1 monochromatic! Interference Colors What changes for other colors (wavelengths)?What changes for other colors (wavelengths)? Δ = d (n s - n f ) Δ = d (n s - n f ) 1  1.5  Δ=  /2 d fast slow

2 Interference Colors Story above was for one wavelength (color) of lightStory above was for one wavelength (color) of light Retardation distance (Δ) is ~same for all colors, but:Retardation distance (Δ) is ~same for all colors, but: – Δ = m  -> no ray (rotation = 0° or 180°) – Δ = [(m-0.5)  ] -> max.ray (rot.=90°, 270°) Certain wavelengths get blocked at analyzer, others passCertain wavelengths get blocked at analyzer, others pass –produces an “interference color” –show Excel calculation

3 Thickness effect Δ = d (n s - n f ) Δ = d (n s - n f ) quartz wedge demoquartz wedge demo –  = (n s - n f ) = (a small value) –shows change in set of transmitted wavelengths (i.e., color) with increasing retardation, Δ

4 Birefringence effect Δ = d (n s - n f ) = d  Δ = d (n s - n f ) = d  –can get same set of colors by varying  at constant d maximum  is characteristic of mineral!maximum  is characteristic of mineral! –e.g., calcite  = (a large value) –quartz  = (a small value) orientation-dependentorientation-dependent –  (=n s -n f ) ranges from 0 to a maximum –0 is looking along “optic axis”

5 Interference Color Chart thickness, d (µm) retardation, Δ (nm) birefringence,  quartz? range of colors - same as quartz wedgerange of colors - same as quartz wedge measuring birefringencemeasuring birefringence

6 Interference Color Chart thickness, d (µm) retardation, Δ (nm) birefringence,  OrdersOrders Every 550 nm (≈  blue )Every 550 nm (≈  blue )

7 Interference Color Chart Two kinds of whiteTwo kinds of white –low-order –high-order  = d = 30 µm what color is out here?