Relationship between change in quantum energy and photon energy:

$\overline{){\mathbf{\u2206}}{\mathbf{E}}{\mathbf{=}}{\mathbf{h}}{\mathbf{f}}{\mathbf{=}}{\mathbf{h}}\frac{\mathbf{c}}{\mathbf{\lambda}}}$

We are looking for the wave whose photon energy is equal to the ionization energy of hydrogen.

Plank's constant, h = 6.63 × 10^{-34} J/s

Speed of light, c = 3.0 × 10^{8} m/s

What is the longest wavelength light capable of ionizing a hydrogen atom in the ground state?