It's very simple.
Energy = [tex] \frac{hc}{\lambda} [/tex]
where, h= Planck's constant = 6.6 x [tex] 10^{-34} [/tex] [tex]m^2 kg/s[/tex]
c= speed of light = 3 x [tex] 10^{8} [/tex] m/s
[tex] \lambda [/tex] = wavelength
So, energy = [tex] \frac{6.6 \times 10^{-34} \times 3 \times 10^{8}}{120} [/tex]
= 1.65 x [tex] 10^{-27} [/tex] J
=1.65 x [tex] 10^{-30} [/tex] KJ
