Thin-Film Interference
A soap bubble is colourless — so why the swirling rainbows?
- Soap and water are clear, yet a bubble shimmers with swirling colours.
- A thin film of oil on a wet road does the same.
- The colours come from light reflecting off both surfaces of the thin film and interfering.
- It is interference again — this time from a film only a wavelength or two thick.
Two reflections interfere
- When light hits a thin film, part reflects off the top surface and part off the bottom.
- The two reflected waves travel slightly different distances, then overlap and interfere.
- Depending on the film's thickness and the light's wavelength, they add or cancel.
- The result is bright or dark for each colour, giving the shimmering hues.
The colours of a soap bubble come from:
Reflections off the top and bottom surfaces interfere, producing colours.
Why the colours change
- The extra path in the film is set by its thickness, so different thicknesses favour different colours.
- As a bubble's wall thins and swirls, the thickness changes and so do the colours.
- One wavelength cancels while another adds, so white light splits into shifting hues.
- Where the film is thinnest (about to pop), it can look black — all colours cancel.
Which property of the film decides which colours add or cancel?
The film's thickness sets the extra path, so it decides the interference for each colour.
Select all true statements about thin-film interference.
Thin-film colours come from two-surface interference set by thickness, and power anti-reflection coatings. No pigments involved.
Useful thin films
- Anti-reflection coatings on camera lenses and glasses use thin films to cancel reflected light.
- The coating is made exactly the right thickness to make reflections interfere destructively.
- That lets more light through the lens and cuts glare.
- The same physics that colours a bubble makes your glasses reflect less.
Bright or dark film?
A thin film looks bright or dark depending on how the reflected waves combine. Sort each case.
A soap film about to burst can look black because:
When very thin, the two reflections cancel for all colours, so little light reflects.
Lens coatings that use thin films to cancel reflected light are called ____-reflection coatings.
Anti-reflection coatings make reflections interfere destructively.
The colours of a soap bubble are not pigments — soap is colourless. They come from interference between light reflected off the two surfaces of the film. As the film's thickness changes, which colours cancel and which add changes too, so the colours swirl.
Soap-bubble colours are caused by pigments in the soap.
Soap is colourless; the colours come from thin-film interference.
Why does a soap film sometimes look black just before it bursts?
- The film has thinned to almost nothing, so the two reflections are essentially in step to cancel.
- Every visible colour interferes destructively, so little light reflects — it looks dark.
Thin-film interference colours soap bubbles and oil slicks: light reflecting off the film's two surfaces overlaps and interferes. The thickness decides which colours add or cancel, so the hues shift as the film changes. The same effect powers anti-reflection coatings on lenses.