Soap bubble iridescence. 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.