Where do the colours in soap bubbles come from?

The colours do not come from the soap bubble itself, but from the light reflected in the bubbles. Because white light actually consists of all colours, we can see all kinds of different colours if we make bubbles in the white light of the sun.

On the light table in the Bubblearium at Experimentarium, the light changes colour continuously, which is why the colours of the bubbles also change. If we only shine one colour, we will only be able to see that colour and only in some places on the bubble.

Perhaps the thinnest thing you can see

The colours are visible because the soap bubble is extremely thin. In fact, it is one of the thinnest things we can see with our eyes. When you blow a soap bubble, it will be about 100 times thinner than a strand of hair. But a soap bubble has different thicknesses in different places, which is why we see different colours.

They are visible due to a phenomenon called thin-film interference, which occurs because the thickness of the soap film (5-5,000 nanometres) is of the same order of magnitude as the wavelength of light (380-760 nanometres).

Where do the bands come from?

After a while, the colours of the soap bubble settle into beautiful horizontal bands. This happens because gravity pulls the water in the bubble downwards, making the bubble thicker at the bottom and thinner at the top. After a while, the bubble will have the same thickness (and the same colour) at a certain height above the tabletop. But when we move up and down on the bubble, the thickness will change.

The illustration below shows an illustration of a soap bubble cut in half, zoomed in significantly to show the thickness of the bubble. Notice how it gets thicker and thicker the further down the bubble you look.

In this video, you can learn more about light and colours in soap bubbles: