Acoustic properties of soap films. Soap film placed in front of a loud speaker whilst Roxanne by the Police is playing. The film vibrates in response to the propagation and frequencies of emitted sound waves. The colours correspond to variations in film thickness as the liquid drains, due to gravity. Iridescence is a property where certain surfaces change colour depending on the angle of light refraction or reflection, or the angle of view. The colours are due to interference of light waves reflected from the upper and lower surfaces of the soap bubble film. The interference patterns appear as bands of colour, related to the thickness of the film. Thicker walls cause interference at longer (redder) wavelengths, so appear bluer and greener. As the film thins by evaporation and draining, the reflected light cancels out yellow then green then blue light, appearing blue, magenta and golden yellow, before becoming too thin to support itself, and popping.