Ncert Exercises & Solutions

2.8 What is the number of photons of light with a wavelength of 4000 pm that provide 1J of energy?

Energy (E) of a photon = hν
Energy (En) of ‘n’ photons = nhν
$\Rightarrow n = \frac{E_{n} λ}{hc}$
Where, λ = wavelength of light = 4000 pm = 4000 × $10^{- 12}$ m c = velocity of light in vacuum = 3 × $10^{8}$ m/s h = Planck’s constant = 6.626 × $10^{- 34}$ Js.

Substituting the values in the given expression of n:
$n = \frac{(1) x (4000 x 10^{- 12})}{(6.626 x 10^{- 34}) (3 x 10^{8})} = 2.012 x 10^{16}$
Hence, the number of photons with a wavelength of 4000 pm and energy of 1 J are 2.012 × $10^{16}$ .

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