A film of soap dance with music

Whirlwinds, waves, undulations ... We would say almost that this film of soap vibrates of feelings under the influence of the acoustic waves! A series of videos realized by researchers of the CNRS combines physical appearance and music, light and sound.

The device is simple, as illustrates this video. A film is created at the end of a tube of Plexiglas plunged a few moments into a bath of soapy water. On the other side, instead of blowing to form a bubble, we place a loudspeaker which emits some music. A camera records the movie of soap enlightened by a diffuse white light.

Seeing this video we can ask 2 questions. Why there are colors in the film of soap and why the music moves these colors?
We'll try to answer to these two questions using drawings and a little of theory.

Why there are colors in the film of soap ?

We see colors because the film of soap is enlightened by a diffuse white light and a part of this light is reflected by the film.

Both beams which stand out from this blade of water did not travel completely the same way, because one of them made a distance in the film of soap, which is not the same material as the air which surrounds him. We say that they are out of phase.
A beam arrives, several leave.

It is necessary to remember that every color is a wave which moves, every tint is characterized by the distance between two consecutive crests (wavelength: l). The short wavelengths are blue, and the biggest are red. So, 2 beams going out of the blade can be out of phase as on the following plan:

If they had gone out as bellow (change of thickness of the film, or the slope of the beam), they would have cancelled their effects ( destructive interferences), the observer would not have seen red.

If, finally, they go out as bellow ( constructive interferences), they accumulate their effects, the red is very visible.

Why the music moves these colors?

Sound is also a wave and he progresses by moving air molecules as shows the following video :

So, molecules near the film of soap are moved by the loudspeaker and they hurt the film. This has the consequence to change the thickness of the film in some areas and this change interferences and thus colors.

sources :
https://www.science-et-vie.com/video/regardez-danser-un-film-de-savon-au-rythme-de-la-musique-8977
http://philippe.boeuf.pagesperso-orange.fr/robert/physique/bullesavon.htm

Don't hesitate to correct me if I make mistakes in English!

@fibonacco