This Demonstration shows some general qualitative features of absorption spectroscopy, common to all types of spectroscopy. Given a stack of quantized energy levels—atomic, molecular or nuclear—radiation will be absorbed when its frequency matches one of the energy level differences, according to the Bohr condition , assuming the transition is allowed by the appropriate selection rules.
The upper slider allows you to simulate a spectrometer sweeping across the relevant frequency range. You will observe a transition, shown by an orange arrow, near the resonance frequency for each transition. No arrows will appear if the frequency is not close to one of the transition frequencies.
The quality of spectrometers is limited by their frequency resolution, which the second slider allows you to vary. At low resolution, the spectral peaks are broadened and the energy levels can be determined with less accuracy. It is the task of the spectroscopist to assign the peaks of the spectrum to specific transitions. If you check the "assignments" box, the results are given.