac{(2.52\times {10}^{19})(6.62\times {10}^{-34})}{500\times {10}^{-9}}=3.33\times {10}^{-8}$ N