The Science of Soap Bubbles

Have you ever noticed the swirling colours in a soap bubble? I wondered if it was possible to photograph them under the microscope using washing up liquid. I put a few drops of washing up liquid  and water on a watch glass and  added a drop or two of glycerine to make the bubbles last longer. I used a pipette to create various sized bubbles by blowing air through the mixture. I was able to create suitable sized bubbles about half inch across.

Using 60x magnification with the stereomicroscope I could see patterns starting to form after several seconds but the lighting was causing unwanted patterns in the picture. I needed a diffuser so I used a plastic diffuser plate from an old slide viewer and focused on the bright reflection of the light on the bubble surface. The resulting photos are shown here. The patterns are beautiful starting with pinks and greens and progressing to blues and yellows before the bubble pops.

These two images show a predominance of pinks and greens because the bubble film is relatively thick.

Here yellows and pale blues are starting to form as the bubble thickness decreases with time.

Now for the science.  A soap bubble consists of 3 layers. A top layer of soap molecules, a central layer of water molecules and an inner layer of soap molecules. When light strikes the outer surface of the bubble about 4% of the light is reflected back. More light is reflected back from the inner surface and because the thickness of the film is in the region of light wavelengths interference between the two can occur. The type of interference depends on the thickness of the bubble film and the particular wavelength of the light.  If two reflected waves line up (ie in phase) the interference is constructive. If they don't then the interference is destructive. So the beautiful colours you see in a soap bubble are intereference patterns.

Therefore, everywhere you see yellow, the film is just the right thickness to destructively remove the blue light waves. Where you see cyan, the red light has been destructively removed. And where you see magenta, the green light has been destructively removed. The progression from pinks and greens to blues and yellows occurs as the bubble thickness decreases with time.

This image was taken shortly before the bubble burst. Note the blues and yellows.

Just before the bubble bursts deep blues and yellows appear as the film gets thin.

A bubble planet.