[Make it Lady (OP)]

I love it when it rains on petrol/gas station forecourts. I like the rainbows the petrol makes. But why does it happen? What makes the rainbow in petrol?

[lightarrow]

I love it when it rains on petrol/gas station forecourts. I like the rainbows the petrol makes. But why does it happen? What makes the rainbow in petrol?

Sorry, but do you mean the colors in a water pond with oil on it?
In this case it's thin film interference:

http://images.google.it/imgres?imgurl=http://farm1.static.flickr.com/42/84121850_bca0ca1618.jpg&imgrefurl=http://www.flickr.com/photos/leviathor/84121850/&usg=__ie2UMG4sRMWzwWSe7ac08SVc3wk=&h=323&w=500&sz=134&hl=it&start=16&um=1&tbnid=76kvXn-pi9o1hM:&tbnh=84&tbnw=130&prev=/images%3Fq%3Dthin%2Bfilm%2Binterference%26hl%3Dit%26sa%3DN%26um%3D1
http://www.abdn.ac.uk/physics/geddes/darren1.jpg
http://physics.bu.edu/py106/notes/Thinfilm.html
http://physics.bu.edu/~duffy/semester2/c26_thinfilm.html
http://hyperphysics.phy-astr.gsu.edu/HBASE/PHYOPT/interf.html
http://dev.physicslab.org/Document.aspx?doctype=3&filename=PhysicalOptics_ThinFilmInterference.xml
http://www.physicsclassroom.com/Class/light/u12l1c.cfm
http://www.mwit.ac.th/~ampornke/Content_University/Light%20Interference.htm
http://images.google.it/imgres?imgurl=http://www.olympusfluoview.com/theory/images/interferencefiltersfigure6.jpg&imgrefurl=http://www.olympusfluoview.com/theory/interferencefilters.html&usg=__mUL09fmZkZabkLFNNZbv_xp89YQ=&h=236&w=399&sz=35&hl=it&start=32&um=1&tbnid=hUDfIYlgrAV-BM:&tbnh=73&tbnw=124&prev=/images%3Fq%3Dthin%2Bfilm%2Binterference%26ndsp%3D18%26hl%3Dit%26sa%3DN%26start%3D18%26um%3D1

[Make it Lady (OP)]

Oh, what a hackneyed question. I'm sorry for being so predictable.

[RD]

I love it when it rains on petrol/gas station forecourts. I like the rainbows the petrol makes.

I believe they are referred to as "dead rainbows", (because they are lying on the ground).  []

[paul.fr]

This, just like the author, is an oldie...

http://www.thenakedscientists.com/HTML/content/questions/question/1708/

    

Why does petrol and oil make a rainbow effect on water ? James, Email, Cambridge

   

This is because the oil spreads out to form a very thin film on the surface of the water, but of varying thickness. In some places it is literally a molecule thick, whilst in other places it is much thicker. When light passes through the oil some of it is reflected back off the different layers of oil, whilst some carries on and is reflected off the surface of the water lying below. Because the light waves have now travelled different distances before being reflected they mix together producing a spectrum of colours - because the thickness of the oil layer varies. For the super intelligent the light spectrum occurs because, having travelled slightly different distances, some of the waves are now 'out of phase' and cancel each other out, producing dark spots, whilst others add together, producing lighter spots.

[lightarrow]

Oh, what a hackneyed question. I'm sorry for being so predictable.

Don't worry, it's not hackneyed, but for whom that already know it. []

[Madidus_Scientia]

Well i've learnt something, the word "hackneyed"

[Vendess1991]

Damn, i never heard of "dead rainbow" name. This is actually such a cool and interesting name!

[adianadiadi]

Ah. I am new to this petrol rainbow concept. Are they different from regular rainbows? Any picture?

[Colin2B]

Ah. I am new to this petrol rainbow concept. Are they different from regular rainbows?

Regular rainbows are formed by refraction ie splitting the spectrum, thin film rainbows occur by diffraction ie an interference pattern.

[chiralSPO]

Ah. I am new to this petrol rainbow concept. Are they different from regular rainbows?

Regular rainbows are formed by refraction ie splitting the spectrum, thin film rainbows occur by diffraction ie an interference pattern.

To follow up: "regular rainbows" are white light getting split into individual frequencies ("pure colors"), while "dead rainbows" are the whole spectrum minus some distinct wavelengths (depending on the thickness of the layers, one or more visible wavelengths can be removed). This means that many of the apparent colors are like magenta, cyan, and yellow, rather than red, green, and blue (at least in the first band, where only one wavelength is removed). See the Michel-Lévy chart shown below:

michel-levy-chart.jpg (163.24 kB . 859x590 - viewed 9575 times)