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The '''absorption spectrum''' of a material is collection of frequencies of light absorbed (not reflected) by that material. The absorption spectrum is determined by the atomic and molecular composition of the material. Radiation is more likely to be absorbed at frequencies that match the energy difference between two quantum mechanical states of the molecules. 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.
The frequencies where absorption lines occur, as well as their relative intensities, primarily depend on the electronic and molecular structure of the molecule. The frequencies will also, though, 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). The lines will also have a width and shape that are primarily determined by the spectral density or the density of states of the system.