Green, blue, purple and gold: a fresh bubble is enchantingly beautiful, shining there in the sunlight. Like a pearl, but dynamic. With every breath of wind, the colors dance across the surface, like an oil layer on water. But then the colors go down. What remains is deep black: a harbinger of the inevitable snapping.
What causes those colors? And why are they sinking? We ask Huib Bakker, professor of physical chemistry at the UvA and director of Amolf, the institute for fundamental physics of complex matter. “The wall of a soap bubble consists of a shell of water, sandwiched between two layers of soap molecules,” says Bakker. “Each layer of soap is only a nanometer thick: a millionth of a millimeter. The water layer is thicker: several hundred nanometers.” That thickness is in the order of magnitude of the wavelength of visible light. Blue, green and red light have wavelengths of about 450, 550 and 650 nanometers, respectively.
We see things because light is reflected on them. The light reaches our eye through the object. However, a bubble does something special. “Light falling on the bubble is reflected from both the outside and the inside of the bubble,” says Bakker. “The light reflected from the inside travels a slightly longer distance on the way to our eye. This creates interference.”
Weaken or even extinguish
Interference is the interplay between waves. Waves can amplify each other if they act in the same direction at the same time, but also weaken or even cancel each other out if they work against each other. This applies to light waves, but also to water waves. Try it with a cork in a bath: if you let waves run into each other, you can make the cork bounce up and down more or less.
So it is with the light that falls on the bubble. That consists of all colors. But the water layer causes interference in all those colors in the reflected light. “Which colors are enhanced or weakened depends on the local thickness of the bubble wall,” explains Bakker. “For example, in a certain place the wall is so thick that blue light is amplified and red light is attenuated. In that spot you see the bubble blue. In another place the wall is a bit thicker and you can see the bubble red.”
This same phenomenon explains the colors of the pearl, of oil layers on water and of iridescent bird feathers and beetles. With soap bubbles, it also explains the dynamics of the colours. A gust of wind pushes dents into the wall, changing the interference and causing the colors to dance. And due to gravity, the water sinks down between the two layers of soap. Bakker: “This also changes the thickness of the wall and you will see the colors change.”
Read an episode of the Everyday Science section: Colored and colorless air bubbles
But why does the bubble appear black after all? If light reflects off the transition from air to water, a so-called phase jump occurs, explains Bakker. ‘Up’ becomes ‘down’, and vice versa, in the example of the cork. At the other transition, that from water to air, there is no phase jump. You will notice that as soon as the water layer becomes negligibly thin. Then the waves reflecting on the outside and on the inside begin to cancel each other out.
Bakker: “As the wall becomes thinner, the extinction becomes more and more effective and the reflected light becomes increasingly weak. That applies to all colours. Just before it bursts, the bubble is at most black.”
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