the solar spectrum; the prominent Fraunhofer lines at 5890 and 5896 A are due to sodium. Figure 4-15(b) shows qualitatively what a plot of intensity versus wavelength looks like; the intensity dips sharply at the wavelength of the Fraunhofer lines, but is not zero. (It was not Fraunhofer who first observed the absorption lines, but it was he who first recognized that some of them coincided in wavelength with bright emission lines produced by laboratory sources. It remained, however, for Kirchhoff and Bunsen in 1861 to make a detailed comparison of the solar spectrum with the arc and spark spectra of pure elements.) One can be sure that the Fraunhofer lines are the result of resonance absorption processes. The picture is that the continuous radiation from hot and relatively dense matter near the sun's surface is selectively filtered, as it passes outward, by atoms in the more tenuous vapors of the solar atmosphere. It would be satisfying if one could trace out the detailed shape of an optical absorption line and relate its width to the characteristic time (= 1/) for the decay of the spontaneous emission. This, however, is extremely hard to do. The chief enemy is the Doppler effect. Both direct and indirect evidence show that a typical lifetime for an excited atom emitting visible light is about 10 sec, so that Y is about 10 sec. The angular frequency of the emitted light, as defined by 2arc/X, is about 4 X 10' sec'. Thus we can calculate a line width 8N as follows:
õእ õq» ን' 10*
−—− ARSZ −— : −—
ARS ---- X ado wo 4 X 1015
(Hence 3x s 10A for x s 5000 A.) But, unless special precautions are taken, the emitting atoms have random thermal motions of several hundred meters per second, and we can estimate a Doppler broadening of the spectral lines: Δλ
as 2 x 10
10-0 یہ بط = Ը
The Doppler effect is thus about 100 times greater than any effect due to the true lifetime of the radiating atom. Interatomic collisions also disturb the situation, so that the resonance shapes of spectral lines are more a matter of inference than of direct spectroscopic observation.