BY A. L. HcAULAY. Tl.Sc. H.A.. Ph.D. 83 



SECTION 3. DIFFRACTION PATTERN DUE TO 

 SINGLE SLIT. VISIBILITY CURVE. 



The slit C is first completely closed, and then siowly 

 opened. When its width is about .1 mm. tha field of the eye- 

 piece is seen to be slightly illuminated. On increasing its 

 width the illumination increases, and at a csrtain stago dark 

 vertical bands appear, one from' each side of the field, and 



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move towards the centre. The slit is opened till it com- 

 pletely uncovers one aperture of the grid D, i.e., until its 

 width is about .4 mm. The appearance is now as indicated 

 in Fig. II., where abscissai represent distances from the cross- 

 wire of V, and ordinates the intensity of the light. This 

 curve in what follows will be referred to as the visibility 

 curve. 



Measure the distance, f, from E to F, the width of the 

 slit, e, and with the micrometer eyepiece the corresponding 

 distance, 2d. between the first njinima on the visibility curve. 

 Repeat with* three diffex-ent slit widths, ending by op;.>ning 

 the slit till one whole aperture of the grid is exposed. Show 

 that in each case <i^^ f/e. "here /\ is the mean wave length 

 of the light used (about •' x 10-'> (.,„!,.) 



