The phenomenon of retardation (phase retardation) of light in one direction compared to its perpendicular direction knows as "birefringence" or double refraction, which rises due to polarizability anisotropy in two direction. There is another concept named "dichroism". What is this phenomenon and what is the molecular origin of it?
Also, when an optically anisotropic transparent sample is seen colorful, that is due to birefringence or dichroism?
In anisotropic materials generally two different waves travel which have two different complex indices of refraction. If the focus is on differences in the real parts the phenomenon is called "birefringence", if it is on the imaginary parts then it is called "dichroism". However, one cannot be without the other as they are related by the Kramers-Kronig relations. There are, nevertheless, transparency regions where a difference in the real parts remains while the imaginary parts are practically zero. Concerning color, it is introduced by linear dichroism, but for layers I could imagine that also interference could play a role.
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