Some of the light falling onto a transparent film is reflected from the top surface. The unreflected light travels through the film until it meets the lower surface where again some is reflected. This light reflected from the bottom surface travels back and rejoins that reflected from the top surface.

Due to its journey within the film and through reflection from the bottom surface, this reflected light wave may have a different phase to the light reflected from the upper surface.

The extent of phase difference depends on the thickness and refractive index of the film, the angle at which the light strikes the film surface and the colour of the light.

If for a particular colour the two reflected rays are in phase with each other when they rejoin, then they will combine to produce a bright reflection (constructive interference). If they are out of phase, they will largely cancel each other and that colour will be weakly reflected (destructuve interference).

A stack of such films spaced by air layers will, when viewed in reflection using white light, give vivid iridescent colour. The wings of butterflies and moths have multilayer structures on their surfaces that act in this manner to give the observed iridescence.

Morpho butterfly wings

Left

Morpho butterfly multilayer structure.

Right

Schematic diagram showing how light rays reflect from a Morpho multilayer system.

Urania moth wings

Left

Urania moth multilayer structure.

Right

Schematic diagram showing how light rays reflect from an Urania multilayer system.

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