Bubbles

An image by Brian Compton.

Bubbles enchant the young and the young in spirit. They soar aloft to dance on zephyrs. They shimmer with ever shifting iridescent shades. They reflect curiously the surrounding scene. We wish them life but know they will pop.

The upper half of this bubble reflects the landscape but the lower part is not the scene’s mirror reflection in a lake. Instead it is a second completely inverted image of the same view.


©Brian Compton, shown with photographer's and parental permission.
Interference

The bubble skin is only a few wavelengths of light thick.   Light reflects to us from the front and back film surfaces.  

The two sets of reflected waves combine.   For some colours and directions the waves are in phase and we see that colour.   Other colours are out of phase and are not seen.

As the film drains and evaporates the thickness decreases and the colours change.

When a black region appears it is not a hole. However, it signals that the film is less than a wavelength thick and will soon pop. The film is so thin that there is almost no phase difference - but the 180° phase shift at the front reflection renders them out of phase and gives blackness.

Reflections

Two sides to the bubble, two reflections.

The more distant side acts as a concave mirror. It focusses the scene into an inverted real image (put a screen there and it would be visible) inside the bubble.

The near face is a convex mirror like a car's wing mirror.  Rays reflected from it appear to come from a virtual erect image.

Atmospheric
Optics
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