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Question

A 2 mW laser operates at a wavelength of 500 nm. The number of photons that will be emitted per second is
[Given Planck's constant h=6.6×1034 Js, speed of light c=3.0×108 m/s]

A
2×1016
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B
1×1016
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C
1.5×1016
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D
5×1015
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Solution

The correct option is D 5×1015
Given:
Power of laser =2 mW
wavelength =500 nm.
Planck's constant h=6.6×1034 Js
speed of light c=3.0×108 m/s

Energy of photon (E) is given by

E=hcλ

Number of photons of wavelength λ emitted in t second from laser of power P is given by

n=Ptλhc

n=2×103×1×5×1076.6×1034×3×108 ( t=1S)

n=5×1015

Hence option (A) is correct.

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