PHYSICAL ASPECTS OF IMAGE FORMATION 



27 



are coherent, the "range of coherence", merely shrinks. Without 

 altering a, let us take a large source (Fig. 1.30). This figure shows 

 the diffraction disk related to the source centre 5*0 and the diffraction 

 disk produced by any point Si. If the latter is close to So, both 

 diffraction disks are slightly offset. Both points A and B are illuminated 





\i 



Fig. 1.30. Range of coherence with an extended source. 



in coherent light by So and Sj too, but the two latter are incoherent. 

 In both cases, A and B can produce interference fringes. The two 

 fringe systems are incoherent but, as they are superimposed, nothing 

 is altered. As point Si recedes from So, phenomena remain unaltered 

 until the two diffraction disks are offset to such an extent that neither 

 of points A and B are illuminated by Si. Consequently, if a large 

 source is imaged on the plane P by means of the lens C, the two 

 points A and B are always illuminated in coherent light provided that 



Fig. 1.3 1. Points A and B are illuminated in incoherent light. 



the distance AB hQ small in relation to the diffraction-disk diameter 

 of C (diameter depending solely on a). When distance AB is greater 

 (Fig. 1.31) the two points A and B are no longer overspread by the 

 same diffraction disk in relation to a point of the large source and, 

 therefore, are illuminated in incoherent light. Hence, the coherence 

 in relation to two points A and B in the plane P depends solely on 

 the aperture a of the lens C. 



