Soap Bubbles ~ Imaged by astrophotographer Göran Strand (web,blog).

Each bubble reflects the surrounding scene twice, once upright and another inverted. All are suffused by shimmering colours.

©Göran Strand, shown with permission
The contraction force of surface tension increases the pressure inside the bubble. The smaller the bubble the greater the excess pressure. How then can bubbles ever form when a liquid boils? The pressure inside a zero radius bubble would be infinite!

The difference in excess pressure with size show when two bubble intersect. More or less equal sized bubbles - as in Göran's image have a near flat common surface.

A small-large bubble union has a common surface bulging into the larger of the bubbles.

Foams get really complicated. Each surface pulls to minimise its energy.

Calculating the structure of a foam that has the lowest energy is no trivial matter. Lord Kelvin in 1887 tackled the problem and considered a foam of equal volume cells. He derived an ideal low energy structure of packed 14-sided polyhedra, tetrakaidecahedra.

There the matter stood until 1994 when Denis Weaire and Robert Phelan at Trinity College, Dublin derived a less regular structure of 12 and 14 sided polyhedra that had an energy 0.3% less than that of a Kelvin foam.

Is a
Weaire‑Phelan foam the least energy? We do not know.

For a really fascinating account of foam physics try this video.

Interference - The colours come from overlap and interference between light waves reflected from the closely spaced surfaces of the bubble skin. The skin is only a few wavelengths of light or less in thickness. In a particular direction, rays of one colour might be in-phase and bright while other colour waves are less in phase and dimmer. The interference colours shift and change with viewing direction and as the bubble skin evaporates and thins.

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Small bubbles are spherical because their skins "act" like sheets of elastic that try to reach a minimum energy configuration where the stretching is least - a sphere. Surface tension is the true agent, the unbalanced attraction at the surface between soap or water molecules.

Reflections - The two reflections arise from the bubble back and front sides.

The front presents a convex surface to the scene. Think of it as similar to the convex wing mirror on a car. In it the image is erect. It is a virtual image that cannot be captured anywhere on a screen.

The back surface is concave to the scene and reflects it to form a real but inverted image within the bubble