Q.

A photon of wavelength 6630 Å is incident on a totally reflecting surface. The momentum delivered by the photon is equal to

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a

10–27 kg-m/sec

b

None of these

c

6.63 x 10–27 kg-m/sec 

d

2 x 10–27 kg-m/sec

answer is B.

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Detailed Solution

The momentum of the incident radiation is given as p = \frac{h}{\lambda } . When the light is totally reflected normal to the surface the direction of the ray is reversed. That means it reverses the direction of it’s momentum without changing it’s magnitude
\therefore \Delta p = 2p = \frac{{2h}}{\lambda } = \frac{{2 \times 6.6 \times {{10}^{ - 34}}}}{{6630 \times {{10}^{ - 10}}}} = {\text{ }}2 \times {10^{--27}}kg - m/sec

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