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Why does oil show so many colors?

  1. May 16, 2010 #1
    I noticed that when i was working on my car today that when i spilled oil on the ground in the sun it looked as if it had a bunch of colors in it. I know that used oil is black and black absorbs light. So why does it show all of these colors?
     
  2. jcsd
  3. May 16, 2010 #2
    Thin film interference.
     
  4. May 17, 2010 #3

    Entropee

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    Gold Member

    The crests and troughs of certain colors of light cancel eachother out, so the oil displays all the colors that aren't canceled out. I may have something wrong there but that was my understanding of it, same goes for soap bubbles if you were wondering about those too.

    Also please correct me if I have said something wrong here.
     
  5. May 17, 2010 #4

    diazona

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    No, that's more or less the right idea. Wikipedia actually has a decent explanation:
    http://en.wikipedia.org/wiki/Thin-film_interference
    Much of the article might be a bit technical but the pictures should give a good idea of what's going on.

    Basically, whenever you have a thin film of something like oil, light reflects off both the top surface and the bottom surface of the film. If you have a specific relationship between the thickness of the film, the angle of the incoming light rays, and the wavelength of the light, those two reflections will destructively interfere (cancel each other out). For a slightly different specific relationship of those three quantities, the two reflections will constructively interfere (add together to produce a stronger wave).

    For any given film thickness and angle, certain wavelengths undergo constructive interference and others undergo destructive interference. The constructively interfering waves will have the strongest intensity when they get to your eyes, so they mainly determine the color you see.
     
  6. May 17, 2010 #5
    So does it do the same thing for new oil and old oil?
     
    Last edited: May 17, 2010
  7. May 17, 2010 #6
    Pretty much yes.
     
  8. May 18, 2010 #7
    I just wonder why you didn't notice such thing until recently
    I noticed early in my childhood
     
  9. May 20, 2010 #8
    i did notice it a long time ago but, never thought to ask why it did it.
     
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