THIN FILM INTERFERENCE

Thin film interference occurs when light reflects from two surfaces of a thin layer. The optical path difference is 2nt·cos θ, where n is the refractive index and t is the film thickness. Constructive interference (bright) happens when 2nt·cos θ = (m+½)λ (accounting for phase flip at the top surface); destructive when 2nt·cos θ = mλ. This produces the vivid colors seen in soap bubbles, oil slicks, and butterfly wings.