|Bubbles by Matthew Adam. See also books by John Adam (X and the City ,Mathematics in Nature).
©Matthew Adam, shown with permission
|Bubbles are fascinating. Old style geometric optics describes their reflections but we must use wave optics to explain their colours. Surface tension forces battle to keep bubbles envelopes spherical. To understand why they exist at all needs some attention as to why certain molecules stick together while rejecting others.
Reflections: Look carefully at the reflections. There are two images. The upper one is upright and the lower inverted. The bottom picture shows the upright image most clearly. The upright image is from the bubble's front surface that acts as a convex mirror. The inverted image is from the bubble's rear surface acting as a large concave mirror.
Interference colours: The vivid iridescent and ever changing colours are from thin-film interference seen because soap bubble films are a mere few wavelengths of light across.
Imagine skylight reaching the outer bubble surface. Some is directly reflected. Some enters the film and part of that is then reflected back by the inner film surface to leave the bubble in the same direction as the directly reflected waves.
The two outgoing wave trains overlap and interfere. In some directions - and for some colours - the wave crests correspond. That particular direction and colour will be bright. In other directions or colours the two outgoing waves might be out of phase and cancel. The rear bubble surface independently contributes a second set of interference colours.