NOTES 



8 9 



Then momentum represented in magnitude and direc- 

 tion by DC must be added to AB to convert it to BC, 

 though it acts through B. 



Then a force, CD, acts through B on the beam, and 

 an equal and opposite force, DC, acts on the medium at 

 B, that is, we have a normal pressure inwards. 



Now let A 1 B 1 (fig. 38) represent the momentum per 

 second brought to E l by the part of the incident beam 

 which is refracted along BjCj. 



FIG. 38. 



If E is the energy in length i of AjBj, and if E' is that 

 in length i of BjCj, the equality of energy passing along 

 the two beams gives 



EV = E'V 



where V and V are the velocities in the two media. 



But if M and M' are the momenta per second carried 

 by the two beams, our assumption gives 



M = E and M' = E' 

 Then MV = M'V 



