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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=6.6×10−34 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=6.6×10−34 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=2×10−3×1×5×10−76.6×10−34×3×108 (∵ t=1S) ⇒n=5×1015 Hence option (A) is correct.

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