188 THE QUANTUM THEORY 



provides the energy of these explosions, but the phe- 

 nomenon is governed by a remarkable rule. Firstly, the 

 speed of the electrons is not increased by using more 

 powerful light. Concentration of the light produces 

 more explosions but not more powerful explosions. 

 Secondly, the speed is increased by using bluer light, i.e. 

 light of shorter period. For example, the feeble light 

 reaching us from Sirius will cause more powerful ejec- 

 tions of electrons than full sunlight, because Sirius is 

 bluer than the sun; the remoteness of Sirius does not 

 weaken the ejections though it reduces their number. 



This is a straightforward quantum phenomenon. 

 Every electron flying out of the metal has picked up just 

 one quantum from the incident light. Since the /z-rule 

 associates the greater energy with the shorter vibration 

 period, bluer light gives the more intense energy. 

 Experiments show that (after deducting a constant 

 "threshold" energy used up in extricating the electron 

 from the film) each electron comes out with a kinetic 

 energy equal to the energy of the quantum of incident 

 light. 



The film can be prepared in the dark; but on ex- 

 posure to feeble light electrons immediately begin to 

 fly out before any of the collection-boxes could have 

 been filled by fair means. Nor can we appeal to any 

 trigger action of the light releasing an electron already 

 loaded up with energy for its journey; it is the nature 

 of the light which settles the amount of the load. The 

 light calls the tune, therefore the light must pay the 

 piper. Only classical theory does not provide light with 

 a pocket to pay from. 



It is always difficult to make a fence of objections so 

 thorough as to rule out all progress along a certain line 

 of explanation. But even if it is still possible to wriggle 



