Thin-film interference – soap films, oil slicks and anti-reflection coatings

What it shows

When light meets a thin transparent film, part of it reflects from the top surface and part from the bottom surface. The second wave travels an extra distance 2nt inside the film. A reflection off a medium of higher refractive index also turns the wave over, a phase change of half a wavelength. If the two reflected waves meet in phase the film looks bright; if they are out of phase they cancel. Because the condition depends on wavelength, white light gives the shifting colours of soap bubbles and oil slicks, and a quarter-wave coating makes glasses reflect less.

How to use

Pick a Film: Soap, Oil on water or Coated lens. With Light set to One colour, move Thickness t and watch the Reflected waves inset and the R – t graph. Switch to White to see the Colour seen in reflection and the colour band under the graph. Change Film index n, press Thin the film to let it drain, and press Record to log rows.

Parameters you can change

• Film Soap film, Oil on water, Coated glass
• Light White light, One colour
• Wavelength 380–750 nm
• Refractive index of the film 1.2–2.4
• Film thickness 0–1000 nm
• Graph R against thickness, R against wavelength

Questions to explore

1. Why does a soap film show different colours where its thickness is different?
2. How thick should a coating with n = 1.38 be to cut the reflection of green light, and why?
3. Why does a soap film turn black just before it bursts?