Atomic Absorption at Characteristic Wavelengths

Why free atoms of a given element absorb light only at that element's own narrow, characteristic wavelengths, letting the amount absorbed reveal how much of that element is present.

References

• Massmann, H. "Comparison of Atomic-Absorption and Atomic-Fluorescence Spectroscopy." Spectrochimica Acta 23B (1968): 215–226.