barrier will sxifflce to demonstrate diffraction effects behind 

 the breakwater gap. 



When the gap width is less than the wave length, the gap 

 acts as the source of a spherical wavelet and the diffracted 

 waves spread into the geometrical shadow, even though very faint- 

 ly (figure 22), Upon increasing the gap width, the diffracted 

 waves also extend far into the geometrical shadow, and the effect 

 of diffraction is greater than before, since more energy is 

 admitted by the breakwater gap (fig. 23), However, when the gap 

 is made very large compared with the wave length, the phenomenon 

 becomes an edge effect, so that the part of the wave passing 

 through the center of the gap progresses without being disturbed 

 for a very great distance (fig. 24), 



15. Diffraction of light by two noncoplanar parallel straight 

 edges 



Heirtzler (1949) used the Kirchoff diffraction formula to 

 determine the intensity of illumination as a function of distance 

 along some plane of observation, for the case of diffraction by 

 two noncoplanar parallel straight edges. The method is essentially 

 the same as that employed by Drude (1922) in the case of a slit. 



It is seen from figure 25 that the opening lies between the 

 source Q and the point of observation P. The projection of the 

 line QP on the xz plane coincides with the x-axls and Q'; the 

 projection of Q on the xy plane is located midway between the 

 two edges which have coordinates x' and x". The expression for 

 the intensity of illumination is then found to be 



8. Heirtzler, J. R., 1949: Diffraction by two noncoplanar 

 parallel straight edges, American Journal of Physics , 

 V. 17, no. 7, pp. 419-422. 



51 



