I am doing vision therapy for one of my kids, and one of the therapy exercises for this particular week is for child to wear an eye patch on one eye, and pop soap bubbles with a fingertip for 5 minutes, then switch eyes. Kid has virtually no depth perception, and has difficulty making eyes work together. In short, she generally sees double and can’t judge where things are in front of her - and that’ with her glasses ON. You can imagine how this wreaks havoc with her ability to read.
Anyway, I’ve noticed something as I blow innumerable bubbles for her every day:
When I dip the wand and draw it out, the first few bubbles are essentially ‘white’. Then they begin turning ‘yellow’ and ‘green’ and eventually mostly ‘blue’ on the surface. The last one, the one that breaks as I try to blow it, appears very blue indeed.
I can only assume that this has to do with the thickness of the soap film, which thins as more and more of the soap on the wand is used up making bubbles. I’m not clear on why thinner film translates into these different colors of the spectrum, but it’s consistent with every wandful.
The colors are caused by interference between light reflecting off the inner and outer surfaces of the bubble. As you correctly surmise, the color varies depending upon the thickness of the bubble film. This page explains the phenomenon nicely.
You’re seeing white light interference. The “white” bubbles are so thick that the path length twice through the bubble exceeds the coherence length of the light, and it can’t interfere with itself. As the bubble gets older, more of the soap is pulled to the bottom and the top gets thinner, and the light can interfere.
Your sequence of colors is off – thick films tend to be either bluish green or pinkish purple. These are the characteristic colors of white light interference (they’re the colors of thiick opil films and of supernumerary rainbows, for the same reason). Only when the film starts to get really thin, only a couple of wavelengths thick, will you see blue and yellow. You’ll never see pure red. Just before it breaks, it’ll go “dark”.
The first guy to work out the colors of white light interference was J.M. Pertner, back in the 19th century. There’s a cute chart, based on his work, at the beginning of C.V. Boys’ book on making soap bubbles (still in print, last I checked, from Dover Books).
