The light and colour of soap bubbles

Archana Ghode, IIT (Madras), Chennai

The fascination of bubbles lies in their myriad iridescent colours.  These colours do not come from the color of the soap or the bubble solution. They come from a property of the light that bounces off its surface. In particular, the thickness of the bubble wall determines the colour that you will see.

The colours are caused by what is called interference between light waves. You may have seen similar colour patterns when looking at some oil spilt on a wet road.

How does interference cause the beautiful colours? 

You may already know that ordinary sunlight looks white but is made up of many colours, from deep indigo and blue to orange and red. You may also have seen sunlight being broken into its colours by a prism.

When light hits a bubble surface, two things happen. It can be reflected off it, or it can pass through. Since the surface is so thin, very soon, the light that passes through hits the back surface of the bubble. At this point, it can also be reflected back. So there are two light rays that reach the eye: one reflected from the outer surface and the other reflected from the inner surface of the bubble. This is easily understood from the figure.

To understand interference, it is easiest to think of light as a wave rather than as a ray. The wave has ups and downs, called crests and troughs. The light wave is just like the ripple that is formed when a pool of water is struck by a stone. The separation between two crests (or two troughs) is different for different colours and is called the wavelength.

So when two different reflections reach the eye, what the eye sees depends on how the crests and troughs of the two waves match up against each other. If the thickness of the soap bubble is such that the reflected wave from the front and back surfaces are exactly an integer number of wavelengths apart, then the crests of the two waves will add.

The interference is called constructive and an enhanced light will reach the eye (See figure). If, on the other hand, the wall thickness is such that the crest of one wave falls on the trough of the other, the entire wave cancels out and no light reaches the eye. Since different colours have different wavelengths, it is clear that different colours are enhanced for different thicknesses.

So white light falling on the surface is reflected as a colour that changes with thickness. When the bubble is newly formed with a thick surface, red colour cancels out so the bubbles appear blue or blue-green. Due to evaporation, the surface starts to thin so one by one yellow, green and blue light is not reflected, and the bubble looks blue, magenta and a golden yellow respectively. 

Finally, when the bubble's wall becomes much thinner than the wavelength of visible light, all the waves in the visible region cancel each other and the bubble looks black. The bubble soon pops after this.

Can you have coloured bubbles

Adding coloured dye to bubble mixtures fails to produce coloured bubbles, because the dye attaches to the water molecules as opposed to the surfactant. Therefore, a colourless bubble forms with the dye falling to a point at the base. Dye chemist Dr. Ram Sabnis developed a lactone dye that sticks to the surfactants, thus enabling brightly coloured bubbles to be formed. An example of this dye is crystal violet lactone.