Look closely at the bubble.  Optically, there is a lot going on.    Complex iridescent colours produced by thin-film interference swirl over its surface.   The micron thin bubble walls mirror the surroundings.   The upper part of the bubble has an upright image of sun and dark landscape.     The same scene is upside-down in the bubble's lower section.   There are other, fainter, images showing at least two more suns.  


Reflections:
The lower inverted image is from the bubble's rear surface acting as a large concave mirror. The image is real in that it would be visible on a screen if it could be held inside the bubble between its centre and back surface.

The upper upright image is from the bubble's front acting as a convex mirror.    The reflected rays appear to come from an image of the sun and landscape placed between the bubble centre and its front surface. This image is virtual and cannot be captured on a screen.

Extra images originate from light reflected back and forth between the back and front surfaces. They are faint because of light loss at each reflection. Only the bright sun reveals their presence.



Interference colours:
Geometric optics explains the reflections. The colours can only be accounted for by invoking the wave nature of light. They are from thin-film interference, seen because soap bubble films are a mere few wavelengths of light across.

Imagine (lower right) 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 (wavelengths) - the wave crests correspond. The waves are in-phase as shown at right. That particular direction and colour will be bright.   In other directions or colours the two outgoing waves might be out of phase and cancel. There are actually multiple reflections and outgoing waves but the physics is the same.

The rear bubble surface contributes a second set of interference colours.

The overall result is a fascinating patchwork of iridescent colours shifting and changing as the soap film drains and thins by evaporation.



Why are large bubbles wobbly and non-spherical?
Water molecules attract one another. At a surface these intermolecular forces are unbalanced with the net effect that the surface acts as though it is stretched and under tension.

The minimum free energy condition is when the surface has the smallest area - a sphere.   Small bubbles are held spherical by this surface tension force and it also slightly increases their internal air pressure.

The small forces and internal pressure are inadequate to maintain the sphericity of large bubbles. They wobble and are easily distorted in air currents.



Blow your own bubbles?
Recipes here.