Thin Film Interference Calculator

About this calculator

When light hits a thin transparent film — a soap bubble, an oil slick, an anti-reflective lens coating — part reflects off the top surface and part off the bottom, and the two reflected waves interfere. Whether that interference is constructive (bright color) or destructive (dark) depends on the film's thickness relative to the wavelength, following 2nt = (m + ½)λ for constructive reflection (with corrections for phase shifts at each interface), where n is the film's refractive index and t its thickness.

This is the physics behind the shifting rainbow colors on soap bubbles and oil on wet pavement — different thicknesses reinforce different wavelengths. Optical engineers use the same equations deliberately when designing anti-reflective and dichroic coatings, and materials scientists use thin-film interference (via techniques like ellipsometry) to measure coating thickness non-destructively.