Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: thedoc on 02/02/2015 16:34:24

Title: Why does petrol create a rainbow effect on water?
Post by: thedoc on 02/02/2015 16:34:24

Why does petrol oil  shows different colours when poured on water?

Title: Re: Why does petrol create a rainbow effect on water?
Post by: chiralSPO 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 = 1x10^–9 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.

Title: Re: Why does petrol create a rainbow effect on water?
Post by: RD on 02/02/2015 22:20:51

The dark "holes" in thin film interference patterns (http://en.wikipedia.org/wiki/Thin-film_interference) are interesting ...

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F2%2F20%2FInterference_colours_in_soap_film_1.jpg&hash=77c94725334ada58ddce09d5d033e83c)
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 (http://en.wikipedia.org/wiki/Interference_%28wave_propagation%29#Mechanism).