The soap solution experiences “surface tension” - an elastic tightening of its surface. The surface tension in plain water is just too strong for bubbles to last for any length of time, but soap decreases the pull of surface tension to about one-third of plain water. In a soap-and-water solution, some soap molecules find their way to the surface and squeeze their way between the surface water molecules, pushing their hydrophobic (grease) ends out of the water. This separates the water molecules from each other. When a soap solution has trapped some air to form a bubble, that solution can no longer shrink into a tiny droplet. The air keeps the bubble large and it can’t avoid having lots of molecules on its surface.

A soap bubble appears iridescent under white light, because light waves reflected from the front and back surfaces of the film travel different distances. This may cause destructive interference for some particular wavelength, and the hue or colour associated with that wavelength will be absent from the reflected light. If film thickness or direction of illumination changes, interference occurs at different wavelengths and the reflected light changes colour.

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Bernard Sadehanbi and Sheyda Saneinejad
St. Elizabeth Catholic High School, Richmond Hill, Ontario
1st Prize ( High School/CEGEP Class Project )

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