To physicists, the proof of Bohr's model of the hydrogen atom was its ability to explain exactly how atoms absorb and emit energy (called their atomic spectra). If we heat a substance to the point at which its molecules are broken down into gaseous atoms, and then pass electromagnetic radiation of many frequencies through it, most of the frequencies will pass through the gas unimpeded, and only a certain set of frequencies of radiation will be absorbed.

Conversely, if we add energy to the atoms by heating them even further, they will emit radiation at this same characteristic set of frequencies. (These same general comments apply at lower temperatures to molecules, but molecular spectra are more complicated than atomic spectra.)

We now know that the different frequencies of radiation absorbed or emitted by atoms represent jumps between different energy states that the electrons can be in (right).