Bubbla

Meet Bubbla, a soap film whose colours come from light interfering with itself.

When light hits a thin film, some reflects from the front surface and some from the back. The two reflected waves overlap. The back-surface wave travels an extra distance 2d inside the film, which counts as 2nd once the film's index is included. Reflection from the front of a soap film (air to soap) flips the wave's phase by half a cycle, while reflection from the back (soap to air) does not. So the reflection is brightest when 2nd = (m + ½)λ at normal incidence.

Because the condition depends on wavelength, a film of a given thickness reflects some colours strongly and others weakly. As the film drains and thins unevenly, the bands of colour swirl and shift.

The same physics colours oil slicks and is used deliberately in anti-reflection coatings on glasses and camera lenses.

Where a soap film becomes much thinner than a wavelength, all colours cancel in reflection and it looks black just before it pops. The formula here assumes light arriving straight on.