36 BIOLOGICAL EFFECTS OF RADIATION 



The general term for the production of hght by hght is, of course, the 

 famiUar word "fluorescence," which is often applied to most if not to all 

 of the phenomena described in this section. "Phosphorescence" is a 

 word applied by tradition to cases in which the emission of light endures 

 for a readily perceptible time after the irradiating light is cut off. It 

 is not exhibited by gases and therefore is still comparatively obscure. It 

 appears to be most strikingly (if not exclusively) exhibited by mixtures 

 in which one component is present in very small proportion compared 

 with another. 



SCATTERING WITHOUT CHANGE OF FREQUENCY 



As has just been said, a beam of monochromatic light traversing a gas 

 provokes the emission, from the gas, of photons of identical frequency, 

 provided that the common frequency of incident and of emitted light is 

 coincident with that of one of the absorption lines of the gas. We 

 interpret this as meaning that photons of the incident beam are absorbed 

 by the atoms and put these into excited states, from which they return 

 to the normal state by emitting photons identical with the absorbed ones. 



An apparently similar phenomenon occurs when there is no coinci- 

 dence between the incident frequency and that of any of the absorption 

 lines: it is known as "scattering without change of frequency." It is 

 very much feebler than the other; in order to perceive it with the eye 

 under laboratory conditions, one must send a very strong beam of light 

 through dust-free gas in a chamber with blackened walls, and look at 

 the path of the beam from the side with a well-rested and dark-adapted 

 eye. Dust in the gas will conceal the effect entirely by superposing on 

 it a very much stronger scattering from the dust particles. 



Scattering without change of frequency is one of the phenomena of 

 light which are most readily interpreted by the classical undulatory 

 theory. The electrons in the atoms are supposed to be set into forced 

 vibrations by the cyclically varying electric field in the light waves, and 

 thereby to become radiators of light themselves. If the intensity of the 

 light is held constant and its frequency is varied, the amplitude of the 

 forced vibrations of the electrons should increase rapidly with frequency 

 (as the square thereof). This accounts for the blue color of the clear, 

 fog-free, and dust-free sky; for the color is chiefly dominated by that part 

 of the visible spectrum for which the scattering is strongest, and that is 

 the blue part. In the X-ray region of the spectrum, the agreement 

 of the classical theory with observation is so good that estimates of the 

 total number of electrons in atoms of various metals were made over a 

 quarter of a century ago from observations on the intensity of the scatter- 

 ing of X-rays, these two being proportional to one another and the factor 

 of proportionality being given by the classical theory, (This last state- 



