PHYSICAL ASPECTS OF IMAGE FORMATION 25 



that energy exchanges between matter and radiation (electromagnetic 

 waves) occur by quanta of value hv {h = Planck's constant, v = fre- 

 quency of the vibration emitted). It is usually considered that electro- 

 magnetic waves are emitted by wave-trains generated from the various 

 atoms of the source. 



Atoms emit for a very short time r. If a long time elapses in relation 

 to T, as is always the case with our experiments, the initial vibrations 

 shall have vanished. Other atoms will emit new vibrations having no 

 relationship with the initial ones since their direction, amplitude and 

 phase have changed. The various elements of a source are therefore 

 incoherent. 



Let us first consider the well-known experiment of Young's twin 

 apertures. The latter (Fig. 1 28), A and B, are identical and illuminated 

 by the very minute source Si. Interference fringes are seen on the 

 screen E. Notwhhstanding the very minute dimensions of the source Si, 

 the vibrations it emits are originated by a large number of atoms each 

 of which emits at a given moment a vibratory motion unrelated to 





M2: 



I 

 I 

 I 



0,' 



Fig. 1.28. Young's experiment. 



those emitted by the other atoms of Si. However, every atom-emitted 

 vibratory motion gives rise to vibrations that reach A and B in the 

 same condition because, provided 5*1 be sufficiently small, it can be 

 said that, from all its points, the optical paths Si A and SiB are 

 equivalent. Phenomena occur as if apertures A and B emit coherent 

 vibrations. Therefore the contrast of fringes is maximum on the 

 screen. If A and B are illuminated by the second source S.. alone, 

 it would show well-contrasted fringes on the screen E that, 

 however, would be offset in relation to the interference fringes 

 originated by Si. Let us assume that sources Si and Sa are illumi- 

 nating A and B concurrently. Since both sources are incoherent, 

 the resulting intensity, at any point of the screen E, is the sum of the 

 intensities generated by both phenomena. When the angle d is very 

 small, offset of the two phenomena is negligible : apparently the latter 



