Form birefringence
= Form birefringence
<Birefringence> produced by subwavelength structure rather than solely by intrinsic crystalline anisotropy. For ideal dielectric lamellae, continuity of tangential <electric field> gives $\epsilon_\parallel=f\epsilon_1+(1-f)\epsilon_2$, while continuity of normal <electric displacement field> gives $\epsilon_\perp^{-1}=f/\epsilon_1+(1-f)/\epsilon_2$. The unequal effective <permittivities> give different principal <refractive indices>.