Why does a soap bubble or a film of oil on water show swirling rainbow colours?
Light reflects off both the top and bottom surfaces of the thin film.
Light reflects off both the top and bottom surfaces of the thin film. The two reflected waves travel slightly different distances and interfere. For each colour (wavelength), some thicknesses interfere constructively and others destructively, so different colours are reinforced at different film thicknesses — giving the shifting rainbow pattern as the film thins and moves.
Key point
Thin-film interference: reflections from the two surfaces reinforce different colours.
Common mistake
thinking the colours are pigments in the soap or oil.
Memory tip
colours come from interference, not dye — they shift as thickness changes.
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