Question

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

Class XI Structure of Atom Page 65

Answer 1

concept : number of photons is calculated by dividing total energy by energy of one photons .
but we know, energy of one photons can be calculated with help of E = hc/λ
and then, use formula for number of photons (N) :
$N = \frac{total\: energy}{energy\:of\:one\:photon}$

Here,
λ = 4000 pm = 4 × 10^-9 m
h = 6.626 × 10^-34 Js
c = 3 × 10^8 m/s
E = hc/λ
= 6.626 × 10^-34 × 3 × 10^8/4 × 10^-9
= 4.9695 × 10^-17 J

now,
number of photons = Total energy /energy of one photon
= 1 J/4.9695 × 10^17 J
= 2.0122 × 10^16 photons