The Royal Institution Demo Team has released a demonstration explaining how light interference creates the colorful swirls seen on soap bubbles [1].
Understanding this phenomenon clarifies the fundamental physics of light and matter. It demonstrates how the thickness of a transparent layer can change the visible properties of an object, a principle used in various industrial and scientific applications.
The colors appear due to a process called thin-film interference [1]. When light hits a soap bubble, it reflects off both the outer surface and the inner surface of the thin soap film [1]. Because the film is extremely thin, these two reflections interfere with one another [1].
Different thicknesses of the soap film cause constructive and destructive interference of various wavelengths [1, 2]. This process filters the light, producing the shifting rainbow patterns that move across the surface of the bubble [1, 2]. As the film drains or stretches, its thickness changes, which alters the colors observed by the viewer [1].
This mechanism differs from why bulk soap foam appears white [2]. While a single bubble shows distinct colors, foam consists of many tiny bubbles clustered together [2]. These numerous surfaces scatter all wavelengths of light simultaneously [2]. This collective scattering results in a white appearance, rather than the distinct rainbow swirls seen on individual bubbles [2].
The demonstration highlights the interaction between light waves and the physical structure of the soap layer [1]. By visualizing these changes, the team illustrates how wavelength interference determines the perceived color of a surface [1].
“Colourful swirls on soap bubbles are caused by light interfering with itself inside the thin soap film”
This explanation bridges the gap between a common household observation and the complex physics of wave optics. By distinguishing between the interference patterns of a single thin film and the scattering effects of bulk foam, the demonstration provides a clear example of how structural geometry affects light perception.
