<P> A soap bubble is an extremely thin film of soapy water enclosing air that forms a hollow sphere with an iridescent surface . Soap bubbles usually last for only a few seconds before bursting, either on their own or on contact with another object . They are often used for children's enjoyment, but they are also used in artistic performances . Assembling several bubbles results in foam . </P> <P> When light shines onto a bubble it appears to change colour . Unlike those seen in a rainbow, which arise from differential refraction, the colours seen in a soap bubble arise from interference of light reflecting off the front and back surfaces of the thin soap film . Depending on the thickness of the film, different colours interfere constructively and destructively . </P> <P> Soap bubbles are physical examples of the complex mathematical problem of minimal surface . They will assume the shape of least surface area possible containing a given volume . A true minimal surface is more properly illustrated by a soap film, which has equal pressure on inside as outside, hence is a surface with zero mean curvature . A soap bubble is a closed soap film: due to the difference in outside and inside pressure, it is a surface of constant mean curvature . </P>

Colours seen in soap bubbles are due to