Saturday, 28 September 2019

optics - What causes multiple colored patches on a wet road?


I was going to school (after a rainy hour) when I saw some patches of shiny colours lying on road. Some small children surrounded that area and thought that it's a rainbow falling on the Earth. (For some 5-6 year old children it is a serious thing.)


I have a similar photo from Google:
enter image description here


It is definitely not a rainbow falling on the Earth but what is it? The road is a busy road and hence I think that's because of some pollutants, oil, or grease.



Answer




There is a thorough explanation of the phenomenon on the Hyperphysics site.


What you are seeing is an oil film floating on the water, and the thickness of the oil film is around the wavelength of visible light i.e. around a micron. The film behaves as an etalon because light reflecting from the upper air-oil interface interferes with light reflecting from the lower oil-water interface.


If you were viewing the road in monochromatic light, for example under sodium street lamps, you would see a pattern of light and dark fringes. Because daylight spans a range of wavelengths from 400 to 700nm the fringes from the different wavelengths of light overlap and you get a coloured pattern.


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