1!Y A. L. M.AILAY, H.Sc. IJ.A.. IMi.l). 93 



SECTION 5. THE TWO LINE GRATING. POSITION 



OF SPECTRA FORMED BY A GRATING. 



The slit C is widened till a second aperture of the 

 grid begins to be uncovered. When a very narrow strip is 

 uncovered the pattern of section 3 is still clearly seen in the 

 eyepiece F, but it is furrowed by dark lines. A typical curve 

 connecting illumination with distance from the crosswire is 

 shown in Figure V. 1. On further widening the slit till the 

 vv'hole of the second aperture is uncovered, the dark bands 

 deepen, and the pattern splits into bright strips, all but the 

 centre one being coloured at the edges. The colour ", are dis- 

 persed more the further the strips are from the crosswire. 

 These bright strips with coloured edges correspond to the 

 spectra formed by gratings, and are, in fact, the spectra 

 formed by a grating of two lines. Figure V. 2 shows the 

 way in which V. 1 would develop on widening the slit. As 

 will presently be shown, these spectra should be evenly 

 spaced. Measure the distances of their centres from the 

 centre of the central uncoloured sti-ip with the micrometer 

 eyepiece, and calculate the corresponding values of «iii o. 

 having the same meaning as in the last section. It will be 

 noticed that some gaps are double others, indicating that cer- 

 tain spectra are missing. This is due to the fact that they 

 should appear where there is a minimum of the visibility 

 curve, and where c:;nsequenlly there is no light available for 

 their production. c Figure V. 2 represents such a case. 



Draw and dimension curves similar to Figures V. 1 and 

 V. 2 from your observations, and compare them with the 

 dimensioned visibility curve you obtained as in section 3. 

 Make a special note of any spectra that are missing. Com- 

 pare the values you obtain for sin for the centres of the 



spectra with '" ' where 1 is the distance between two 



successive apertures of the grid, m is an integer, and A is a 

 mean value for the wave length of white light, ray •' x 10— ■> 

 cms. 



SECTION (5. THEORY OF SECTION 5. 



The case taken for con.'-ideration is that observed when 

 the two apertures are equal in width, i.e., when two complete 

 apertures of the grid D are uncovered by the slit C. Con- 

 sider the illumination at a point on the field uf F correspond- 

 ing to an angle of diffraction o. Then from what has gone 

 before, the illumination due to each slit separately is that 



