Relation Between Refractive Index and Density of a Glass at Constant Temperature

Data are presented on the changes in the room‐temperature refractive index and density of a borosilicate crown glass produced by different heat‐treatments. Measurements on a large number of samples with widely varying thermal history show that the refractive index is a single‐valued function of the density. The increase in refractive index for a given density change is smaller than that calculated by the Lorentz‐Lorenz theory; the apparent contribution of the induced polarization to the local electric field is very small. This may be due to a reduction in the ionic polarizabilities that accompany the density increase; alternatively, the Lorentz‐Lorenz form of the theory may not be valid in solid materials such as glass. Data of other investigators are shown to agree with the data reported here.