60 Problem of Diffraction by a Semi-infinite Screen. 



The third case investigated is that shown in fig. 3. </>' is a 

 small positive angle and <£ is nearly equal to it. 



In this case practically the whole of the field is divided 

 equally between the region of shadow and reflexion (marked 

 I. and III. in the figures) and only a comparatively small 

 portion (marked II. in the figure) is that occupied by the 

 region of transmission. Both of the dividing boundaries fall 

 within the region of the field under observation ; and so far 

 as this part of the field is concerned, the approximate theory 

 of diffraction gives the same result as that found from the 

 complete analytical solution satisfying the boundary con- 

 dition 5 = 0. The theoretical form of the illumination curve 

 from a typical case of this kind is shown in fig. 2 (a), and 

 a photograph of the diffraction fringes is reproduced in 

 PI. II. fig. 4 (a). Table IV. shows the calculated positions 

 of the minima of illumination and the experimental data 

 for comparison. The agreement is satisfactory. 



Table IV. 





3 cm. \ = 4410A.U. 



<£' = !< 



Calculated widths of the 



successive bands between Observed widths. 



he minima of illumination. 



0-0071 cm. 



0-0071 



00052 



0-0053 



00046 



0-0048 



0-0044 



0-0042 



0-0039 



00041 



0-0037 



0-0038 



Summary and Conclusion, 



The results arrived at from this investigation may be 

 summarized as follows : — 



When plane waves of light are diffracted by the edge 



