Thinning soap bubbles. Time-lapse macro photographs of equally-sized bubbles in a cylindrical glass tube. Gravitational liquid flow is made visible by soap bubble iridescence. Iridescence is the property of certain surfaces to 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, 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.