Dispersive power and crossover temperature, T-CO, of two polar nematic liquid crystals in the visible spectral region

Refractive index studies are carried out on two highly polar liquid crystals: 1. N-(p-n-methoxy benzylidene)-p-amino benzonitrile, PmBAB, 2. N-(p-n-ethoxy benzylidene)-p-amino benzonitrile, PeBAB. The experimental investigations are carried out in the visible region at four different wavelengths, namely, 633, 589, 546 and 436 nm. The two compounds exhibit only the nematic liquid crystalline phase in between the isotropic and crystalline solid. The dispersive power omega is estimated for two consecutive wavelengths for the case of <n>, n(e) and n(o) for different wavelengths and found to be constant with temperature. Further the temperature gradients of n(e) and n(o) are estimated, and the crossover temperature is obtained using dn(o)/dT for all the wavelengths.