Refractive Index and Thickness of Ultrathin Sections of Coals and Graphite by Interferometry

Abstract

The development of techniques for preparing ultrathin sections of coals now permits the direct measurement with an interference microscope of the refractive index of coal components. The change in phase of polarized monochromatic light (5460 A) upon passage through an absorbing ultrathin section is a function of refractive index, thickness, and extinction coefficient. A graphical procedure has been developed for calculating these parameters from phase change and independent absorption measurements. The effects of multiple reflections on the latter data were carefully considered. The ultrathin sections were usually between 500 and 1000 A thick. The refractive indices obtained agreed well with those previously derived from reflectance measurements. Values of 2.20 and 0.51 for indices of refraction and absorption of graphite at 5460 A were obtained by these direct measurements as compared to 2.15 and 0.66 by reflectance measurements.