[Physics] If blue light has a higher energy than red light, why does it scatter more

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As $E=hf=\frac{hc}{\lambda}$, blue light – with a smaller wavelength – should have a higher energy. However, it is the case that blue light scatters the most. Why is it that higher energy rays scatter more?

Best Answer

In general, the scattering of light from some object depends on the how close the wavelength of light is to the size of the object.

To make an analogy, if a tidal wave with a wavelength of several kilometers hits a telegraph pole with a radius of 15 cm it isn't going to scatter very much. On the other hand, waves with a wavelength of a few cm, e.g. generated by you throwing a stone into the water, are going to be strongly scattered.

As you've said in your question, blue light has a smaller wavelength than red light. Assuming you are talking about the sky, the scattering is from particles much smaller than the wavelength of light. That means you'd expect light with the smaller wavelength to be scattered more strongly because it's nearer to the size of the objects doing the scattering.

The formula you quote is for the energy of a photon, but this is not relevant for Rayleigh scattering.

To expand the discussion a bit, when the particle size approaches or exceeds the wavelength of light the difference in the wavelengths disappears. If you look at scattering from e.g. a colloidal suspension with a one micron particle size the scattering is (mostly) wavelength independant.

If "scattering" can be extended to include diffraction you find the wavelength dependance is inverted. Red light is diffracted more strongly than blue light.

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