STELLAR LABORATORIES DUNHAM 267 



arc in which many atoms collide so hard with one another that their 

 electrons are forced into excited orbits from which they fall back 

 with the emission of light darts. 



If a light dart is thrown out by an atom whose electron has 

 fallen a long distance, i, e., from a very large orbit to a small one, 

 then the light dart must carry away a large amount of energy, and 

 it does this by vibrating at high frequency. But if it w^as thrown 

 out by an atom which had less energy to unload because its electron 

 did not have so far to fall, then the light dart will vibrate more 

 slowly. Since both darts travel at the same speed, the high-energy 



7a 



-O- 



\ 





X 



FiGDRH 2. — The formation of bright spectral lines 

 Two atoms are shown, each with an electron in an excited orbit. Such atoms have 

 In store an amount of energy which depends on the size of the orbit which the 

 electron occupies. When the electron returns spontaneously to its normal orbit, 

 the stored energy is thrown out in the form of a light dart. An electron falling 

 from a relatively small orbit emits a small amount of energy. The resulting light 

 dart has a low frequency of vibration, a long wave length, and appears at the red 

 end of the spectrum. An electron falling from a larger orbit emits more energy, and 

 the corresponding light dart has a higher frequency, a shorter wave length, and 

 appears in the violet region of the spectrum. The light darts affect the photographic 

 plate where they strike. At a and & the plates are turned through a right angle 

 to show the resulting spectral lines. 



dart will have a short wave length, while the low-energy dart will 

 have a long wave length. Figure 2 shows two such light darts 

 approaching the slits of two spectrographs. 



The prism bends a high-energy light dart more than a low-energy 

 dart, and the two will strike a photographic plate at entirely differ- 

 ent places. Thus by measuring the image on the photograph we have 

 an absolutely direct method of knowing how big a jump the electron 

 made in the atom. 



There are all conceivable kinds of jumps between the many possi- 

 ble orbits in which the electrons may start and end, but each sends 

 out a slightly different light dart, which, after passing through the 



