The 1913 Bohr model of hydrogen explained the observed spectral lines by proposing that electron energies are quantized.
Niels Bohr argued that an electron in hydrogen could occupy only certain allowed stationary states. It emitted or absorbed light when it moved between those states, with the photon’s energy equal to the difference between the two levels.
This idea accounted for the discrete wavelengths in the hydrogen spectrum, including the well-known Balmer series. It also connected the spectrum’s lines with a simple mathematical formula for hydrogen energy levels.
Bohr’s model was an important step toward quantum mechanics, but it was not the final theory. It works especially well for hydrogen-like atoms and does not fully explain the behavior of more complicated atoms or modern quantum descriptions of electrons.