Soap water has no color, but soap bubbles shine with rainbow colors. Why? The skin of a bubble is very thin, much thinner than a hair. Light bounces off the outside and the inside of this skin.

The two lights meet, and some colors become stronger. The colors change as the skin gets thinner. When the top looks dark, the bubble will soon pop.

Although soap solution is clear, soap bubbles show swirling rainbow colors. The film of a bubble is only a few hundred nanometers thick, hundreds of times thinner than a hair. When light hits the film, some of it reflects from the outer surface and some from the inner surface.

Because the second light travels a little farther, the two light waves may line up and become stronger, or cancel each other out. Which color becomes stronger depends on the thickness of the film. The colors move because liquid flows down and water evaporates, constantly changing the thickness. Colors like this, made by structure instead of pigment, are called structural colors.

Soap bubbles contain no pigment, yet they glow with shifting colors. The explanation lies in thin-film interference. Light hitting a film only a few hundred nanometers thick is partly reflected from the outer surface and partly from the inner one; the second ray travels an extra distance, made effectively longer because light slows down in water. Depending on the film's thickness, the two reflected waves reinforce or cancel particular wavelengths, and when one color is removed, we see its complement, producing the characteristic magenta, yellow, and cyan hues.

Isaac Newton carefully recorded these colors in his 1704 work Opticks but, viewing light as particles, explained them with his theory of "fits"; Thomas Young later accounted for them in 1802 by treating light as a wave. Because liquid drains downward and evaporates, the film's thickness keeps changing, so the colors swirl. As the top thins to tens of nanometers, it turns dark: light reflecting from the outer surface undergoes a half-wave shift that the inner reflection does not, so when the path difference nearly vanishes, the two waves cancel. A dark spot therefore warns that the bubble is about to burst.