Thin-Film Interference and Iridescence

When light hits a thin transparent film, part of it reflects off the top surface and part travels through and reflects off the bottom surface. Those two reflected waves then overlap. Because the bottom wave travelled a little farther (twice through the film), the two are shifted in phase. Where the shift lines crests up with crests, that wavelength is reinforced (constructive interference); where crest meets trough, it cancels (destructive).

The catch: the phase shift depends on wavelength. So for a given film thickness, some colors are amplified and others erased - and the reflected light comes back tinted. No molecule absorbed anything; the color is manufactured purely by the geometry of two surfaces a few hundred nanometres apart.

Vary the film thickness and you sweep through the whole interference sequence. A vanishingly thin film reflects almost nothing - the famous black film a soap bubble shows just before it pops. Thicken it and color climbs through silver, gold, magenta, blue, green, and then repeats in ever-paler "higher orders." Drag the thickness and watch the reflected color and its per-wavelength reflectance.

Iridescence is defined by one thing: the color changes with viewing angle. The reason is geometric - when you look at the film at a slant, the light's path through it is different, so the effective optical thickness changes (it scales with the cosine of the angle inside the film). As you tilt to a steeper angle, the constructive wavelength shifts shorter - toward blue. Hold the thickness fixed and change the angle.

On a real bubble the film is not one thickness - gravity drains it, so it is thinnest at the top and thickest at the bottom, and that gradient of thickness becomes a gradient of color: stacked bands that drift downward as the film thins, with a dark patch creeping in at the top just before it bursts. Render a draining film and watch the bands move.

Thin-film interference is one kind of structural color - color from physical microstructure rather than absorbing chemicals. The simplest diagnostic: tilt it. A pigment looks the same from every angle; a structural color shifts. Compare a pigment swatch against an interference swatch as you change the angle.

Six questions on interference, thickness, angle, and structural color. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.

Interference is one of the deep ways color is made. These articles cover the colorants, the nature, the light, and the other structural effects around it.