Thin Film Interference opal dogbane beetle peacock y

Thin Film Interference opal dogbane beetle peacock

y k 1 k 2 if then Maxima at:

Reflection from a dielectric slab: Reflectance: Io IR 1 . 04. 0369 (near normal incidence) (n = relative index) n . 96 . 0384 R = 0. 04 for glass To consider interference between the front and back reflection, assume they have the same amplitude: 0. 04 Io

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

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?

white n t t = 400 nm, n = 1. 515, q = 0 R lo (nm)

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

4% per interface adds up!

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

E no nf ng k Antireflection coating

Antireflection coating: two requirements 1. Destructive interference: no = 1. 0 nf = x ns = 1. 5 t

1. Equal amplitudes:

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.