The Determination of the Refractive Indices of Inhomogeneous Solids by Interference Microscopy

Abstract

The refractive indices of an optically inhomogeneous specimen will generally vary in directions both parallel to and perpendicular to the microscope axis. It is, however, possible to define a mean index μs such that (μs × specimen thickness) represents the optical thickness in a direction parallel to this axis. Interference microscopy reveals those parts of the specimen for which the mean index μs is equal to the refractive index of the immersion liquid. Thus, by gradually changing the index of the liquid relative to that of the specimen, one can determine the manner in which μs varies from region to region of the specimen. These changes in relative indices can be effected by altering either the wavelength of the incident light or the temperature of the system. In the wavelength variation method, which is applicable to specimens of thickness less than about 0.1 mm, the incident beam is of white light, and the fringe pattern is observed with the aid of a spectrograph. In the temperature variation method, which is useful for thicker specimens, monochromatic light is employed, the fringes being observed by direct microscopic examination. If the specimen is of convenient geometrical form it is possible to utilize these observed values of μs in order to calculate the refractive indices at any point in the interior of the specimen, but with irregularly shaped specimens one might have to resort to section cutting in order to achieve the same results. Application of these methods to anisotropic bodies enables conclusions to be drawn concerning the degree of molecular orientation and the closeness of molecular packing at different regions of the specimen.