It is an optical property possesed by some materials like certain crystals and polymers. In the case of birefringent materials, they bend the light in two different directions. These directions depend on the polarization state of the light. There is a different direction for horizontal and vertical polarization states.
Researchers can tell what the light is doing inside the material by measuring the birefringence.
This gives them scientific information about how the material's molecules are behaving without having to disturb them. The following picture is the optical-system elements showing polarization orientations. The cartoon of the ERE experiment section below shows the setup for the Extensional Rheology Experiment. The position of the moving RDD is recorded to verify the imposed velocity. It also quantifies the extent of undesirable shear strain in the fluid near the RDD's.