Experimental Methods to Measure Refractive Index and Birefringence

3 Experimental Methods to Measure Refractive Index and Birefringence 

The classical meftiod to measure ftie index of refraction of transparent materials is based on total internal refraction. The instrument based on this method is the Abbe refractometer designed by Ernst Abbe in the early 1900s. This typically yields index accuracy of two units in the fourth decimal place, but it works only for materials where the refractive index is smaller than the glass prism (about n = 1.89). To measure anisotropic fluids with the Abbe refractometer, the material has to be uniformly (usually homeotropically) aligned between the prisms. Ordinary and extraordinary rays can be selected by using polarization filters polarized perpendicular and along the optic axis, respectively. 

Basically, the same principle can be used to measure An in samples of uniform thickness by observing the transmitted spectra under white light illumination. The transmitted intensity between crossed polarizers for a given wavelength becomes zero for Y/2 = n mod n, which gives An = IXJd where I = 1,2,. The measurements can be conveniently done by placing UV-VIS spectrometer (e.g., from OceanOptics) in the eye-piece of a polarizing 

Experimental setups to measure birefringence using wedge cell, (a) Refraction of laser tight beam, (b) Formation of fringes under monochromatic plane wave Ulumination. 

In PM the textures are mostly observed in transmission under collimated (parallel) illuminated light beams, where the light enters the film normal to its surfaces (orthoconic observations). The microscope in orthoconic observations gives a magnified, inverted 2D image of the texture. 

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