Absorption spectrum

In an absorption spectrum, portions of a continuous spectrum are missing because they have been absorbed by the medium through which the light has passed; the missing wavelengths appear as dark lines or gaps.

A material's absorption spectrum is the fraction of incident radiation absorbed by the material over a range of frequencies. The absorption spectrum is primarily determined by the atomic and molecular composition of the material. The absorption that occurs due to a transition between two states is referred to as an absorption line and a spectrum is typically composed of many lines.

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Read more: https://www.khanacademy.org/partner-content/nasa/measuringuniverse/spectroscopy/a/absorptionemission-lines

Relation to emission spectrum
Emission is a process by which a substance releases energy in the form of electromagnetic radiation. The emission spectrum will typically have a quite different intensity pattern from the absorption spectrum, though, so the two are not equivalent.

Within absorption spectrum, the frequencies where absorption lines occur, as well as their relative intensities, primarily depend on the electronic and molecular structure of the sample. The frequencies will also depend on the interactions between molecules in the sample, the crystal structure in solids, and on several environmental factors (e.g., temperature, pressure, electromagnetic field).