polarized light. polarizing filters natural polarization
TRANSCRIPT
Retardation• One ray is fast, one slow• v = c/n (n = index of refraction)• Time to traverse thin section = h/v = hn/c• Fast ray takes t = hnf/c• Slow ray takes t = hns/c• Time lag = Δt = hns/c - hnf/c = h(ns – nf)/c• Fast ray leads slow ray by c Δt = h(ns – nf)• This quantity is called retardation• The quantity ns – nf is called birefringence
Retardation• If the retardation = integer number of
wavelengths, light recombines with no change, and is blocked
• If the retardation = integer number of wavelengths plus 1/2, light recombines perpendicular to its original direction, and is fully transmitted
• Materials with zero birefringence (isometric or noncrystalline) are called isotropic
Vibration Direction
• The optical properties of a mineral are determined by vibration direction
• The ray path has little role
• We have to look at light differently
The Indicatrix
• How can we summarize optical behavior in all directions?
• The indicatrix is an ellipsoid with radius equal to refractive index for that vibration direction.
• Shape of the indicatrix reflects symmetry of crystal
The Indicatrix
• Isometric or noncrystalline materials have the same RI in all directions (isotropic). The indicatrix is a sphere.
• Hexagonal, trigonal and tetragonal minerals have one high symmetry axis (uniaxial). The indicatrix is an ellipsoid of revolution
• All other minerals have an indicatrix with 3 unequal axes (biaxial)
Optic Axes• If RI doesn’t vary, there is no retardation
and no interference color.
• This happens if cross section of indicatrix is a circle.
• Every mineral has at least one circular cross section.
• Direction perpendicular to a circular cross section is called an optic axis.
What Optical Behavior?
• Isotropic minerals are easy – they never show interference colors
• Can we somehow see the optical behavior in many directions at once?– If we could turn a grain at will, that would be
great– Universal stages are expensive and laborious
to use– Can we send light through in many different
directions at once and see what happens?
Conoscopic Observation
• Retardation increases away from optic axis– Higher birefringence– Greater thickness of material– We see concentric color bands (isochromes)
• Some areas of the field go extinct– Extinction areas are called isogyres