The Naked Scientists

The Naked Scientists Forum

Author Topic: Why does petrol create a rainbow effect on water?  (Read 7683 times)

Offline thedoc

  • Forum Admin
  • Administrator
  • Sr. Member
  • *****
  • Posts: 511
  • Thanked: 12 times
    • View Profile
Why does petrol oil  shows different colours when poured on water?
« Last Edit: 30/09/2016 08:11:09 by chris »


 

Offline chiralSPO

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 1879
  • Thanked: 144 times
    • View Profile
Re: Why does petrol create a rainbow effect on water?
« Reply #1 on: 02/02/2015 16:58:51 »
I believe it is an interference pattern, like what is seen on soap bubbles. If the petrol forms a layer on top of the water that is a few hundred nm thick (1 nm = 1x109 meters), then visible light of the right wavelength reflecting off the top of the layer will interfere with light of the same wavelength reflecting off the bottom of the layer. The thickness of the layer and the angle of the light will determine which colors (wavelengths) are muted.

Note: this type of rainbow is different from that formed by raindrops, or a prism, or a diffraction grating--these familiar rainbows separate light into single wavelengths. The rainbow observed on a petrol/water puddle shows all the light except for one (or two) wavelengths. That's why the colors are so different between the two types of rainbow effects.
 

Offline RD

  • Neilep Level Member
  • ******
  • Posts: 8132
  • Thanked: 53 times
    • View Profile
Re: Why does petrol create a rainbow effect on water?
« Reply #2 on: 02/02/2015 22:20:51 »
The dark "holes" in thin film interference patterns are interesting ...


http://en.wikipedia.org/wiki/Interference_%28wave_propagation%29

They are where the film layer is a very small fraction of the wavelength of visible light , ( about 10nm) , so there is almost total destructive interference
« Last Edit: 02/02/2015 22:36:50 by RD »
 

The Naked Scientists Forum

Re: Why does petrol create a rainbow effect on water?
« Reply #2 on: 02/02/2015 22:20:51 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums