60 PRESSURE OF LIGHT 



P v P 2 constitute a counter-clockwise couple or 

 torque, acting on the air, and there must be an 

 equal and opposite couple acting on the tube, 

 turning it round clockwise. 



FIG. 26. 



Since a beam of light is just as much a stream 

 of momentum as a blast of air, if we shift a beam 

 of light parallel to itself we must exert a couple or 

 torque upon it, and the light will react with an 

 equal and opposite torque on the system by which 

 the change is effected. 



An experiment to show this was made by 

 suspending two small glass prisms, with refracting 

 angle 34, at the ends of a rod 3 cm. long, hung 

 up by a quartz fibre in an exhausted case. The 

 plan is shown in fig. 27, C being the point of 

 suspension. The sloping side of each prism was 

 2*15 cm. long and r6 cm. high. The ray AB 

 was refracted along BD, and then emerged along 

 DE, turning a corner in each prism. It exerted 

 a force outwards against each corner, and the 

 suspended system turned round in the direction 



