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Polarisation

A parallel be...

Question

A parallel beam of monochromatic light of wavelength 663 nm is incident on a totally reflecting plane mirror. The angle of incident is $60^{o}$ and the number of photons striking the mirror per second is $1.0 \times 10^{19}$ . If the force exerted by light beam on the mirror is $Y \times 10^{- 8} N$ . Find Y.

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Solution

momentum $p = \frac{h}{λ}$ = $10^{- 27} K g m / s$

Force exerted by 1 photon = change in momentum = $2 * p C o s i = p$
Force exerted by $10^{19}$ photons = $10^{- 27} * 10^{19} = 10^{- 8} N$

So force extered in $10^{- 8} N$ is 1

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