1

monochromatic!

Interference ColorsInterference Colors• What changes for What changes for

other colors other colors (wavelengths)?(wavelengths)?

• ΔΔ = d (n = d (nss - n - nff)) 1 λ

1.5 λ

Δ=λ/2

d

fastslow

2

Interference ColorsInterference Colors

• Story above was for one wavelength Story above was for one wavelength (color) of light(color) of light

• Retardation distance (Retardation distance (ΔΔ) is ~same for ) is ~same for all colors, but:all colors, but:– ΔΔ = m = m λλ -> no ray (rotation = 0° or 180°) -> no ray (rotation = 0° or 180°)– ΔΔ = [(m-0.5) = [(m-0.5) λλ] -> max.ray (rot.=90°, 270°)] -> max.ray (rot.=90°, 270°)

• Certain wavelengths get blocked at Certain wavelengths get blocked at analyzer, others passanalyzer, others pass– produces an “interference color”produces an “interference color”– show Excel calculationshow Excel calculation

3

Thickness effectThickness effect• ΔΔ = d (n = d (nss - n - nff))

• quartz wedge demoquartz wedge demo– δδ = (n = (nss - n - nff) = 0.009 ) = 0.009

(a small value)(a small value)– shows change in set shows change in set

of transmitted of transmitted wavelengths (i.e., wavelengths (i.e., color) with color) with increasing increasing retardation, retardation, ΔΔ

4

Birefringence effectBirefringence effect

• ΔΔ = d (n = d (nss - n - nff) = d ) = d δδ– can get same set of colors by varying can get same set of colors by varying δδ at at

constant dconstant d

• maximum maximum δδ is characteristic of mineral! is characteristic of mineral!– e.g., calcite e.g., calcite δδ = 0.172 (a large value) = 0.172 (a large value)– quartz quartz δδ = 0.009 (a small value) = 0.009 (a small value)

• orientation-dependentorientation-dependent– δδ (=n (=nss-n-nff) ranges from 0 to a maximum) ranges from 0 to a maximum– 0 is looking along “optic axis”0 is looking along “optic axis”

5

Interference Color ChartInterference Color Chartth

ickn

ess,

d (

µm

)

retardation, Δ (nm)

birefringence, δ

birefringence, δ

quartz?

quartz?

• range of colors - same as quartz wedgerange of colors - same as quartz wedge• measuring birefringencemeasuring birefringence

6

Interference Color ChartInterference Color Chartth

ickn

ess,

d (

µm

)

retardation, Δ (nm)

birefringence, δ

birefringence, δ

• OrdersOrders

• Every 550 nm (≈ Every 550 nm (≈ λλblueblue))

7

Interference Color ChartInterference Color Chart

• Two kinds of whiteTwo kinds of white– low-orderlow-order– high-orderhigh-order

δ = 0.172

d = 30 µm

what color is out here?