ILLUSTRATING CARRIAGE OF MOMENTUM 53 



absorbing surface it exerts a force which has a 

 component along the surface. 



Let a beam of light or radiation AB fall on the 

 absorbing surface S, fig. 21. Then it has momentum 

 in the direction AB. Let the length of AB 

 represent the momentum which it brings in one 

 second. Resolve AB into NB perpendicular to 

 the surface and TB along it. If S cannot be 

 pushed back, NB will have no visible effect. But 

 if S can slide on its own plane TB will push it 

 towards S/ 



o 



FIG. 22. 



To test this two discs of glass, one blacked, the 

 other silvered, each about 2 cm. in diameter were 

 fixed at the ends of a thin glass rod 5, cm. long, 



