Question

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

Answer 1

Energy (E) of a photon = hν
Energy (En) of ‘n’ photons = nhν
$\Rightarrow n=\frac{E_0\lambda }{hc}$
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:
$n=\frac{\left(1\right)\times (4000\times {10}^{-12})}{(6.626\times {10}^{-34})(3\times {10}^8)}=2.012\times {10}^{16}$
Hence, the number of photons with a wavelength of 4000 pm and energy of 1 J are 2.012 × ${10}^{16}$ .