The working equation for this is the Planck's Law:
E = hc/λ
where
E is the energy
h is the Planck's constant equal to 6.62607004×10⁻³⁴ m²·kg / s
c is the speed of light equal to 3×10⁸ m/s
λ is the wavelength
Assuming 1 mol of Cl-Cl bond,
242 kJ * 1000 J/1 kJ = (6.62607004×10⁻³⁴ m²·kg / s)(3×10⁸ m/s)/λ
λ = 8.214×10⁻³¹ m
