Oil slicks

It rained this morning, and when I went to the grocery store the water had mixed with oil in the parking lot. I’m not sure why, in this picture, the different drops are different colors, but I’m sure some readers will know. 


  1. Posted July 21, 2018 at 9:02 am | Permalink

    The color depends upon the thickness of the oil film upon the water. Very minute differences result in the different colors.

    • moleatthecounter
      Posted July 21, 2018 at 9:08 am | Permalink

      So, essentially, the light is refracted by different amounts according to the water thickness, giving us a sort of rainbow spectrum effect via the different drops… Something like that I guess?

      • infiniteimprobabilit
        Posted July 21, 2018 at 9:44 am | Permalink

        I think (from memory) it depends on the wavelength of each particular colour of light – the thickness of a particular area of the oil film is a multiple of some particular wavelength.

        (Googles: Here’s an explanation –
        https://www.scientificamerican.com/article/why-do-beautiful-bands-of/ )

        Interestingly, some of the colours generated by e.g. butterfly wings are caused by a related phenomenon. The dimensions of physical structures correspond to a wavelength of light. See ‘structural coloration’


        • infiniteimprobabilit
          Posted July 21, 2018 at 9:47 am | Permalink

          I see lydifeline (below) linked to the same SciAm article before I did. Sorry, my plagiarism was unintentional. 🙂


        • infiniteimprobabilit
          Posted July 21, 2018 at 9:53 am | Permalink

          What is unusual, of course, is the uniformity of colour within each drop of water.

          I would assume this is because the oil has had a long, long time to distribute itself and the film within each drop has therefore attained a very uniform thickness. Obviously each drop has ended up with a slightly different thickness of oil on its surface, hence the different colours.


          • jahigginbotham
            Posted July 21, 2018 at 11:42 pm | Permalink

            Perhaps it is because the water is isolated into droplets that the oil layer is of more or less homogeneous thickness. On a larger water surface the oil would continue spreading and the edges would be thinner than the center; the resulting gradient would produce the rainbow effect which is more commonly observed.

            • infiniteimprobabilit
              Posted July 22, 2018 at 1:32 am | Permalink

              Yes, I’d agree with that.


        • Posted July 21, 2018 at 10:07 am | Permalink

          Stop unweaving my rainbows!

        • nicky
          Posted July 22, 2018 at 2:06 am | Permalink

          Yes, structural colours on butterfly wings and some feathers. They are generally much brighter than pigment colours, the latter absorbing light (of the other wavelengths).

      • Posted July 22, 2018 at 6:07 am | Permalink

        In classical wave theory of light, the white light (composed of light of many different wavelengths) reflects off the front surface of the oil layer and the back surface of the oil layer.

        The two different reflections interfere with each other. For most wavelengths, the interference is destructive but for wavelengths where the thickness of the oil is an exact multiple, the two reflections interfere constructively.

        In ,a href=”https://en.wikipedia.org/wiki/QED:_The_Strange_Theory_of_Light_and_Matter”>QED< Richard Feynman gives quantum mechanical explanation of the phenomenon which amounts to the same thing, except, the wave, rather than being a physical thing is a probability function for a photon to bounce off the front or back of the oil and arrive at your eye.

    • Serendipitydawg
      Posted July 21, 2018 at 9:13 am | Permalink

      … and applicable to soap films and bubbles… Thin film interference.

      • moleatthecounter
        Posted July 21, 2018 at 12:40 pm | Permalink

        Thanks all! Fascinating…

    • ThyroidPlanet
      Posted July 22, 2018 at 3:25 pm | Permalink

      very precisely put

      I’d suggest :

      observed color: red —— purple
      oil thickness : thin —– thick

      … the water having evaporated to different degrees, depending on location.

      … and then, there could be fluoescin mixed in, from, as some readers correctly note, anti-freeze – which has the fluorescent green fluorescin mixed in in small amounts. otherwise, anti-freeze is, I’d guess, colorless.

      • ThyroidPlanet
        Posted July 22, 2018 at 3:47 pm | Permalink


        … also my choice of words like “degree” wasn’t good, in this case, but I’ll leave it alone…

  2. Janet
    Posted July 21, 2018 at 9:17 am | Permalink

    That’s such a cool photo!

  3. Posted July 21, 2018 at 9:23 am | Permalink

    I think it is God’s way of promising not to flood the earth with motor oil.

    • Ken Phelps
      Posted July 21, 2018 at 10:14 am | Permalink

      You, my friend, will spend eternity enjoying The Prince of Peace’s hospitality in Hell.

  4. Posted July 21, 2018 at 9:24 am | Permalink

    Well I looked it up, because I am curious as well! —> https://www.scientificamerican.com/article/why-do-beautiful-bands-of/

    • S.K.Graham
      Posted July 21, 2018 at 10:10 am | Permalink

      Going to take issue with one gross oversimplification in that article: “Each color of light has a different wavelength.”

      A color does not have a wavelength.

      A wavelength has a color.

      Also, each mixture of wavelengths(spectrum) has a color, which may be the same color as another mixture of wavelengths, these are called metamers of each other. Every color has infinitely many metamers.

      Some colors, i.e. purple, are not had by any single wavelength spectrum.

  5. Randall Schenck
    Posted July 21, 2018 at 9:27 am | Permalink

    Different colors might also be caused by different types of oil – synthetic verses regular oils or different oil weights.

  6. Brian
    Posted July 21, 2018 at 9:34 am | Permalink

    Oh, all that science stuff! I thought the oil companies were simply showing support for the LGBTQ community. .

  7. Posted July 21, 2018 at 11:19 am | Permalink

    Nice photo!

    • rickflick
      Posted July 21, 2018 at 5:03 pm | Permalink

      Agreed. I think it was Richard Dawkins who pointed out that a scientific understanding can enhance our sense of beauty and feeling of wonder at the natural world.

  8. Posted July 21, 2018 at 11:58 am | Permalink

    Fantastic optical illusion! So surprising that all these drops are the same color!

    Ok, not really. Great picture!

  9. ploubere
    Posted July 21, 2018 at 12:18 pm | Permalink

    There might also be different chemicals, from antifreeze probably, as well as motor oil additives.

    • Posted July 21, 2018 at 10:39 pm | Permalink

      Then the question become, why is each drop a separate distinct color?

      • Posted July 21, 2018 at 10:39 pm | Permalink


      • jahigginbotham
        Posted July 21, 2018 at 11:38 pm | Permalink

        Because the oil is a different thickness on each droplet corresponding to constructive interference from different wavelengths or colors of light.

        • infiniteimprobabilit
          Posted July 22, 2018 at 1:35 am | Permalink

          Yes, I think so. It’s easy to postulate how the oil film might have a different thickness in each tiny pool of water. Much harder to conceive a mechanism whereby the composition of the oil would be different in different drops.


  10. W.T. Effingham
    Posted July 21, 2018 at 12:26 pm | Permalink

    Phickle fotons.

  11. Eli Siegel
    Posted July 21, 2018 at 1:12 pm | Permalink

    Isn’t this similar to X ray diffraction?

    • jahigginbotham
      Posted July 21, 2018 at 11:35 pm | Permalink

      In the sense that it is constructive interference of waves which is observed, yes. But the light is reflected from two layers whereas xrays are scattered from the electrons in atoms in a 3-dimensional lattice.

  12. Posted July 21, 2018 at 4:46 pm | Permalink

    Thanks for sharing one of the many beauties all around us, if we take time, as you did, to see and wonder.

  13. Posted July 21, 2018 at 9:32 pm | Permalink

    Some people’s cars are leaking oil, anti-freeze and brake fluid.

    • infiniteimprobabilit
      Posted July 22, 2018 at 1:44 am | Permalink

      Mine does. 😉

      Though much less since I took the oil pump off and carefully resealed the gasket face with silicone gasket goo.

      But it takes remarkably little oil to produce a film a few molecules thick on water drops. I would guess those colours come from water which has trickled over an oily surface and picked up just a minuscule amount of oil on the way.

      I don’t think the colours would come from antifreeze, since antifreeze is designed to be compatible (soluble) in water. And brake fluid is too, in fact it’s hygroscopic (absorbs water), which is a nuisance. So neither would form the colourful film, I think.


  14. Bob
    Posted July 22, 2018 at 7:27 am | Permalink

    Although ubiquitous in the United States, oil contamination like this from cars and trucks are seldom seen in most of Europe. This was one of the first things I noticed when I lived in Germany. Parking lots there were nearly completely free of these terrible stains. Streets and highways were also free of the black center line in each lane that we accept as normal. The lack of vehicle safety inspections in the US permits far too many cars and trucks to remain on the roads here. Sorry if I ruin your swooning over what is contamination.

  15. Posted July 23, 2018 at 11:51 am | Permalink

    A “thin film” problem, which my “Waves and Modern Physics” instructor in CEGEP was fond of.

    There seems to be more going on here though, since I see “non-spectral” colours.

  16. Posted July 29, 2018 at 12:06 am | Permalink

    Love the droplets and the colorplay. Want to hang them on a wall. No really. A great wall art.

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