Depth 1
How soap bubbles make rainbows without rain
Soap bubbles do not actually contain any paint, yet they swirl with brilliant reds, greens, and blues. This happens because a bubble wall is unimaginably thin, sometimes only a few hundred nanometers across. When white sunlight hits the outer film, some light bounces right off, while the rest plunges inside and bounces off the inner film. As these two light waves race back out, they crash into each other. Depending on the exact thickness of the soap film, certain colors cancel each other out while others team up and glow. Because gravity constantly pulls the soapy water downward, the film thins out like a wedge, creating those moving rainbow bands right before it pops.