98 PRESSURE OF LIGHT 



Let aN be the normal meeting in N the hemisphere 

 through A with a as centre. Let P be a point on 

 the hemisphere at from N and at i^ from A. Let 

 ANP = <. 



The velocity of a towards P is v cos \J/, so that the wave- 

 length is altered from A to A where 



. f V COS \I/ 



- A A ( ] -^ r - 



FIG. 41. 



But cos i/r = sin 6 cos <. 



rr,, \ ' \ ' f sin cos 



Then A = A A i v 



V 



The energy density at P is 



E _^V_Rco./ + E sinf , cos 



The distance between two hemispheres one second 

 apart is V v cos \f/ = V v sin cos <. The momentum 

 between them per unit area resolved in the direction of 

 motion is 



cos \l/ (V v sin cos <J>) 



