What causes the iridescent sheen on a pool of petrol (gasoline)?

zbeckabee

This relates to water, however, it would most likely hold true for gas: When certain iron-containing minerals (for example hematite or limonite) are present in water that has a lot of organic matter in it, the minerals dissolve. However when the dissolved minerals come into contact with air at the surface of the water, they can react (oxidize) and mix with un-dissolved iron ions to form the sheen you see on the water. Iron oxide films are naturally occurring and generally do not indicate a problem, however the presence of an oil sheen on the water can indicate a leaking underground storage tank, or some other source of contamination that could be affecting other water sources in the area. One way to tell the difference is to take a stick and stir up the water. If the film breaks into pieces, then it is most likely an iron oxide film. If the film merely swirls around but remains intact, then the film is most likely caused by oil. http://www.fredericktownelabs.com/residentialfaq.htm Jul 22 08, 5:19 AM

Sevorak

This answer applies to water with a thin layer of oil/oil based substance floating on top, not a pool of just gasoline, but it may be the answer you're looking for. The sheen is cause by thin film interference. Incoming light first hits the thin film of oil, and some of the light is reflected, the rest is refracted and passes through the oil. The refracted light then hits the oil-water barrier, and some of this light is reflected, and the rest is refracted. This reflected light passes back through the oil, and most is refracted through the oil-air barrier. This reflected light is now parallel with the light that reflected off the oil first, but they are slightly out of phase with each other, so an interference pattern is formed, which causes all the pretty colors. The same process causes the iridescent colors of soap bubbles. http://physics.bu.edu/py106/notes/Thinfilm.html Jul 22 08, 3:11 PM