Thermooptic Coefficients of Soda–Lime–Silica Glasses

The thermooptic coefficients, d n /d T , of two series of soda–lime–silica glasses, 25Na₂O· x CaO·(75 – x )SiO₂ and (25 – y )Na₂O· y CaO·75SiO₂, have been measured. When CaO is substituted for SiO₂, the magnitude of d n /d T passes through a maximum negative value. When CaO is substituted for Na₂O, there is a monotonic change from negative to positive d n /d T . These effects are accounted for by the changes in relative values of the coefficient of thermal expansion and the temperature coefficient of electronic polarizability, Θ. The effect of composition on Θ is explained by the changes in density of various nonbridging and bridging oxygens in the glass.