That this particular case of diffraction is pxorely an edge 

 effect can also be seen from the two views, in which different 

 angles of incidence were used. 



The analogous problem, that of diffraction of sound waves 

 by a semi-infinite screen, has been studied by many investi- 

 gators. Although a somewhat different approach was used by 

 Lamb (I906), his results were the same, because it was shown 

 (Friedlander, 1946) that although the solution differed in form 

 from the one obtained by Baker and Copson (1950) » the two solutions 

 are related by a suitable transformation. It has further been 

 shown (Sivian and O'Neill, 1931-1932) by means of the solution 

 obtained by Lamb, that essentially the same situation holds for 

 any angle of incidence for which the plane of the wave front is 

 parallel to the edge of the screen. This means that diffraction 

 by a straight edge is an edge effect, which is in complete agree- 

 ment with Baker and Copson and with our ripple tank investigation 



(fig. 19). 



14. Di ffraction by an infinite slit '^ 



"Partially opened doors and windows in a house or' 

 other building afford apertures which may be small 

 compared with the wave length of many common sounds. 

 Thus the openings may be a few inches wide, and the 

 wave length of the speaking voice of a man may be 

 eight feet or more. Hence such sounds spread in all 

 directions beyond those openings, as is well known, 

 instead of proceeding as straight lines and giving 

 sharp sound 'shadows' as in the case of light through 

 the same opening." 



Barton. 



Consider the incidence of a wave upon an infinite slit 



of very small width compared with the wave length. If the slit 



7. Drude, P. K. L. (1922): Theory of Optics. Longmans, Green 

 and Co., pp. 198-20^. Translated by C. Riborg Mann and 

 R. A. Millikan. 



46 



