WAVKS AND CORPUSCLES — DE BROGLIE 247 



bound U|) with matter. Our conception of the physical worlil there- 

 fore will not be complete unless another phenomenon independent 

 •of matter be added to it, namely, \\fi,ht. 



But what is li<^ht ^ Of what is it constituted? 



The j)hilosophers of antitjuity and many scientists up to the be- 

 ginning; of the last century maintained that li«rht was made up of 

 small corpuscles in rapid movement. The propa'ijation of light in 

 straight lines under usual conditions, as well as the reflection of 

 light by mirrors, was easily explained by such an hypothesis. 



The corpuscidar theory of light had been abandoneil for a century 

 following the experiments of Young and Fresnel. Young and Fres- 

 nel had discovered a large class of light plienomena — those relating 

 to interference and diffraction — the interpretation of which was im- 

 possible by the corpuscular theor}'. However, another conception, 

 the wave theory of light, as Fresnel admirably showed, explains both 

 the older class of })henomena — the rectilinear propagation of light, 

 its reflection and refraction — as well as the later class connected with 

 interference and ditfraction. 



The uiidulatory conception of light had been upheld previously 

 by certain farseeing geniuses like Christian Hu3'ghens, who thought 

 that light should be compared to the propagation of a wave in an 

 elastic medium; for instance, like the movement of a ripple upon the 

 surface of a sheet of water upon which a stone has been cast. Since 

 light passes through a vacuum, Fresnel imagined a sort of subtle 

 medium, the ether, which impregnated all matter, lilling the empty 

 spaces and serving as a basis for the propagation of the luminous 

 wave. 



Let us now consider how a luminous wave is conceived. Light 

 freely propagated is analagous to a succession of waves of which the 

 crests are separated by a constant distance called the '* wave length." 

 A group of these waves moves in the direction of propagation with 

 a definite velocity known as the '* velocity of light." For light waves 

 in vacuo this should be taken as 300,000 km./sec, as has been meas- 

 ured by experiments made since the death of Fresnel. At an\' given 

 point in space the difl'erent waves with their crests and troughs pass 

 successively. The magnitude which is ])ropagated by the waves 

 therefore varies at a given point periodically and the period of that 

 variation is evidently equal to the time which elapses between the 

 passage of two successive crests. 



We have just seen how a wave moves forward in a region where 

 there is nothing to hinder its progress. A different action occurs 

 when the wave in its motion impinges u])on an obstacle, for example, 

 if it encounters a plane surface which stops and reflects it; or if it 

 must pass through apertures pierced in a screen ; or further if it en- 

 counters a point wliich diffuses it. Then the disturbance will be- 



