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peacock

peacock. dogbane beetle. Thin Film Interference. opal. k 1. k 2. y. then. if. Maxima at:. Reflection from a dielectric slab:. Reflectance:. I o. I R. 1. .04. (near normal incidence). .0369. ( n = relative index). n. .0384. .96. R = 0.04 for glass.

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peacock

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  1. peacock dogbane beetle Thin Film Interference opal

  2. k1 k2 y then if Maxima at:

  3. Reflection from a dielectric slab: Reflectance: Io IR 1 .04 (near normal incidence) .0369 (n = relative index) n .0384 .96 R = 0.04 for glass To consider interference between the front and back reflection, assume they have the same amplitude: 0.04 Io

  4. Dielectric Films n t What is d? Assume near normal incidence: Physical path difference: 2t Optical path difference:

  5. Wait!! Reflections can cause phase shifts! First reflection: p phase shift Second reflection: no phase shift “Reflection” path difference: Total phase difference: Constructive Destructive How does this lead to color?

  6. white n t t = 400 nm, n = 1.515, q = 0 R lo (nm)

  7. normal and 45 degrees 45 0 For varying angle of incidence:

  8. 4% per interface adds up!

  9. E k Goal: R = 0, T = 1 (Impedance Matching) no ng Can only adjust n

  10. E Antireflection coating k no nf ng

  11. Antireflection coating: two requirements 1. Destructive interference: no = 1.0 t nf = x ns = 1.5

  12. 1. Equal amplitudes:

  13. Reflections from the front and back surfaces of a dielectric slab result in two plane waves with an optical path difference and therefore exhibit interference. The interference effects can be engineered to minimize reflectance or produce pretty colors.

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