Birefringence

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In an anisotropic particle, light in the higher refractive index direction is slowed relative to light in the lower refractive index direction. As a consequence, fewer waves of light will get through the slower direction per unit time. Light is retarded in one direction relative to the other. Retardation (nm) = thickness (nm) x (n -n ) When retardation occurs the light coming out of the crystal in one direction is out of phase with light coming out in the other direction and destructive interference occurs producing the observed polarization colors.