All these phenomena affect the spectral lines, and therefore the actual observed spectrum from a sample of hydrogen gas is more complex than the theory predicts.
If white light passes through a gas of excited hydrogen atoms, the electrons of which are revolving around the proton in the second Bohr orbit (n = 2), as in stellar atmospheres, these excited electrons will absorb from this white light those photons the wavelengths of which are given by the Bohr formula for the Balmer lines, and the absorbing electrons will be thrown into higher orbits. A spectrum analysis of this white light after it has passed through the gas will show dark lines against a bright background at just those positions where the bright Balmer lines are found. This is called an absorption spectrum.
The phenomena of fluorescence and phosphorescence (see LUMINESCENCE) result from the absorption of photons of a particular wavelength followed by the emission of photons of a longer wavelength. In both fluorescence and phosphorescence, the photon absorbed from the illuminating radiation excites an electron that is initially in the ground state to a higher state. This excited electron then falls to a lower level, but not immediately back to the ground state, emitting a longer-wavelength photon than it absorbed. In fluorescence, the emission and absorption follow each other quite rapidly, so that fluorescence lasts only as long as the illuminating radiation is on. In phosphorescence, however, the emission occurs quite slowly and persists for a long time after the illuminating radiation has been turned off.
The sodium atom produces a more complex spectrum than does the hydrogen atom. The sodium atom has 11 orbital electrons: an inner group of 2, a middle group of 8, and an outer group of 1. If the spectrum of sodium is excited by the electric spark, many of these electrons can be responsible for production of lines; if it is excited by the electric arc, or by a flame, the outer electron is responsible for most of the lines, and to some extent, in its broad features, it behaves like the electron in the hydrogen atom. But complexities in the motion of this outer electron arise from its interaction with the ten core electrons that occupy the closed shells of the sodium atom. Moreover, the electron not only can move to orbits other than its own, but the orbits can have varying eccentricities, and in any orbit, the electron can have different orbital magnetic moments and different orbital angular momentum. These variations produce not only several series of lines, but also doublets and triplets, groups of two or three lines that differ only slightly in wavelength. The most important series of lines are called sharp, principal, diffuse, and fine (abbreviated S, P, D, and F in theoretical work; further series are abbreviated G and H without being named).

Absorption Spectra
Most of the information that physicists have gained about the structure of the atom has been obtained through spectroscopy.