J %, f ,^ (Vl,V2)^ (14.1) 



2(Po > P^) 



where 



A = amplitude 

 p ,p, = distances shown in figure 21 

 V, ,V2 = values corresponding to the limits of 

 integration x, and Xp. 



In order to investigate the distribution of light in a 

 plane which lies a distance, b, behind the screen, it is necessary 

 to take the width of the slit into account. In the case where 

 the width of the slit is snail compared to the wave length, the 

 geometrical shadow cannot be even approximately located , for 

 the light is distributed almost evenly over a large region and 

 there is nowhere a sharp shadow formed. In the case where the 

 width of the slit is large compared with the wave length, the 

 effect will be simple diffraction at each of the edges. 



In the entrance to harbors or bays, breakwater gaps are fre- 

 quently encountered which are physically analogous to the problem 

 just discussed. In order to study the effect of ocean waves on 

 the breakwater gap (finite slit), two strips of plastic, boiinded 

 by straight edges, were placed In the ripple tank so that they 

 were alined in the same plane. The distance between the two 

 edges was then varied and the phenomenon observed and photographed. 



It has been found (Penney and Price, 1944) that in the case 

 of the breakwater gap, the wave pattern is essentially the same 

 whether the barrier is cushioned or rigid. Therefore our rigid 



50 



