46 PRESSURE OF LIGHT 



opposite, and produce no effect. We have, there- 

 fore, only the pressure P of the incident radiation. 



In the second case let us take a disc black on 

 the front face and perfectly reflecting on the back 

 face, and let us send the same beam on to the front 

 face. When the disc has reached a steady tempera- 

 ture the energy of the radiation which it emits is 

 equal to that of the radiation which it receives. 

 The back surface being a perfect reflector does 

 not send out any radiation. All the energy pours 

 out from the front surface. If it were given out 

 along the normal only it would produce pressure 

 P, equal to that of the incident beam, and the total 

 pressure would be 2P. But it is sent out in all 

 directions, and is distributed in the different direc- 

 tions in the same way as the light from a white- 

 hot surface. It can be shown that the pressure is 

 thereby reduced to f P, so that the total pressure 

 of incident and emitted radiation is f P. 



If there were no back pressure of, no recoil from, 

 the issuing radiation, the pressure' on the two discs 

 would be P in each case. What, then, is to be 

 looked for to prove the existence of back pressure 

 is a greater value in the second case than in the 

 first. 



In the experiment four discs were used, the front 

 and back surfaces being respectively black and 

 black, black and silver, silver and silver, silver and 

 black. Supposing that the black was perfectly 



