This photograph is an example of thin-film interference. White light reflecting off soap bubbles displays a swirling pattern of colours. A soap bubble is an air-filled sphere surrounded by an extremely thin film made up of a sheet of water encompassed by two layers of soap molecules. Light behaves as a wave, much like ripples from a stone thrown into a pond, and different colours of light have different wavelengths. White light is a complete mixture of all the wavelengths of the visible spectrum at equal intensity. When white light shines onto the soap bubbles, the light reflected from the outer soap layer interferes with the light reflected from the inner soap layer. Depending on the thickness of the film and the angle of light, the reflected light waves can interfere destructively or constructively with each other, causing the iridescent colours on the soap bubbles’ surfaces.