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# Polarized Light - PowerPoint PPT Presentation

Polarized Light. Polarized Light. Polarizing Filters. Natural Polarization. Double Refraction. Polarized Light in Crystals. Privileged Directions. Fast and Slow Directions. Fast and Slow Rays. Retardation. Retardation = n λ. Retardation = (n+1/2) λ. Retardation.

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## PowerPoint Slideshow about 'Polarized Light' - sana

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Presentation Transcript

### Polarized Light

• 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

• 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

• The optical properties of a mineral are determined by vibration direction

• The ray path has little role

• We have to look at light differently

• 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

• 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)

• 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.

• 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?

• 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