CBSE 2025 · Region 6 · Set 2 · Q19 · 2 marks
A laser beam of wavelength $\displaystyle 500$ nm and power $\displaystyle 5$ mW strikes normally on a perfectly reflecting surface of area $\displaystyle 1 \mathrm{~mm}^{2}$ of a body. It rebounds back from the surface. Find the force exerted by the laser beam on the body.
Marking-scheme solution
No. of photons ejected per second \(\displaystyle =\frac{P}{E}\)
\[=\frac{P}{\left(\dfrac{h c}{\lambda}\right)}
\] Momentum of photon, \(\displaystyle p=\frac{h}{\lambda}\) Force exerted by laser beam
\[\begin{aligned}
F & =2 \times \frac{P}{\left(\dfrac{h c}{\lambda}\right)} \times \frac{h}{\lambda} \\
& =\frac{2 P}{c} \\
& =\frac{2 \times 5 \times 10^{-3}}{3 \times 10^{8}} \\
F & =3.33 \times 10^{-11} \mathrm{~N}
\end{aligned}
\]
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CBSE Class 12 Physics past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.