What causes the phase shift in thin film interference?

When light reflects from a boundary going from a lower to a higher refractive index medium, it picks up a half-wavelength (π) phase shift. This calculator accounts for phase shifts at both surfaces automatically based on the refractive indices you enter.

What is the constructive interference condition for reflected light?

For a film with phase shifts at one surface (e.g. air–film–air with n_film > n_air), constructive reflection occurs when 2 n t = (m + ½) λ, and destructive when 2 n t = m λ. The calculator handles all four cases depending on how many phase shifts occur.

Why does an oil slick show different colors?

Oil film thickness varies across the surface, so different thicknesses cancel different wavelengths via destructive interference. The remaining wavelengths add constructively and appear as vivid colors.

What is a quarter-wave anti-reflection coating?

A film of thickness t = λ / (4 n) causes destructive interference for reflected light, minimizing reflectance. Lens coatings use this principle — the green-tinted look of camera lenses comes from a coating tuned for green light.

Does this assume normal incidence?

Yes. The formulas used here assume light strikes the film perpendicular to its surface. At oblique angles the effective path length is longer (replace t with t·cos θ_r), which shifts the interference condition.

Thin Film Interference Calculator

Name: Thin Film Interference Calculator
Availability: InStock
Author: Nham Vu

Enter film thickness, refractive index, and wavelength to find whether reflected or transmitted light undergoes constructive or destructive interference.

Film Parameters

Film Thickness (t) nm

Soap film: ~100–1000 nm; AR coating: ~100–200 nm

Film Refractive Index (n_film)

Glass ≈ 1.5 · Water ≈ 1.33 · MgF₂ ≈ 1.38

Surrounding Medium (n_medium)

Air/vacuum = 1.0 · Same on both sides assumed

Wavelength (λ) nm

Visible: 380–700 nm (green ≈ 550 nm)

Constructive Wavelengths for Reflected Light (orders m = 0–5)

Order (m)	λ constructive (nm)	λ destructive (nm)	Visible?

Enter film parameters above and click Calculate.

Summary

Enter film thickness, refractive index, and wavelength to find whether reflected or transmitted light undergoes constructive or destructive interference.

How it works

Enter the film thickness in nanometers.
Enter the refractive index of the film (e.g. 1.5 for glass, 1.33 for water).
Enter the wavelength of incident light in nanometers.
Select the surrounding medium (air/vacuum or a custom refractive index).
Click Calculate to see whether reflected and transmitted light interfere constructively or destructively, along with the nearest constructive and destructive wavelengths.

Use cases

Predict the colors seen on soap bubbles and oil slicks.
Design anti-reflection coatings for camera lenses and solar panels.
Analyze thin-film optical filters and beamsplitters.
Solve optics homework problems involving phase shifts at boundaries.
Determine the minimum film thickness for a quarter-wave anti-reflection coating.
Explore how iridescence works in butterfly wings and beetle shells.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu