Soap Bubble Thin-Film Interference
Light reflecting from the inner and outer surfaces of a soap film travels different path lengths. When those paths differ by whole wavelengths, colors reinforce; when they differ by half wavelengths, colors cancel. As the bubble drains and thins, the interference bands shift — producing the swirling rainbow patterns we see in real soap bubbles.
About this lab
Thin-film interference occurs when light reflects from both surfaces of a thin transparent layer, such as a soap film, oil slick, or anti-reflective coating. The two reflected beams travel slightly different distances — the optical path difference is 2nt cos(theta), where n is the refractive index of the film, t is the thickness, and theta is the angle of refraction inside the film. Because different wavelengths of light experience constructive or destructive interference at different thicknesses, a thin film illuminated by white light displays vivid colors that depend on the local film thickness.
A soap bubble is a thin film of water (n approximately 1.33) with surfactant molecules at both surfaces. Gravity causes the film to drain downward, making it thinner at the top and thicker at the bottom. This thickness gradient creates horizontal bands of color. As draining continues, the top of the bubble becomes so thin that the optical path difference approaches zero for all visible wavelengths. At this point, a phase shift of pi upon reflection from the denser medium causes destructive interference across the entire visible spectrum, and the top of the bubble appears black — the so-called "black film" that often precedes a bubble popping.
The physics of reflection includes an important subtlety: light reflecting from a surface where it enters a denser medium (air to soap) undergoes a half-wavelength phase shift, while reflection from a surface where it enters a less dense medium (soap to air) does not. For a soap film surrounded by air, only the first surface introduces a phase shift, so the condition for constructive interference becomes 2nt cos(theta) = (m + 1/2) lambda, where m is an integer and lambda is the wavelength in vacuum. This simulation computes the reflected intensity for each wavelength of visible light and combines them using CIE color matching functions to produce the perceived color at every point across the film.