Ask A Scientist© Physics Archive

name         Mary D.
status       educator
age          50s

Question -   How thick is a soap bubble?  Of course the thickness must
change; I understand that.  But when you see the colors so vividly, does
that mean you have thicknesses on the order of microns, and does the
black come when you are below .4 microns, the wavelength of blue?  Or
perhaps below .2 microns?  Or perhaps even much less?  When I consider
that the bubble needs only to be as thick as the length of a soap
molecule, it could actually be much less than 200 nm, perhaps as little
as 20 nm.  But I can not find the sizes of these small molecules or the
thicknesses of soap films.  This is for our study of magnitudes.

As the thickness of a soap bubble gets smaller than any wavelength of
visible light, its ability to reflect visible light decreases, and it
stops preferentially reflecting any particular wavelength of visible
light.  You see the bubble looking sort of gray in this case. The most
striking colors occur when the thickness of the film is greater than,
but still comparable to, the wavelengths present in visible light.

Tim Mooney
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The brilliant colors observed in soap bubbles occur because of
interference of light reflected from the outer and inner surfaces of the
bubble. So at that stage, the thickness of the bubble is the order of
magnitude of the wavelength of visible light.

In addition, there is a stage in the life of bubbles referred to as "black
bubbles". This occurs when the thickness of the bubble is less than the
wavelength of visible light, but still a coherent film. When that happens,
the bubble appears to disappear, but its existence is known because droplets
can be observed when the bubble pops.
Vince Calder
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