We know that the energy of a wave particle can be found using the formula:
E = hf
Where f = frequency (Hz), E = Energy (J), h = Planck's constant
We also know that:
c = fλ,
where λ =wavelength (m), and c=3 × 108m/s
Reorganizing c:
c = fλ
λ
=c/f,Therefore:
E = hc/λ
Plugging in the values:
E = (6.63 × 10-34 ) . (3 × 108 ) /(420 × 10-9 ) =4.74 ×10-22 J
λ =c/f,