32 PRESSURE OF LIGHT 



they must press against B in being absorbed as much 

 as they pressed against A in being emitted. The 

 pressure against B is therefore equal to the energy 

 per cubic centimetre in the beam. 



But if B is a reflector, a perfect reflector let 

 us suppose, the reflected waves press back just as 

 much as the incident waves and so the pressure is 

 doubled. Or, to put it in another way, the incident 

 waves bring up momentum in the direction AB 

 and give it to B. B must give up momentum to 

 the reflected waves in the direction B to A, which 

 has the same effect as if it received momentum in 

 the direction A to B. That is, it receives a double 

 dose of momentum and the pressure is doubled. 

 Since we have twice as much energy in the space 

 just outside B where reflection is going on, as when 

 we have only the single beam, the pressure is still 

 equal to the energy per cubic centimetre. 1 



We can now see what the pressure amounts to. 

 In full sunlight outside our atmosphere the sunlight 

 received on a square centimetre of a black surface 

 would suffice to raise I gramme of water about 

 2-5 C. per minute 2 or about 0*041 7 C. per second 

 according to recent experiments. This is equi- 



1 Note 3, p. 85. 



2 There is still great uncertainty as to the value of this 

 heating effect the "Solar Constant." We shall take it 

 throughout our calculations as 2-5 calories per sq. cm. per 

 minute. 



