Thomas Young 19 



water waves, but generally to all small displacements such 

 as those we suppose to occur in the propagation of light. 

 The important point to notice is, that two rays of light 

 falling on the same point can neutralize each other's effect, 

 so that there is darkness, where each ray separately produced 

 illumination. 



The colours of thin plates could not be explained on 

 Newton's theory, unless the corpuscles of light were endowed 

 with some peculiar attributes, and it occurred to Young 

 that a more natural explanation presented itself by con- 

 sidering the overlapping of waves which occurs whenever 

 two rays of light meet at a point. This led him to design 

 new experiments in which two sets of light waves could 

 be made to overlap in such a manner that the crest of 

 one set falls exactly over the hollow of the other, so that 

 the two waves neutralize each other. By measuring the 

 distances of the dark regions from each other, he showed 

 how the lengths of waves could be determined. All seemed 

 simple and straightforward, when a formidable difficulty 

 arose, through the discovery of a new property of light, 

 now called polarization. This seems to have baffled Young 

 to such an extent that he began to be doubtful of his 

 theory. It was only when the French engineer, Fresnel 

 (who rediscovered the cause of the " interference " of light 

 and corrected Young's explanation of " diffraction "), had, 

 in conjunction with Arago, formulated more precisely the 

 experimental conditions under which polarized light may 

 interfere, that the clue to the solution was found. In a 

 letter to Arago, dated 12th of January 1817, Young 

 suggested that the peculiarity of waves which gave rise 

 to polarization might be due to the direction in which the 

 motion takes place. In a wave of sound, each particle 

 of air moves backward and forward in the direction in 

 which the sound is propagated, so that if the sound 

 spreads out from one point, the motion is directed every- 

 where to or from the centre. In a wa' er wave propagated 

 over a horizontal sheet of water, on the other hand, the 

 direction is mainly up and down. It occurred to Young 

 that if a wave of light resembled that spreading over a sheet 

 of water, two disturbances propagated in the same direction 



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