Thin Film Interference

Soap Film – Why Color?

iridescence.

Reflection and Phase

t

Thin Film Interference

Some light reflects off top layer, some off bottom layer.

If crest meets trough destructive interference

If crest meets crest (trough meetstrough) constructive interference

If remove (by destructive interference) a color from white light, will see the complementary color.

ex., if yellow light is cancelled, will see blue

Thin Film

What determines the color seen?

Film material (n) and thickness (t)Angle viewed at

(assume light strikes perpendicular)

t

extra distance (ABC) = 2t

Both rays

One ray

1λ, 2λ, 3λ, …

½λ, 3/2 λ,

5/2λ,…

phase shift

2t

Both rays

One ray

1λ, 2λ, 3λ, …

constr. inter.

destr. inter.

½λ, 3/2 λ,

5/2λ,…

destr. inter.

constr. inter.

phase shift

2t

λ is the wavelength in the film

λair

λn = --- n

A soap bubble (n = 1.35) appears green (λ = 540 nm) at the point on its front surface nearest the viewer. What is its minimum thickness?

1.0 x 107 m

http://en.wikipedia.org/wiki/Image:Blue_morpho_butterfly.jpg

Iridescent Paint

Stealth Bomber

Eliminate radar through thin-filminterference

Anti-Reflective Coating

Coated lens reduces reflectionsthrough thin-film interference.