14 PRESSURE OF LIGHT 



just the opposite motion. For the light falling on 

 the unblackened surfaces is partly reflected and 

 would therefore press not only in falling on the 

 surface but would give, as it were, another kick 

 back on being reflected, while the light falling on 

 the blackened surface would press only in falling 

 on it, for it is absorbed there and does not rebound. 

 The unblackened surfaces would therefore retreat. 



It was soon found that the action of the radio- 

 meter is due to the residual air still remaining in 

 what we call the vacuum in the bulb. The black- 

 ened surfaces absorb the light and so become hotter 

 than the unblackened surfaces. The molecules of 

 air in the bulb are rushing about in all directions, 

 and those which hit the hot black surface get a 

 little extra energy from it and go back faster than 

 they came up, and so give a greater kick back 

 against it than if they rebounded at the same 

 speed. Those which come up on the other cooler 

 side go back with the same velocity with which 

 they came up and do not give an increased kick 

 back. Thus the residual air presses more against 

 the black surface and the vanes spin round. 



For reasons which we cannot enter into here, 

 this "radiometer action," as it is termed, only 

 comes into serious consideration when the air is 

 very much rarefied. But no doubt it was present 

 in the early attempts made to detect light-pressure 

 on a disc in an exhausted vessel. 



