Thin Film Interference Factors
What variables can you use to change the light fringes in
a soap bubble?
The light fringes are caused by variations in the thickness of the bubble
film. So in principle, anything that causes the thickness to change will
alter the fringes. The concentration of soap is an obvious factor. In
addition, there are other ingredients such as glycerin that slow the
evaporation of water from the film will also be a factor. The temperature
are relative humidity both affect the evaporation of water and so will
alter the fringe behavior. When the wall gets thin enough the colors will
appear to disappear, just before the bubble pops. The wall has not really
disappeared, it is just too thin to cause interference of visible light. If
you view the bubble with light of a single wavelength, like a laser
pointer, you will not see colored fringes, but you should be able to see
ripples as the monochromatic light constructively and destructively
interferes. I have to admit I'm anticipating this result, I have not
actually tried it, but it would be an interesting "experiment".
There are always two variables there:
1) the thickness of the walls of your bubble
2) the refractive index of the bubble-walls
2a) the ratio of water-to-other stuff in the bubble-walls
2b) the refractive index of that "other stuff"
(1) and (2) depend on what you do, and on time after blowing the bubble.
It is possible to blow a soap bubble that is dripping from its underside,
even while it floats away in the breeze.
This bubble will have thicker walls than the last bubble blown off a ring
that is almost starved of soapy water.
The humidity of the air you are working in probably matters in the
evolution of your bubble.
As a bubble flies away, it is also drying up and getting thinner walls.
The water evaporates, but the soap cannot.
A bubble made of 99% water /1% soap will get very thin indeed before it pops.
I think I recall some very thin bubbles looking colorless, almost
whitish. But I am not sure.
A bubble made from 70% water / 25% glycerin / 5% soap cannot thin out very
Because pure glycerin grabs water from the air, it cannot even dry all
It will dry to a certain point, then just sit there with fringes
playing across its surface.
Color-fringes require the index of refraction of the bubble wall to be
different than air.
The larger that difference, the stronger the colors.
Water has almost the lowest refractive index you can find, 1.33.
Air's index is 1.0, so that is 0.33 above air.
Glycerin's index is 1.40, substantially better.
Soap itself is probably similar to glycerin.
So having a very thick bubble-soap solution probably helps.
Glycerin makes water more viscous, which makes the bubbles you blow from
it a little thicker.
Bubbles which are much too thick-walled will have almost no color,
or perhaps many very narrow stripes that move too fast and do not show
up very much.
Bubbles which are thinner than about 0.1 micron should have no fringes at all.
Bubbles which are almost that thin will have only about one faint color
or at least the fringes will be very few and very broad.
There you have it. With those ideas you can find the best.
Definitely get some glycerin-containing bubble solution.
I think the large jugs of Toys-R-Us bubble solution have enough.
If you put one drop on a plate and let it dry one day in a sunny
windowsill, most of the water is gone.
What's left is mostly glycerin and soap.
If it seems like the whole drop is still there, that solution had a high
percentage of glycerin in it.
Weighing a known volume is also good. Glycerin's density is 1.26gm/cm3,
vs. water's 1.00.
Sugar should be somewhat similar to glycerin. You could try it.
It is water-soluble, it makes water more viscous, and it's index is 1.42.
However, because it makes solid crystals, it might help your bubble pop
I suspect sugar will be least detrimental when the humidity is high.
There are a couple of water-soluble plastics which I suppose may have less
of this problem.
(poly-vinyl-alcohol (PVA), poly-vinyl-pyrolidone(PVP)).
They dry to solids, but the solids are non-crystalline,
more likely to make a clear, flexible bubble-skin than a prickly-pointed
crystal. I hope.
PVP's refractive index is even higher than glycerin or sugar.
A bit of the clear-gel school-glue might have a good effect too. Not sure
how much dissolves in water.
Click here to return to the Physics Archives
Update: June 2012