Atomic Spectra, Energy Levels, and Photons

When a gas is heated or an electric current is passed through it at low pressure, it emits light. If that light is passed through a prism or a diffraction grating, it does not spread into a continuous rainbow. Instead it produces a set of sharp, bright, colored lines separated by darkness. This is called a line emission spectrum, and each element produces its own unique pattern of lines, a kind of atomic fingerprint. Hydrogen, for example, shows a characteristic set of lines in the visible region. The existence of these discrete lines is direct experimental evidence that the energy of electrons inside atoms cannot take just any value; it is quantized.

To explain line spectra we use the idea of atomic energy levels. An electron bound to an atom can only occupy certain allowed energy states,