The absorption lines (not just sodium, but others - Ca, Mg and many others) appears as sharp, dark stripes in the vertical direction - and were originally called Fraunhofer lines. When a photographic spectrum of the Sun is obtained it is usually in the form of a strip, with wavelength along the x-axis. Because stars like the Sun and cooler usually form the bulk of the light seen from a galaxy, this line is easily seen in galaxy spectra, and the difference between it's wavelength and 589nm can be used to estimate the redshift (or occasionally blueshift) of galaxies.Īs for why they are called "lines" - I think this is just historically how they were first detected. For all ionization stages of sodium, at least some experimental data are available how- ever, for those for which only a few transitions have been measured, theoretical calcula- tions or values obtained by isoelectronic tting are reported. The expected interference fringe pattern from a single spectral line is a 'bullseye' of concentric light and dark rings. Only with transition probabilities Only with energy level classifications Only with observed wavelengths. Format output: HTML (formatted) ASCII (text) CSV (text) Tab-delimited. (a) A sample of excited hydrogen atoms emits a characteristic red light. available for neutral ions and 380 < < 780. Figure 7.3.1: The Emission of Light by Hydrogen Atoms. In cool stars like the Sun, the sodium lines (a good spectrograph will show you it is a doublet, separated by 0.6 nm, caused by LS coupling in sodium atoms) at 589nm is one of the most prominent absorption lines. A Fabry-Perot interferometer is used to separate the spectral lines of hydrogen from those of deuterium. With sodium, however, we observe a yellow color because the most intense lines in its spectrum are in the yellow portion of the spectrum, at about 589 nm. Because the outer atmosphere is usually in equilibrium, the excited atoms do re-radiate photons at these frequencies, but in all directions, so that light is removed from the original beam. The relatively cool gas on the outside has sodium atoms in lower energy states, and these are reasonably opaque to photons of particular frequencies that excite them to higher energy levels. The absorption is caused because stars are hotter in the middle than they are on the outside. Although the photons may be re-emitted, they are effectively removed from the beam of light, resulting in a dark or absorption feature. Photons of the appropriate energies are absorbed by the atoms in the gas. An absorption spectrum is produced when a continuum passes through 'cooler' gas. The sodium line that is being referred to is a reasonably sharp, dark absorption feature that is seen in the orange part of the visible spectrum of the bulk of stars that make up the light from a distant galaxy. an emission spectrum or emission-line spectrum.
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