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How do absorption spectra reveal which atoms or molecules are present in a sample?

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A horizontal gradient bar displays the visible light spectrum from violet to red, labeled with corresponding frequency in terahertz, wavelength in nanometers, and photon energy in electron volts. — light When white light is spread apart by a prism or a diffraction grating, the colours of the visible spectrum appear. The colours vary according to their wavelengths. Violet has the highest frequencies and shortest wavelengths, and red has the lowest frequencies and the longest wavelengths.

Absorption spectra reveal the presence of specific atoms or molecules by measuring the loss of electromagnetic radiation at characteristic wavelengths.123

When a broad band of light passes through a sample, the particles within it—whether atoms, ions, or molecules—absorb only those specific wavelengths that correspond to the energy required to excite them from one quantum state to another. Because these energy levels are unique to the structure of each species, the resulting pattern of missing wavelengths (dark lines or bands) acts as a distinct "fingerprint" that allows for the identification of the absorbing substance.4315

This process is fundamental to spectrochemical analysis, where the measured absorption is used to determine both the identity and the concentration of the sample. For example, infrared spectrophotometry specifically monitors vibrational energy changes to identify organic compounds, while electronic transitions are often studied in the visible and ultraviolet regions to characterize molecular structures.2365