双折射现象，光学现象的一种，可以用光的横波性质来解释。当光照射到各向异性晶体（单轴晶体，如方解石、石英、红宝石等）时，发生两个不同方向的折射；其中一个遵守折射定律的称为o光（ordinary ray、寻常光），另一束不遵从折射定律的称为e光（extraordinary ray、非常光），这两束光都是偏振光。光线从一个特殊的角度射入晶体是不会发生双折射现象，这一角度称为晶体的光轴。对于双轴晶体（如云母、蓝宝石、结晶硫磺等），两束光都不满足折射定理。

波片是这种现象的一个应用。

Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with asymmetric crystal structures are often birefringent, as are plastics under mechanical stress.