By looking at the specific wavelengths of light that are either absorbed or emitted from a sample of H atoms, we discover something about the energy of the electrons in the atom. First, we notice that only specific wavelengths are associated with transitions. This means that there are discrete energy levels that the electron is moving between. The energy of the light of the transition corresponds to the difference in energy between two of these levels. If the energy of the electron is increasing, this is from absorption of the light energy. If the light is being emitted, this is from the energy of the electron decreasing.
Here is a picture of the visible line spectrum for H atom emission. Notice the discrete wavelengths of emission above a zero background.
Here is a picture of the visible line spectrum for H atom absorption. Notice the lines appear in the same place, but they are now dark rather than light. That is because the energy has been removed from continuous white light background. Before encountering the H sample, the light source had intensity at all wavelengths. The only wavelengths that could excite the electrons were ones with energies that corresponded to the difference between energy levels in the H atom. These are the same differences as were observed in emission. Only now, the energy is exciting the electron up rather than being emitted after the electron is excited.
