We have studied the phenomenon of light propagation in an optically active linearly birefringent medium with particular reference to the electro-optic crystal of Bi12SiO20. Analytic expressions are given for the ellipticity and the orientation of the emerging light vibration when linearly polarized light is incident normally on the crystal. From these expressions we derive the interesting results that there exists an orientation of the incident light vector for which the output beam is linearly polarized and that the input and output linear vibrations are then symmetrically oriented with respect to the principal birefringent axes of the crystal. This particular angle of orientation is expressed in terms of the linear and circular birefringence of the crystal. Since ellipticity is a function of the electric-field-induced linear birefringence and hence of the electro-optic coefficient of the crystal, we have determined the electro-optic coefficient r41 from a measured value of the ellipticity. The coefficient r41 is found to be (4.1±0.1)×10−12 m/V.

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We present measurements of the spectral transmittance and photochromic response of CdCl(2):CuCl coatings. The coatings are also examined with the aid of electron micrographs. We explain the features observed in the spectral transmittance of darkened photochromic films, using a colloidal model obtained from the generalized Maxwell-Garnett theory. The colloidal model indicates the presence of ellipsoidal copper particles with a shape factor (minor axis/major axis) between 0.39 and 0.42 and mean ratios between 2.03 and 1.73 nm. The optical transmittance curve obtained in the faded state shows a peak near 280 nm attributed to Cu(+), and the curve in the darkened state shows two bands, a band located at 375 nm attributed to Cu(2+) and another at 600-610 nm assigned to colloidal copper particles.