When it absorbs electromagnetic radiation the number of photons passing through a sample decreases.
The measurement of this decrease in photons, which we call absorbance, is a useful analytical signal.
Note that the each of the energy levels in Figure 10.4 has a well-defined value because they are quantized.
Absorption occurs only when the photon’s energy, hν, matches the difference in energy, ΔE, between two energy levels.
A plot of absorbance as a function of the photon’s energy is called an absorbance spectrum.
Figure 10.5, for example, shows the absorbance spectrum of cranberry juice.