Why does an absorption spectrum occur

Thus, each spectral line corresponds to one particular transition between energy states of the atoms of a particular element. An absorption line occurs when an electron jumps from a lower energy state to a higher energy state, extracting the required photon from an outside source of energy such as the continuous spectrum of a hot, glowing object.

An emission line is formed when the electron falls back to a lower energy state, releasing a photon. The diagram on the next page demonstrates absorption and emission of photons by an atom using the Neils Bohr model of a hydrogen atom, where the varying energy levels of the electron are represented as varying orbits around the nucleus.

We know that this model is not literally true, but it is useful for describing electron behavior. The varying series of absorption and emission lines represent different ranges of wavelengths on the continuous spectrum. The Lyman series, for example, includes absorption and emission lines in the ultraviolet part of the spectrum. Hydrogen Atom. Emission and absorption lines are also seen when oppositely charged ions recombine to an electrically neutral state.

The thus formed neutral atom is highly excited, with electrons transitioning between states, emitting and absorbing photons. The resulting emission and absorp-tion lines are called recombination lines.

Some recombination lines occur at relatively low frequencies, well within the radio range, specifically those of carbon ions. 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.

Absorption line The pattern of absorption lines in a spectrum is diagnostic of the types of atoms and molecules present, for example, in the surface layers of a star or the atmosphere of a planet. Absorption band An absorption band is a series of very closely spaced absorption lines resulting from the absorption of light by molecules.

How an absorption spectrum is formed Atoms and molecules may change states when they absorb specific amounts of energy. When this decay occurs, the photon produced is not 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 is statistically unlikely to travel along the original path to the observer. These gaps appear as black lines in an image of the spectrum.

The light that leaves the cloud right shows absorption lines in the spectrum at discrete frequencies. According to quantum mechanics, an atom , element or molecule can absorb photons with energies equal to the difference between two energy states. Photons with specific energies will be absorbed by an atom, ion or molecule if this energy is equal to the difference between the energy levels.