Question

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

Answer 1

Energy (E) of a photon = $hν$
Energy (En) of 'n' photons = $nhν$
⇒ $n =\frac {E_nλ}{hc}$
Where, λ = wavelength of light = 4000 pm = 4000 ×10-12 m
c = velocity of light in vacuum = 3 × 108 m/s
h = Planck's constant = 6.626 × 10-34 Js
Substituting the values in the given expression of n:
$n =\frac {(1)×(4000×10^{-12})}{(6.626×10^{-34})(3×10^8)}$
$n = 2.012×10^{16}$

Hence, the number of photons with a wavelength of 4000 pm and energy of 1J are 2.012×1016.