Light from Atoms

When an electron falls from a higher shell to a lower one, the atom gives out the energy as a photon of light.

The gaps are fixed, so each element gives out its own colors, its line spectrum. Sodium glows yellow at $589\ \mathrm{nm}$.

Sodium's yellow line

A photon's energy is $E = \frac{hc}{\lambda}$, with $h = 6.63 \times 10^{-34}\ \mathrm{J\,s}$ and $c = 3.0 \times 10^{8}\ \mathrm{m/s}$: the shorter the wavelength, the more energy.