PHOTONS AND ELECTRONS 37 



merit is, however, valid only if the energy of the photons is considerably 

 greater than the binding energies of the various classes of electrons in the 

 atoms.) 



With the corpuscular image there is no such elegant interpretation of 

 scattering without change of frequency. One cannot say that the inci- 

 dent photon of frequency v is absorbed, raises the atom to a stationary 

 state, and is then reemitted, because we are dealing with cases in which 

 there is no stationary state of energy superior by /iv to that of the normal 

 state. It is not altogether satisfactory to say that the incident photon 

 simply bounces off the atom, for this picture is inadequate for some of the 

 finer details of the process. One may say that the incident photon is 

 absorbed, raises the atom to a "virtual" state, and is instantaneously 

 reemitted; while this sounds like a mere trick of words, it has certain 

 advantages in respect of the next process to be considered. 



SCATTERING OF LIGHT WITH CHANGE OF FREQUENCY: RAMAN EFFECT; 



SELECTION PRINCIPLES 



There are two types of scattering with change of frequency, one of 

 which is observed in the optical spectrum, the other in the X-ray and 

 gamma-ray regions. 



So far as the change of frequency is concerned, the former of these 

 processes — the Raman effect — may be described by a simple picture of 

 the corpuscular type. A photon of energy hv, entering a molecular gas 

 (the effect is almost confined to molecular gases, and to liquids and solids) 

 of which the molecules have an excited state A of which the energy 

 reckoned from the normal state has a value Ea less than hv, impinges on 

 one of these molecules in its normal state; it spends the portion Ea of its 

 energy in exciting this molecule, and flies off with the remainder of its 

 energy {hv — Ea); owing to this reduction in its energy, its frequency is 



reduced from v to ( v j^ )• Thus when a molecular gas is illuminated 



by a beam of monochromatic light, the light emitted from the gas contains 

 various spectrum lines, each of which is shifted downward in frequency 

 from the incident light by an amount equal to the energy value of some 

 excited state of the molecule divided by h. (Cases also occur of lines 

 shifted upward in frequency, for a reason which the reader can easily 

 derive from the explanation given of " anti-Stokesian lines" in a previous 

 section.) 



Objection may be made to this image of a photon striking a molecule 

 and bouncing off with reduced energy, on the ground that we usually 

 think of photons with different energy as being essentially different 

 photons. However, this is probably not a sound objection; when a 

 material object, a tennis ball, for instance, rebounds from the ground to a 



