REFRACTIVE INDICES, LORENTZ FIELD FACTORS, ORDER PARAMETERS IN PURE AND BINARY NEMATIC MIXTURES D. Sunanda, M. S. Madhava, D. Revannasiddaiah, R. Somashekar, and J. Mahadeva Department of Studies in Physics, University of Mysore, Manasagangotri, Mysore-570006 Using the small angle prism technique refractive indices are measured at different temperatures in the nematic phase of (i) two nematogenic com- pounds, viz., hexylbenzoic acid (HBA) and diheptyl azoxybenzene (DHAB), and (ii) binary mixtures of HBA and octylbenzoic acid(OBA) (Mixture-I), and DHAB and nonylbenzoic acid (NBA) (Mixture-II). From the measured re- fractive index data and employing Neugebauer relations, anisotropy of Lor- entz field factors and orientational order parameter have been estimated. The higher order parameter hP 4 i has also been computed using the Humph- ries-James-Luckhurst model and is compared with that of Maier-Saupe model. Keywords: nematics; optical textures; order parameter INTRODUCTION There have been a number of experimental investigations on the birefrin- gence of nematic compounds and their mixtures. Refractive indices and density are the essential parameters required to obtain the accurate value of the polarizability anisotropy. The order parameter [1–5] in nematic liquid crystals is one among the most important parameters. The order parameter of large number of nematic liquid crystals and their mixtures have been reported using optical anisotropy data [6–11]. Here, we have reported the anisotropy of Lorentz filed factors and the orientational order in the cases of: (i) hexylbenzoic acid (HBA), Diheptyl azoxybenzene (DHAB), (ii) two binary mixtures viz., the mixture of HBA and octylbenzoic acid(OBA) (Mix- ture-I), and of DHAB and nonylbenzoic acid (NBA) (Mixture-II). Further, One of the author wishes to thank the University Grants Commission, New Delhi, for the award of a research fellowship. Authors also thank CSIR, New Delhi for the Research Project vide project no. 03(0821)=97=EMR-II dated 2.12.1997 163 Mol. Cryst. Liq. Cryst., Vol. 409, pp. 163–174, 2004 Copyright # Taylor & Francis Inc. ISSN: 1542-1406 print=1563-5287 online DOI: 10.1080=15421400490431156