88 



PRESSURE OF LIGHT 



normal to the surface. They therefore pass through the 

 axis of suspension, and have no moment round it. 



Theory shows that in a vacuum the momentum per 

 length V of a beam, where V is the velocity of light, is 

 equal to the energy per length i . This is fully confirmed 

 by experiment, which shows also that it holds in gases 

 at quite appreciable pressures. 



Let us assume that it holds, too, in a solid medium, 

 such as glass. Then we can prove as follows that the 



FIG. 37- 



force at the point of refraction is a pull outwards on the 

 refracting medium. 



Let us consider separately the reflected and refracted 

 parts of a beam incident obliquely on a refracting 

 medium. Let AB (fig. 37) be reflected along BC. 

 Let AB represent the momentum per second brought to 

 B by the part of the incident beam which is reflected. 

 Then BC equal to AB will represent that in the reflected 

 beam. Produce AB to D, making BD equal to AB or 

 BC 



