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Spectral Lines. .

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Spectral lines.
Spectral lines.

A hot object gives off all wavelegths (see black body) but an energetic gas gives off only certain wavelengths. The gas can be given energy, excited, by heat or by an electric field. If a salt is introduced into a flame it colours the flame and this was the way that the subject was first examined by Kirchoff and Bunsen.

They found that each element had its own unique set of wavelengths which could be used to identify each element. Rubidium and Caesium were first found this way. When the spectrum of the Sun was investigated the element Helium was discovered.

Line spectra occur because the orbits of electrons around the nuclei have only certain well-defined energy values. When an electron, with a higher energy, sheds energy it can only lose a fixed amount and end up with exactly the energy of another lower level. So if it 'jumps' from E2 to E1 it will give out radiation of frequency given by E2 - E1 = hf

It was originally a puzzle why only certain energy levels were allowed for the electron orbits. One of the early triumphs of quantum theory was the explanation for this. The electron has wave properties as well as particle properties and the orbit has to be able to accomodate a whole number of wavelengths. The wavelength of the electron depends on its energy, so the whole effect is that a high energy electron in a standing wave of n lobes drops some energy, moves to a standing wave of n-1 lobes, and the energy appears as a photon of energy equal to the energy given away by the electron. This process can happen in reverse as well, ao that the light of the same energy can be absorbed by the elsectron. In this case the line spectra appears in negative, black lines against a coloured background.

The structure of the atom determines the position of the electron energy levels and hence the spectral lines emitted. Quantum theory has been able to explain the spectra of siple atoms such as Hydrogen with awesome agreement with experiments.

For most atoms the 'jump' between the lowest level and higher ones gives rise to ultra-violet radiation. Visible light comes from jumps between the higher levels, with infra-red given off by jumps within the highest, and closest spaced levels. Splitting light from stars and Galaxies into a spectrum vastly increases the information gleaned from star light; chemical composition, temperature, velocity, rotation rate are the main ones.

Radio astronomy also has the benefit of line spectroscopy but here the emission comes from molecules not atoms. For instance, in cold hydrogen, H2, the two atoms can have their axes parallel or anti-parallel and in switching from one state to the other gives out a signal at 21 cm wavelength. Several other molecules produce similar lines such as OH, CO2, HCN, and this has opened up a whole new branch of astrochemistry, dealing with otherwise invisible cold gas clouds.

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