why do oil slicks appear rainbow?
In: Chemistry
Both the top and bottom surfaces of this thin oil film can reflect light. Because of this, a rainbow-like color pattern is shown on the oil surface
This is due to a phenomenon called “thin-film interference”
The oil layer is extremely thin, on the order of just a few wavelengths of visible light. As the thickness varies, it moves between exact even and odd multiples of specific wavelengths (colors) of light, preferentially reflecting one specific color and absorbing or transmitting the rest.
This produces the rainbow effect.
Note that it only occurs in very thin films – spill the entire jug of motor oil and it goes away.
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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