CHAPTER VI 



Measuring Refraction Indices 

 of Transparent Objects: Unpolarized 

 Two-wave Interference Microscopes 



Two wave interference instruments provide measurement of the path 

 difference between two waves that travelled along different optical 

 paths. The result is therefore always an optical path which is the 

 product of a length multiplied by an index. 



When the object involved is transparent — of thickness e and 

 index n — and immersed in a medium of index /;' the interference 

 instrument shows {n' — n)e. Provided the thickness e and the index n' 

 of the medium in which the object is immersed are known, the measure- 

 ment evinces the index n of the object. The interference microscopes 

 shown in Figs. 3.4, and 3.8 are suitable. 



1. MEASURING PATH DIFFERENCES 



Let us consider the diagrammatic object in Fig. 6.1. It consists of 

 a small rectangular parallelepiped the cross-section of which is shown 

 at BCB'C. Required to measure its index n, considering that it is 



(1) (2) 



' 1 



^r 





I 



Fig. 6.1. Diagrammatic transparent object BCB'C. 



immersed in a medium of index n'. The specimen is illuminated in 

 monochromatic light of wave-length /. Thickness of the object being e 

 and its index //, the optical path travelled by the ray (1) passing through 



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