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The answer lies in an optical physics text. Do a google search for something called "Thin Film Interference." It's the same phenomenon that causes a CD to show raindow colors.
Here's a quip I've found about it:
"In the case of thin films, as light strikes a surface, a portion of the energy re-emerges as a reflected wave while the rest is transmitted through. The energy refracted will experience the same process where part of it is reflected, while the other is transmitted. Thin film will divide the incoming wave into 3 sections: reflected from top surface, from the bottom, and transmitted. The film shears the entire wave front, with these two reflected waves going in the direction of the viewers' retina, they can overlap at a point in the eye. The same thing would happen if this situation involved a thick block of glass, but there is a crucial a crucial difference. If the film is thin in comparison to the coherence length of the light traversing it, the two waves will be correlated; that is, they will be fairly coherent and capable of interfering in a sustained fashion. Because they travel different routes, depending on the film's thickness, they will ultimately interfere in some way that depends on that thickness.
Take for example, a thin film of oil residing on a small body of water, where the dark surface beneath absorbs any light. When the film is at the right thickness, it can produce a color by emerging waves going through constructive interference. Thus, there will be spots of different color on the oil film."
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