By Zencalcy Editorial TeamLast reviewed About us
Light reflecting from the top and bottom of a thin film can reinforce or cancel depending on the film thickness. That is why soap bubbles show coloured bands and why lens coatings are tuned to cancel reflections at one wavelength.
Film thickness for constructive or destructive reflection.
Constructive: 2 n t = (m + ½) λ Destructive: 2 n t = m λ
n is the film index, t its thickness, λ the vacuum wavelength and m a non-negative integer. These forms assume a film in air, with a half-wave phase shift at the top surface only.
| Symbol | Meaning |
|---|---|
| t | Film thickness. |
| n | Refractive index of the film. |
| λ | Wavelength in air. |
| m | Order, a non-negative integer. |
For constructive reflection at m = 0, t = 550 / (4 × 1.33) = 103.4 nm. Destructive reflection at m = 1 needs 206.8 nm.
Continue with the Thin Lens Equation Calculator, Angular Dispersion Calculator, or the Brewster Angle Calculator — or browse more optics and light tools and all physics calculators.
Thickness varies across the film, so different places satisfy constructive interference for different wavelengths. The reflected colour shifts as the film drains and thins.
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
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Content updated: September 2026