PHOTONS AND ELECTRONS 15 



by research in photochemistry but still remains relatively restricted. 

 Beyond it, infra-red light must be detected by its heating effect or by its 

 action on a radiometer. 



From the violet end of the visible spectrum, in the direction of decreas- 

 ing wave-lengths, extends the ultra-violet. Like the infra-red, it formerly 

 was terminated by a gap, beyond which lay the X-rays. This gap was 

 closed at about the same time as that other, and now the limit of the 

 so-called ultra-violet has to be fixed by arbitrary choice somewhere in 

 the zone which is alternatively called that of far ultra-violet or that 

 of soft X-rays. As acceptable a frontier as any is the wave-length 12.3 A 

 corresponding to a photon energy of 100 ev; but no frontier is really good. 

 Subdivisions of the ultra-violet are the "zone of transmission by the 

 earth's atmosphere" extending from the edge of the violet out to about 

 2900 A (4.3 Ev), at which the atmosphere ceases to transmit and beyond 

 which the spectra of the sun and stars cannot be followed; the Schumann 

 region between about 2000 and 1250 A (6.4 and 10 ev), in which fluorite 

 must be used for lenses and prisms and special photographic films are 

 necessary; and the Lyman region, extending thence to about 500 A 

 (about 25 ev), in which still more exceptional films are requisite and 

 only 'the thinnest strata of solid matter (or better, none at all) may be 

 put across the light beam.^ 



Beyond the ultra-violet stretches the X-ray region, and beyond that 

 the gamma-ray spectrum, any boundary imposed between the two being 

 of necessity even more arbitrary than any imposed between the X-rays 

 and the ultra-\iolet. If we could limit the X-ray spectrum to the char- 

 acteristic X-ray "lines" (see page 25), we could put a frontier at about 

 0.12 A (about 10' ev), but this would be doing too great a violence to 

 our classical definitions of X-rays, while on the other hand no other 

 sharp limit can be set except the photon energy corresponding to the 

 highest voltage thus far applied to an X-ray tube, and this leaps upward 

 year by year. If the reader insists on a boundary, he may take 

 25 X (5 • 105 Ey or 0.5 mev). 



People who w^ork with X-ray^ frequently refer to the infra-red, the 

 visible, and the ultra-violet regions collectively as the "optical spectrum." 



ABSORPTION OF LIGHT BY ATOMS 



The absorption spectrum of a not too dense gas consists chiefly, 

 if not entirely, of discrete lines and is known either as a line spectrum 

 or as a band spectrum, the distinction depending on the arrangement of 

 the lines. Band spectra are exhibited by gases in which the atoms are 

 grouped into molecules (notable examples being hydrogen, oxygen, 



6 Schumann and Lyman were the first to devise methods for observing spectra in 

 these respective regions. 



