PRESSURE OF LIGHT 



HOW LIGHT EXERTS PRESSURE 



WHEN we see the havoc wrought on a sea-wall 

 by a storm, it is easy to believe that ocean waves 

 exert a pressure against the shore on which they 

 beat. But it is not easy to think that the tiny 

 ripples of light also press against every body on 

 which they fall, to think that when a lamp is 

 lighted waves of pressure are sent out from it 

 pressing against the source from which they start, 

 pressing against every surface which they illu- 

 minate. Yet we now know certainly that light 

 does exercise such pressure. It is a very minute 

 pressure, far too small, even when it is strongest, 

 to be felt by our bodies, and only to be detected 

 by exceedingly sensitive apparatus. 



In the following pages I shall try to give some 

 account of the reasoning by which the existence 

 of light-pressure was predicted, and shall then 

 describe the experiments by which it was, many 

 years later, actually detected and measured. I 

 shall then point out some consequences of the 



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