The film's colours have the hues and complexity of those seen in a cloud corona. Both are light wave interference effects but there is a major difference.

A corona�s (or a glory�s) colours arise from interference between light waves scattered by single droplets.

The colours of soap bubbles or oil films on puddles are structural colour effects arising from reflection, refraction and interference of light waves as they pass through or interact with bulk matter. The other source of bulk colour is of course that produced by the selective absorption of pigments.

The colours of bubbles are from thin film interference, the simplest type of structural colour. More complex types: multilayer interference, diffraction grating effects ,light scattering and photonic crystal scattering give us the beauty of some butterfly wings, the iridescent panoply of a peacock�s display, the flashes of an opal and the shimmering scales of fishes. Expect OPODs about them.

When light strikes a thin film, some is reflected from the front surface. Some enters the film, reflects off the rear surface and leaves in the same direction as the directly reflected wave. When the film is thin enough the two outgoing waves combine. If their wave crests (shown red for positive and blue for negative at left) coincide then there is light. When they are out of phase there is destructive interference and there is darkness.

Whether there is light or darkness depends on the viewing direction and the thickness and optical properties of the film. We see the colours of soap bubbles changing before our eyes as the film drains or evaporates and alters in thickness.

Films that are thicker or thinner at their centre show coloured rings.

Are Frank Le Blancq's films thicker at the edge or centre?