Birefringence and Retardation. IN THIS LECTURE Isotropic versus Anisotropic Minerals Double Refraction Interference Phenomena Retardation Birefringence Interference at the Upper Polar. Isotropic versus Anisotropic. Anisotropic minerals differ from isotropic minerals because:
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IN THIS LECTURE
d
ts =
Eqn 1
Vs
D
d
+
ts =
Eqn 2
V
Vs
d
D
d
=
+
Eqn 3
Vs
V
Vf
V
V
V

= ns
D = d (
)
and since
Vf
Vs
Vs
D = d ( ns – nf)
Eqn 4
The two rays exiting the mineral are in phase with the slow ray lagging behind the fast ray by one whole wavelength
Upper Polar
Mineral Sample
The vector R, the sum of the two rays, is at right angles to the vibration direction of the upper polariser
=> no light passes the upper polar
Lower Polar
The two rays exiting the mineral are out of phase with the slow ray lagging behind the fast ray by half a wavelength
Upper Polar
Mineral Sample
The vector R, the sum of the two rays, is parallel to the vibration direction of the upper polariser
=> light passes the upper polar
Lower Polar