6U(P) as u* and call the right hand side of (13.1) r^ (the 

 effect of diffraction), the solution may be written 



U = u* + u^ (13.2) 



In the geometrical shadow, U = u , so that the intensity 

 of illumination is measured by 



fTTl 2 I Bl 2 1 .21 z,^ ^% 



when p is large compared with the wave length. The intensity of 

 the incident light having measure unity, the illumination in the 



geometrical shadow is very feeble, but there Is nowhere absolute 



2 1 

 darkness since tan ^9 never is actually zero in the shadow. 



According to (13.1), when p is very large compared with the wave 



length, the diffracted wave is a non-isotroplc cylindrical wave 



propagated outwards from the edge of the screen. Diffraction by 



a black half plane is actually an edge effect, because neither 



■[f noT u depends on the angle of incidence and so U is the same 



for all black screens having the same edge and the sar.e shadow. 



The model tested in the ripple tank is a simple arrangement 



consisting of a vertical strip of plastic bounded by a straight 



edge and supported in such a manner that it protrudes above the 



surface of the water, the plane of the strip being normal to the 



surface. The depth of the water is constant everywhere so that 



the wave front Is only affected by the "wall" and the bounding 



straight edge. 



As was mentioned before, there Is nowhere absolute darkness 



2 1 

 in the geometrical shadow, since tan 2^ ^^ never zero there (13*3) 



This phenomenon may be seen in the prepared photographs (fig. 19). 



44 



