A parallel beam of monochromatic light of wavelength 663nm is inc… - Vidyadip

Question

A parallel beam of monochromatic light of wavelength 663nm is incident on a totally reflecting plane mirror. The angle of incidence is 60° and the number of photons striking the mirror per second is $1.0 \times 10^{19}$. Calculate the force exerted by the light beam on the mirror.

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Answer

$\lambda=663\times10^{-9}\text{m,}\theta=60$$\text{n}=1\times10^{19},\lambda=\frac{\text{h}}{\text{p}}$
$\Rightarrow\text{p}=\frac{\text{p}}{\lambda}=10^{-27}$
Force exerted on the wall
$=\text{n}(\text{mv}\cos\theta-(-\text{mv}\cos\theta))=2\text{n}\text{ mv}\cos\theta.$
$=2\times1\times10^{19}\times10^{-27}\times\frac{1}{2}=1\times10^{-8}\text{N.}$

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