Physics
What makes soap bubbles show a rainbow of colors?
Quick fact
The iridescent colors come from thin-film interference: light reflecting from the front and back surfaces of the soap film overlaps, and some wavelengths cancel out while others amplify, producing vibrant hues.
Why this is interesting
A soap bubble's colors aren't from pigment—they're created by light waves interfering with each other as they reflect off the soap film's two surfaces, creating colors that change as the film thins.
Read the full explanation
Understanding What makes soap bubbles show a rainbow of colors?
When light hits the film, some reflects from the top layer and some from the bottom. The difference in path length creates constructive interference for certain colors and destructive for others, depending on the film's thickness.
A deeper explanation
As water evaporates, the film's thickness changes, so the colors shift—explaining why bubbles are constantly swirling with color. The sequence of colors follows a predictable pattern: from green-yellow to magenta to blue-green as the film thins to just a few hundred nanometers (about 1/1000th of a human hair). Eventually, the film becomes so thin (less than 25 nm) that it appears black—the 'black film' stage—before it pops. This is the same physics that creates the colors on oil slicks and the wings of butterflies, proving that nature's most vivid colors often come not from dyes, but from light playing tricks on microscopic structures.