Why does oil on water turn rainbow colored?


Whenever you see petroleum oil spilled on a wet surface, it turns rainbow colored. That doesn’t happen with olive oil, or other liquids. Why do petroleum products change color that way?

In: 78

For the same reason that a soap bubble appears iridescent: a very thin film interface between materials with very different refractive indices. In the case of a soap bubble it’s mostly water, with the interface between water and air. In the case of an oil slick the oil naturally spreads out into the thinnest possible layer on top of the water.

It’s actually a fairly simple yet complicated process.

– When light hits the layer of oil, some light bounces back (off the surface), and some goes through the surface.
– Then, the stuff that went through the surface continues down, reflects off the water below, and *then* comes back up.
– When the second “part” of the light comes back through the initial surface, there are now two streams of light, with the second one travelling *ever so slightly* farther than the first one.
– This different causes their EMR to go through constructive/destructive interference, which makes certain wavelengths of light show.
– These colors that present themselves are the colors you see.

Edit: u/YourAmazingNeighbor explained this [a little more simply,](https://www.reddit.com/r/explainlikeimfive/comments/11hi1mo/eli5_why_does_oil_on_water_turn_rainbow_colored/jatoy0c/?utm_source=share&utm_medium=ios_app&utm_name=iossmf&context=3) check it out if you’re still confused.

Light is a kind of wave that can bounce off things or go through them. When light hits oil on water, some of it bounces off the oil and some of it goes through the oil and bounces off the water. Then these two bounced light waves meet again and they can either add up or cancel out depending on how they line up. This makes some colors brighter and some colors darker.

Ok. I understand that oil has a low surface tension (I wants to spread out), so it makes a thin film on top of the water. And I get the idea that some light reflects off the oil, while some goes through and reflects off the water underneath. I also understand the concept of a prism splitting light into the separate colors. But I don’t quite get how the two different surfaces reflecting light end up as the whole range of colors. Shouldn’t they all be the same distorted color? Or is it that my eyes are at different angles to the various parts of the oil slick, and the position of my eyes determines the angle the light is traveling from the surfaces, so I get different colors from different parts of the oil slick? And would that explain why the colors change as I move around and look at it from different angles?

Others have already explained the mechanism of _thin film interference_, so read those posts for the physical mechanism. Instead, I want to talk about what you called “rainbow colored”:

The standard [rainbow](https://upload.wikimedia.org/wikipedia/commons/thumb/2/27/WhereRainbowRises.jpg/640px-WhereRainbowRises.jpg) is sun (or any other fully white) light split into its colors, sorted by wavelength: red, orange, yellow, green, cyan, blue, purple. Invisible for us, it is bordered by infrared and ultraviolet at the two ends.

But what [oil-on-water](https://qph.cf2.quoracdn.net/main-qimg-0b09e1839766fa1a471366063e4b3738-lq), [soap bubbles](https://media.sciencephoto.com/c0/27/96/76/c0279676-800px-wm.jpg) and [bismuth crystals](https://i.ebayimg.com/images/g/ve0AAOSwuVpcutz5/s-l1600.jpg) produce is decidedly **not** a rainbow in that sense:

[Here is a picture of the theoretically resulting colors](https://i.stack.imgur.com/AzsSJ.png), and here some more [photographs](https://physicsopenlab.org/wp-content/uploads/2021/09/thinFilmscover.jpg) [of](https://sciencedemonstrations.fas.harvard.edu/files/science-demonstrations/files/thinfilminterference-freeze-bottom-3.jpg?m=1446824435) [the](https://i.ytimg.com/vi/WTxDyYHaYAI/maxresdefault.jpg) [real](https://www.soapbubble.dk/content/2-artikler/9-filmtykkelse/farver-i-saebehinde-cover-lille.png) [thing](https://sciencedemonstrations.fas.harvard.edu/files/science-demonstrations/files/thinfilminterference-freeze-top-2.jpg?m=1446824437).

You get: black, gray, white, orange-brown, purple, blue, light-blue, yellow, pink, blue, green, yellow, pink, green, pink, green, pink, green, pink, …; with it slowly fading into gray further on.

This entire process is similar to how harmonics works for good music, we get those patterns that are not as simple as a rainbow (or playing the chromatic scale). Instead, we get those even more intricate pseudo-rainbow.

Note: There actually is some variation depending on the materials (or more accurately, on the speeds of light in them), not all give exactly the same. But they are close enough, especially early on.