Why does oil sometimes get weird colors in it. It will shine blue, purple, or yellow even. It kind of looks like the colors you get when you case harden metal.
In: Chemistry
iirc, it’s due to the refraction pattern of the light hitting it. [Fatty acids](https://en.wikipedia.org/wiki/Fatty_acid#Types_of_fatty_acids) and oils generally are hydrocarbons (compounds made primarily of hydrogen and carbon). Unsaturated hydrocarbons will have at least one double bond in the structure, whereas a saturated hydrocarbon won’t have one because the carbon chain is saturated with hydrogen and no double bonds are present.
ANYWAY, double bonds in fatty acid chains tend to lower their melting point, for a couple of reasons, the main one being that, should the double bond be a cis double bond (cis meaning on the same side), it creates a bend in the chain, making it harder for the molecules to fit nicely together. Trans double bonds (trans meaning opposite sides in this case) don’t affect the angle of the bonds that much at all so they have higher melting points since the molecules are more closely packed together.
ANYWAY, double bonds and single bonds between molecules have different energy levels and, therefore, absorption and refraction patterns. It gets even more silly when double bonds are spaced in a way that causes [resonance](https://en.wikipedia.org/wiki/Resonance_(chemistry)) where the double bonds will become dynamic as opposed to static. Basically those electrons can alternate to different positions on the chemical, back and forth, functionally making a chemical with resonance, more of a mixture of the same chemical with varying electron positions… at the same time. This makes the light refraction patterns more diverse since certain resonance forms of a chemical may have vastly different positions of electron density and thus, different reactivity.
So… when light hits the surface of an oil spill, the chemistry of the oil is influx since, it’s in multiple different forms that are vacillating back and forth at light speed. The oil is able to absorb multiple different wavelengths and refract them in a [dispersed](https://en.wikipedia.org/wiki/Refraction#Dispersion) pattern that shows more than one wavelength..
I’m starting to think I’m not doing a good job of explaining this to a 5yo, so… uhhh….
It has to do with how thin film diffraction splits up white light into seperate rays. Each color has its own unique wavelength (between roughly 400 and 750 nm) and when white light (light of all visible wavelengths) enters certain mediums the different wavelengths reflects by different measures. This is part of what happens in a rainbow as well as in a crystal.
It’s because the layer of oil is roughly as thick as the wavelength of visible light. Different colours are different wavelengths so variations in the thickness of the oil and in the viewing angle will show different colours, often giving a rainbow effect.
Rainbows happen because different colours are bent (**re**fracted) by different amounts inside rain drops. Oil colours happen because of **dif**fraction; when light reflecting off the surface of the oil interferes with light reflecting off the surface of the water, with the two surfaces being very close together. Diffraction also explains the colours you see on a CD, due to the grooves being so close together.
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