Thin Film Interference
The interference phenomenon occurring when light waves reflected from the top and bottom boundaries of a thin transparent film superimpose.
Governing formula
2 n t cos θ = m λ (Constructive with phase shift)
SI unit
m
In depth
Light reflecting off an optically denser surface undergoes a $π$ ($180^°$) phase shift. Total path difference between top and bottom reflections produces vivid wavelength-dependent color bands.
Examples in the real world
Swirling rainbow color patterns visible on gasoline slicks over wet pavement or soap bubbles.