why do oil slicks appear rainbow?
In: Chemistry
If you have two waves of the same size, if they are out of phase by half a cycle, so that the peak of one matches the trough of the other, they add up to nothing; if they are in phase, they reinforce each other. This is called destructive and constructive interference, respectively.
If the thickness of an oil film, measured in wavelengths of light, is either a whole number or a half number, then the light reflected from the **underside** of the oil is delayed by a whole number of wavelengths relative to the light reflected from the **top** of the oil, so the two light-waves add up and look brighter than they would otherwise. But if the fractional part of its thickness in wavelengths is ¼ or ¾, then the two sources are out of phase by a half-cycle, so they cancel: that bit of oil looks dark (in that wavelength). Other fractions give intermediate results.
The color of light depends on its wavelength. Visible light ranges from red (0.7 microns) to violet (0.4 microns; a micron is a millionth of a metre). The thickness of an oil film on water varies from place to place, because it is disturbed by the breeze and whatnot. So at any given point you see the color whose wavelength is reinforced by the specific thickness of the oil at that point. If the thickness varies smoothly, then the colors will vary smoothly, as in a rainbow.
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