|Oily colours – Colours in an oily puddle captured in Turkey by Rasid Tugral. ©Rasid Tugral, shown with permission.|
Thin film interference produces the kaleidoscopic colours.
Some skylight is reflected back upwards from the oil film's top surface. The remainder enters the oil film and a fraction of that is reflected back up from the oil/water or oil/ground interface.
Some of it emerges in the same direction as that reflected from the upper surface.
The two outgoing waves combine. If their wave crests (shown red for positive and blue for negative at left) coincide we see a bright reflection. When the waves are out of phase there is destructive interference and no reflection is seen.
The relative phase of the two waves depends on the viewing angle, the film thickness and the oil refractive index.
More importantly, the phase is also wavelength dependent. This gives us the coloured reflections.
The wavy and quickly changing colours suggest that the oil film was uneven.
Oil film and soap bubble colours look similar to those of iridescent clouds. Both are produced by interference between light waves but there the similarity ends for cloud colours arise from scattering by individual droplets whereas film colours depend on interactions within the macroscopic film.
| Waves reflected from the oil/air and oil/water junctions combine and interfere. In-phase waves give light. The phase condition is wavelength and angle dependent. Hence we see different coloured reflections.