Filmsinterferens

Filmsinterferens, or thin-film interference, is the optical phenomenon in which light waves reflected from the two boundaries of a thin film interfere with each other. The result is patterns of bright and dark fringes or, more commonly, vivid colors that depend on the film’s thickness, refractive index, the light’s wavelength, and the angle of observation.

Mechanism: When light encounters a thin film, part is reflected at the top surface and part enters the film, reflecting off the bottom surface and then emerging back into the surrounding medium. If the two reflected waves differ in phase by an integer multiple of 2π, they reinforce each other; if by an odd multiple of π, they cancel. A phase shift of π often occurs on reflection from a boundary with higher refractive index, which shifts the interference condition relative to the film’s thickness.

Mathematical note: For normal incidence, a common simplified condition is 2 n t ≈ (m + 1/2) λ for constructive interference in reflection when there is a single phase reversal, and 2 n t ≈ m λ for destructive interference in reflection. Here n is the film’s refractive index, t its thickness, and λ the wavelength in vacuum. Real systems may require angle-dependent formulas and multiple interfaces.

Occurrences and applications: The colorful appearance of oil films on water or soap bubbles is a familiar example. Thin-film interference is exploited in anti-reflective coatings on lenses and solar cells, in decorative finishes, and in optical devices such as Fabry-Pérot etalons and CD/DVD reflectivity structures.

Limitations and notes: The observed colors depend on viewing angle and lighting, making the effect often iridescent and variable. Multiple layers can produce complex spectra.