How it works
Film thickness changes across the bubble as liquid drains and air moves. Different thicknesses strengthen different wavelengths, creating bands that move over time.
What people often miss
The colors are not pigments in the soap. A very dark area can mean the film has become extremely thin and is close to bursting.
Why this question comes up
A bubble turns microscopic changes in film thickness into a visible color map.
Key points
- Soap bubbles show shifting colors because light reflecting from the front and back surfaces of the thin film interferes constructively and destructively.
- Film thickness changes across the bubble as liquid drains and air moves. Different thicknesses strengthen different wavelengths, creating bands that move over time.
- The colors are not pigments in the soap. A very dark area can mean the film has become extremely thin and is close to bursting.
Bottom line
Soap bubbles show shifting colors because light reflecting from the front and back surfaces of the thin film interferes constructively and destructively.
Source and verification
This Answer was checked against Exploratorium. Follow the source for the full official explanation and any later updates.