Everything about Absorption Spectrum totally explained
A material's
absorption spectrum shows the fraction of incident
electromagnetic radiation absorbed by the material over a range of
frequencies. An absorption spectrum is, in a sense, the opposite of an
emission spectrum.Every
chemical element has absorption lines at several particular
wavelengths corresponding to the differences between the energy levels of its atomic orbitals. For example, an object that absorbs
blue,
green and
yellow light will appear
red when viewed under white light. Absorption spectra can therefore be used to identify elements present in a gas or liquid. This method is used in deducing the presence of elements in
stars and other gaseous objects which can't be measured directly.
Explanation
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 quantized, similar to the atomic orbitals, and may be excited by absorbing single photons.
In both the atomic and molecular cases, the excited states don't 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 isn't necessarily emitted in the same direction as the original photon. The most common angle of this has been shown to be about 45 degrees of the original photon. This applies to any situation where gases lie 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 are equally likely to travel in all directions, and it's statistically unlikely to travel along the original path to the observer. These gaps appear as
black lines in an image of the spectrum.
== See also =
Further Information
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