Birefringent is a word that refers to the property of a substance that allows for the splitting of light into two rays with different refractive indices. When it comes to finding synonyms for birefringent, there are a few options that come to mind. One is "doubly refracting," which refers to the overarching concept of a material having this property. Another is "anisotropic," which is a term used to describe a substance whose properties differ based on the direction in which they are measured. Other words that can be used as synonyms for birefringent include "biaxial," "bifringent," and "bifocal." Each of these words carries slightly different connotations and nuances, but they all refer to the same basic idea of a material's ability to split light into two distinct rays.