Thin-Film Interference and Iridescent Colors

What you are seeing: reflectance of a thin oil-like film (n = 1.33) on a glass substrate (n = 1.5) viewed at normal incidence by white light. The film causes wavelength-selective constructive and destructive interference between the front and back reflections. The bottom strip shows the reflected spectrum (sum of reflectance times blackbody-ish illuminant) translated to RGB. The top curve shows $R(\lambda)$ for the current thickness.

Move the thickness slider and watch the reflected color cycle through blue, green, yellow, red as the constructive-interference wavelength $\lambda = 2 n_{\text{film}} d / m$ sweeps across the visible spectrum. This is why oil slicks, soap bubbles, and butterfly wings show iridescent color: even a slight thickness gradient produces a rainbow of reflected hues.

Figure 1. Thin-film interference. Method: Airy formula for two-interface reflectance at normal incidence; spectrum mapped to RGB.

d (nm)350

n_film1.33

speed2

WHAT TO TRY

Vary the film thickness d: the reflected colour cycles through the spectrum as different wavelengths reach constructive interference, the iridescence of soap bubbles and oil slicks.
The extra half-wave on reflection off the denser glass flips the condition, so a vanishingly thin film looks dark, not bright, the two reflections cancelling.
Change the film index: the optical path 2 n d sets which wavelength is enhanced, the principle behind anti-reflection coatings tuned to cancel green light.