Question of the Week Podcast

Question of the Week episode

Mon, 2nd Feb 2015

Why does petrol create a rainbow on water?

Oil rainbow (c)

We find out why petrol creates a rainbow effect when in contact with water, and how the behaviour of light can create all these colours.

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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. chiralSPO, Mon, 2nd Feb 2015

The dark "holes" in thin film interference patterns are interesting ...

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.  RD, Mon, 2nd Feb 2015

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