Modern Physics 2 Question 2

2. A 2mW laser operates at a wavelength of 500nm. The number of photons that will be emitted per second is

[Given, Planck’s constant h=6.6×1034Js, speed of light c=3.0×108m/s]

(a) 1×1016

(b) 5×1015

(c) 1.5×1016

(d) 2×1016

(Main 2019, 10 April II)

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Answer:

Correct Answer: 2. (b)

Solution:

  1. Power of laser is given as

P= Energy  Time  Number of photons emitted × Energy of = one photon  Time P=NEt=NtE

So, number of photons emitted per second

=Nt=PE=Phc/λ=PλhcE=hν=hcλ

Here, h=6.6×1034Js,λ=500nm=500×109m

c=3×108ms1P=2mW=2×103WNt=2×103×500×1096.6×1034×3×108=5.56×10155×1015 photons per second 



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