An atom in which one or more electrons are in energy levels higher than the lowest available ones is said to be in an excited state. An atom in which all electrons are in the lowest possible energy level is said to be in its ground state. This is because the electrons cannot drop to any lower energy level. If the electrons in an atom are at their lowest possible energy level, that atom cannot produce an emission line. In some cases, an excited electron may jump many energy levels in one go, and then cascade to a lower energy level in a series of different steps that each emit its own photon. It is important to note that sometimes the emission lines don't exactly match the absorption lines in a particular cloud of gas. The absorption bands of a substance have the same wavelength intervals as its emission bands. The molecules in the gas can be identified if the absorption bands themselves can be detected and identified. As light penetrates through a cloud of molecules in a gas, transitions of electrons create closely spaced energy levels called absorption bands. Much of the discussion of absorption and emission also applies to molecules. An element can be identified from either its emission lines or its absorption lines. Since the intervals between energy levels in a given atom are the same whether absorption or emission is occurring, the pattern of emission and absorption lines for a given element is the same. While some of the photons do fill in the absorption line a bit, most are cast off to the side, where other observers may see them as forming emission lines against the normal blackness of space. So why doesn't the emission fill in the absorption line and leave no visible feature in the spectrum? The answer is that the majority of those later admitted photons are emitted in a completely new direction, going out to visit a different part of space than the original photon would've reached. This means that for every photon that is absorbed another photon is eventually emitted. When an electron gains energy by absorbing a photon it will later return to a lower energy by re-radiating that photon in a completely random direction. Since atoms have many different allowed energies, there are various possible upward transitions - for example, level 2 to 3, 2 to 4, 1 to 2, 1 to 3, and so on, in one specific type of atom can lead to a whole series of colors being removed from the rainbow.įor a variety of reasons, an excited atom or molecule doesn't stay excited forever. A beam of light passing through a cloud of such atoms will have many of these photons removed, thus creating a noticeable dark line of absorption in the object's rainbow spectrum. Only the specific energies corresponding to an atom or molecule's specific transitions can be absorbed and removed from the beam. This process of energy removal from a light beam is called absorption.
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