96 PRESSURE OF LIGHT 



M = E/V where E is the energy density and V is the 

 velocity of light. 



(i) Pressure on an area moving along its normal and 

 emitting R per sq. cm. when at rest. 



Let a be unit area at the centre of a large hemisphere 

 radius r, fig. 39. 



If a is at rest the energy density at the hemisphere is 



R cos 



for the energy flow given by this is 



E V . 27T7- 2 sin OdO = R 







Let the wave-length of the radiation emitted when a is 

 at rest be A . 



Now give a a velocity v along its normal. The velocity 

 in direction is v cos 0, and therefore the wave-length 

 in that direction is 



x x V v cos 



Assuming that the amplitude is unaltered, the energy 

 density E at the surface of the sphere is as in Note 2 : 



E = E -^! = E V 2 



A 2 (V-v cos 6)* 



neglecting 



R COS ( 2V COS 6\ 



\ ~T > 



The momentum density is E/V, or 



, T R cos Of , 2V cos 



