NOTES 



NOTE i, p. 12. 



LIGHT- PRESSURE ON THE CORPUSCULAR THEORY 



LET a beam of light, supposed to consist of corpuscles 

 moving with velocity V, be incident perpendicularly on 

 a completely absorbing, that is, a quite black surface. 

 Let m be the mass of the corpuscles in a cubic centi- 

 metre. Then the mass coming up to and entering a 

 square centimetre of the surface in one second is that 

 in a column V centimetres long and i sq. cm. cross 

 section. The total mass entering is therefore mV. As 

 it has velocity V the momentum entering per second is 

 wV 2 . But this momentum entering per second is the 

 pressure P per sq. cm. 

 Then P = 



The energy of translation per cubic centimetre is - f 



so that the pressure is twice the energy of translation. 



If the experimenters of the eighteenth century had 

 known of the relation between heat and mechanical 

 energy, they would have measured the heat received per 

 second, and expressing this in mechanical units they 

 would have equated it to the energy of translation 



wV 2 



- x V in a length V of the beam. Dividing by 



V, the known velocity of light, they would have obtained 



or half the pressure they were looking for. 



F2 83 



