[ ' 66 & ] 



LXXVI. On the Diffraction of Light by Cylinders and Spheres 

 immersed in a Medium of nearly equal Refractive Index. 

 By Nehal Karan Sethi, I).Sc, Assistant Professor of 

 Physics in the Benares Hindu University* . 



[Plate XX.] 



1. Introduction. 



rjlHE subject of the present investigation was suggested 

 X by a study of the beautiful and interesting phenomena 

 observed when a mixture of equal quantities of turpentine 

 and glycerine in a flat-sided flask is shaken up and allowed 

 gradually to separate. Some of the effects noticed have 

 been mentioned by writers on Optics f, but the matter does 

 not appear to have been fully investigated. In order to 

 obtain the best possible results, it is necessary to pay attention 

 to the temperature of the liquids. Ordinarily, glycerine has 

 a lower refractive index than turpentine throughout the 

 visible spectrum, and the effects obser\ed are not very 

 striking; but on heating up the mixture, the refractivity of 

 the turpentine falls off more rapi lly and ultimately becomes 

 less than that of the glycerine. Very lively effects are 

 obtained in the intermediate stage.*, the colour of the regularly 

 transmitted li^ht and the general appearance of the mixture 

 as seen by the light scattered by it changing with the temper- 

 ature. The superficial layer of the collection of drops which 

 settle down through the mixture exhibits some particularly 

 beautiful effects, due apparently to the large vertical gradient 

 of: refractive index which exists in the layer. The main 

 features of the phenomenon are analogous to those observed 

 in Christiansen's experiment, and have been dealt with as 

 such in a recent paper by the author %. There are, however, 

 certain special features in the present case depending on 

 the more or less exactly spherical shape of the individual 

 drops which appear to be in the nature of interference and 

 diffraction effects. It is proposed in the present paper to 

 deal with these latter effects more fully and to give a 

 theoretical discussion. 



* Communicated bv Prof. C. V. Raman, M.A. 



t E.g. L. Wright's 'Light,' p. 176, and 11. W. Wood's -Physical 

 Optics.' 2nd ed. p. 112. 



\ " On Wave-Propagation in Heterogeneous Media and the Phenomena 

 observed in Christiansen's Experiment." Proceedings of the Indian Asso- 

 ciation for the Cultivation of Science, vol. vi. parts iii. & iv. (1920). 



