SOUND WAVE AND MICROWAVE SPACE PATTERNS 575 



fluctuations which become more rapid as the radiating source is approached; 

 similar fluctuations are observed in front of large-area microwave antennas. 



Diffracted Waves 



Figure 14 shows the diffraction of waves over a straight edge. The sound 

 waves in the upper half of the pattern are seen to progress with flat phase 

 fronts, but in the shadow region of the f' wooden board circular wave fronts 

 are evident, indicating that the edge is acting as a new Huyghens source. 

 Figure 15 is a similar pattern showing diffraction around a wooden disk. 



Fig. 15 — The circular wave fronts in the shadow of a 10" diameter "opaque" disk combine 

 to produce a lobe structure. 



The circular wave fronts emanating from the top and bottom edges are 

 evident. Similar wave fronts are re-radiated from all around the circular 

 edge of the disk and these combine to produce a concentration of energy 

 along the axis corresponding to the "bright spot" of optics in the shadow of 

 an opaque disk. Figure 16 is a repeat of 15 with the phase signal removed; 

 this amplitude pattern shows the bright spot more clearly. 



Another diffraction effect of optics, the pattern produced by two small 

 slits, can be duplicated by two non-directional sound sources separated 

 several wavelengths and having equal amplitudes and phase. This produces 

 the multi-lobed pattern of Fig. 17. Destructive intereference occurs in those 

 directions for which the two sources are out of phase, and constructive inter- 



