Spectroscopy

Atoms and molecules can only hold certain energies, so they can only emit or absorb photons whose energy exactly matches a gap between two of their levels. Every substance therefore interacts with light at a specific set of wavelengths determined by its structure, and nothing else has quite the same set. A spectrum is that fingerprint, which is why spectroscopy answers "what is this made of" without touching the sample.

Emission and absorption are two views of the same transitions. Heat a gas and it emits bright lines where electrons drop between levels; shine white light through the same gas cold and it absorbs at exactly those wavelengths, leaving dark lines in the same places. Fraunhofer saw the dark lines in sunlight decades before anyone could explain them; they are the Sun's outer gases absorbing the light from below, and reading them is how the composition of a star is known.

Practically, the same spectral position can be quoted as a wavelength, a frequency, a wavenumber or a photon energy, and these are one quantity in four units, tied by c = λν and E = hν. Converting fluently between them is the basic literacy of the subject, because different techniques conventionally use different ones — infrared spectroscopists say wavenumbers, X-ray people say electronvolts, and both mean the same physics. Which transitions a technique probes depends on the energy range: electronic transitions in the visible and ultraviolet, molecular vibrations in the infrared, rotations in the microwave.