= Birefringence
{title2=$\Delta n=n_e-n_o$}
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<Birefringence> is the existence of different optical propagation constants for different eigenpolarizations in an anisotropic material. It provides the propagation-mode separation needed for <adiabatic optical following in a twisted nematic>.
Two different <refractive indices> can produce two displaced images, as in the <calcite double-image experiment>, or a <phase retardation> between overlapping components, as in a <waveplate>. Spatial separation concerns where <energy flux> travels; <phase retardation> concerns the relative <wave phase>. A specimen need not show two visibly separated images to have <birefringence>.
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