Wavelength of light, λ = 4000 pm = 4000 x 10-12 m
Energy provided, E = 1 J
Energy per photon = hc/λ
If N is the number of photons that provide 1 J energy, we can write
1 J = N x hc/λ
or, N = {1 J x λ}/{hc}
= {15 x 4000 x 10-12 m}/{6.626 x 10-34 Js x 3 x 108 m/s}
= 2.012 x 1016
