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 meets trough)  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 λ air λ n = --- 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 10 7 m

Iridescent Paint

Stealth Bomber Eliminate radar through thin-film interference

Anti-Reflective Coating

Coated lens reduces reflections through thin-film interference.