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The energy for ionization for hydrogen is 13.6 eV which corresponds to a frequency (using E=hf) in the UV and not visible to the naked eye. If we send a photon to an atom in some state and the energy of the photon is not enough to ionize it and the energy of the photon does not match any differences of energy between levels, then the photon will just pass through.

Let's shine all the frequencies of visible light Fig. "Continuous spectrum of light" on a hydrogen gas.

Only some special frequencies will be absorbed (Fig. "Absorption Spectrum"). If we look at the light after it went through the gas of hydrogen particle we will see this:

These black lines are the lines of absorption. They are also called spectral lines. Spectroscopy is the science of identifying atoms by their spectral lines. These lines are determined by the energy formula, and this formula is different for each atom. For hydrogen it is E_1 = -\frac{13.6;\rm{eV}}{n^2} but for other atoms it gets more complicated. This spectrum of lines is unique and they can be used as a spectral fingerprint that allows us to determine which atoms are in a sample.

Everyone Wants to Go to Their Ground State

• S: In my case, that would be my bed.
• M: Yes, you are currently in an excited state, but it won't last. We all return to our ground state sooner or later!

If you heat up (with a fire or whatever) our gas of hydrogen, some electrons will move to excited states (n>1). These states are always unstable, and, after a while (usually very short), the electron will tumble back down and, in doing so, the extra energy they had will be emitted as light (see Fig. "Photon Emission").

The electron can go down all the way to the ground state right away, or it can take a leisurely walk passing by every state below emitting a photon for each transition. These steps create special lines of emissions (see Fig. "Emission Spectrum").