Electromagnetic radiation may be characterised by its wavelength. An absorption spectrum is a diagram which shows the wavelengths of electromagnetic radiation absorbed by a material. The material could be a gas, a solute or a solid. An absorption spectrum is, in a sense, the inverse of an emission spectrum.
Atoms and molecules may change states when they absorb specific amounts of energy. Atomic states are defined by the arrangement of electrons in atomic orbitals. An electron in some orbital may be excited to a more energetic orbital by absorbing exactly one photon which has energy equal to the energy difference of the two orbitals.
Molecular states are defined by the molecule's modes of vibration and rotation. These vibrational and rotational modes are quantised, similar to the atomic orbitals, and may be excited by absorbing single photons.
In both the atomic and molecular cases, the excited states do not persist: after some random amount of time, the atoms and molecules revert back to their original, lower energy state. In atoms, the excited electron returns to a lower orbital, emitting a photon. In molecules, the vibrational or rotational mode decays, also emitting a photon.
When this decay occurs, the photon produced is not emitted in the same direction as the original photon. If a cloud of gas (atoms or molecules) comes between a light source and an observer, the observer will see gaps in the spectrum of the light corresponding to the wavelengths of the photons which were absorbed. These gaps occur despite the re-emission of photons because the re-emitted photons do not travel along the original path to the observer. These gaps appear as black lines in an image of the spectrum.
Elements have particular sets of lines at particular wavelengths, corresponding to the energy levels of their atomic orbitals. (Strictly speaking, the lines correspond to the energy differences between the orbitals.) This allows for the identification of elements from absorption spectra. This method is used in deducing the presence of elements in stars and other gaseous objects which cannot be measured directly.
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