Why can you see changing colors in an oil sick on the road

Why can you see changing colors in an oil sick on the road? Also, what can the color you see tell you about the thickness of the oil slick?

Why can you see changing colors in an oil sick on the road? Also, what can the color you see tell you about the thickness of the oil slick?

http://physics.bu.edu/~duffy/py106/Diffr…
There is good explanation at the web site above.
Example - a film of oil on water

http://en.wikipedia.org/wiki/Thin-film_i…

In the case of a thin oil film, a layer of oil sits atop a layer of water. The oil may have an index of refraction near 1.5 and the water has an index of 1.33. As in the case of the soap bubble, the materials on either side of the oil film (air and water) both have refractive indices that are less than the index of the film. nair < nwater < noil. There will be a phase shift upon reflection from the upper boundary because nair < noil but no shift upon reflection from the lower boundary because noil > nwater. The equations for interference will be the same.

There is a diagram below this paragraph!

for constructive interference of reflected light
for destructive interference of reflected light