Generalised theory of interference and its applications. Part IV. Interference figures in absorbing biaxial crystals

Abstract

Phenomena-involving the interference of polarised light-displayed by crystalline plates (in parallel or convergent light) may be given a general method of analysis, using the physical concepts developed in Parts I and II. The subject is discussed under two main heads: the interference phenomena exhibited without and with the aid of an elliptic analyser behind the crystal plate-the former involving the interference of beams in different non-orthogonal states of elliptic polarisation. The cases when the incident light is either unpolarised, partially polarised, or completely polarised are all discussed-the concept of partially coherent beams finding fruitful applications in the first two cases. Illustrative photographs of the interference figures in convergent light shown by the optically inactive mineral iolite accompany the paper. Particular mention may be made of the interference figure with the incident light partially circularly polarised-showing the fading away of the ring system (due to incoherence) at a particular region in the figure; also of the spiral interference figures which are observed between a plane polariser and a circular analyser-even though the crystal is optically inactive.