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As light impinges on the film, some of it is [[Reflection (physics)|reflected]] off the outer surface while some of it enters the film and reemerges after being reflected back and forth between the two surfaces. The total reflection observed is determined by the interference of all these reflections. Since each traversal of the film incurs a [[phase shift]] proportional to the thickness of the film and inversely proportional to the wavelength, the result of the interference depends on these two quantities. Thus, at a given thickness, interference is constructive for some wavelengths and destructive for others, so that [[white#White light|white light]] impinging on the film is reflected with a [[hue]] that changes with thickness.
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Interference effects also depend upon the angle at which the light strikes the film, an effect called ''[[iridescence]]''. So, even if the wall of the bubble were of uniform thickness, one would still see variations of color due to curvature and/or movement. However, the thickness of the wall is continuously changing as gravity pulls the liquid downwards, so bands of colours that move downwards can usually also be observed.
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Soap films seek to minimise their surface area, that is, to minimise their surface energy. The optimum shape for an isolated bubble is thus a sphere. Many bubbles packed together in a foam have much more complicated shapes. See [[Weaire-Phelan structure]] for a discussion of this (called the [[Kelvin problem]]), and [[Plateau's laws]] for a discussion of the structure of the films.
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[[Image:Zubble.JPG|thumb|right|Zubbles]]
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These new bubble mixtures are currently only being sold in the [[USA]] under the trade name ''[[Zubbles]]''.
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