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Thin Film Interference Calculator

Enter the wavelength, refractive index and order to get the film thickness.

How it is calculated

The tool uses t = (m + ½) × λ / (2n), the condition for constructive reflection when exactly one of the two reflections flips the phase of the light (for instance a soap film or water film in air). Here λ is the wavelength of the light in a vacuum or air, in nanometres, n is the refractive index of the film, and m is the order, a whole number starting at 0. If you leave the order blank it is taken as 0, which gives the thinnest film. The wavelength and index must be above zero and the order cannot be negative. The result is a thickness in nm, so thicker solutions for m = 1, 2, … repeat every λ / (2n). The tool does not handle the case where both or neither reflection flips phase, such as an anti-reflective coating on glass, where the half-wave term differs.

Worked example

Green light, λ = 550 nm, in a film of refractive index n = 1.33 (water), order m = 1. Thickness = (1 + 0.5) × 550 / (2 × 1.33) = 825 / 2.66 = 310.15 nm. With m = 0 the thinnest film is 103.38 nm, and each further order adds 550 / 2.66, about 206.8 nm. A film of 310.15 nm therefore reflects 550 nm light strongly, while other thicknesses favour other colours, producing the bands seen on a bubble.

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Common mistakes

  • Entering the wavelength in metres or micrometres. The field is in nanometres, so 550 nm, not 0.55.
  • Entering the wavelength inside the film. The formula expects the vacuum or air wavelength and divides by n itself.
  • Using the formula for an anti-reflective coating on glass. There both reflections flip phase, and the condition for reflection is different, so this result would be wrong.
  • Forgetting that more than one thickness works. Each order m gives another valid thickness, so choose the order that matches your film.
  • Treating the result as the only visible colour. White light contains many wavelengths and each one has its own set of thicknesses.

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.

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