Energy is related to Wavelength as,
E = h c / λ -------- (1)
As,
h = 6.626 × 10⁻³⁴ Js
c = 2.99 × 10⁸ ms⁻¹
λ = 12.5 nm = 1.25 × 10⁻⁸ m
Putting values in eq. 1,
E = (6.626 × 10⁻³⁴ Js × 2.99 × 10⁸ ms⁻¹) ÷ 1.25 × 10⁻⁸ m
E = 1.585 × 10⁻¹⁷ J
E = 1.585 × 10⁻²⁰ kJ × 6.022 × 10²³
E = 9544.87 Kj/mol
