﻿[ 4y± ] 



LV. U asymmetrical Diffraction-bands due to a Rectangular 

 Aperture. By G. V. Raman, Demonstrator in Pliysics at 

 The Presidency College, Madras*. 



"VXTHEN a pencil of monochromatic light coming from a 

 ▼ 1 slit in the focal plane of a collimating lens falls npon 

 the object-glass of a telescope in front of which a narrow 

 rectangular aperture is placed with its sides parallel to the 

 luminous slit of the collimator, the diffraction-pattern seen in 

 the focal plane of the telescope consists of a series of bright 

 and dark bands symmetrically arranged on either side of the 

 geometrical image of the slit, provided that the light falls 

 normally upon the aperture. If, on the contrary, the aper- 

 ture is held inclined to the incident pencil — its sides being- 

 still parallel to the slit — the diffraction-pattern is not neces- 

 sarily symmetrical. The symmetry is not, however, sensibly 

 departed from, unless the incidence be very oblique. The 

 case in which this unsymmetrical pattern was first seen 

 is this : place a prism on the table of a spectrometer and 

 observe the image formed by the light reflected at very 

 oblique incidence from one of the faces of the prism. With 

 a prism of face-width 4*5 cms. and an incidence of 85°, the 

 diffraction-pattern seen in the field is sensibly symmetrical, 

 and the minima of illumination equidistant from one another. 

 If the incidence is greater than 87°, this is no longer true. 

 The bands are wider on one side of the pattern than on the 

 other, those on the side towards the direct image of the 

 slit being broader. This asymmetry increases greatly as 

 the angle of incidence approaches 90°, and at the same time 

 the number of bands on one side of the pattern — the side 

 where they are broader — becomes smaller and smaller till at 

 last they disappear altogether. 



The facts can be explained quite easily. Let a be the 

 width of the face of the prism and A. the wave-length of the 



light, and s — 6 the angle of incidence. Then, in anv direc- 



tion making an angle ^ — cf> with the normal to the face of 



the prism, there is no illumination provided 



a(cos 6 — cosc£)= ±n\ (1) 



where n is any whole number. 



If w = 0, = $, and we have the position of the light 



* Communicated by the Author. 



