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Tyndal Effect

John Tyndall discovered in 1859 that blue light is scattered more than red light when passing through particles suspended in a clear fluid, known as the Tyndall effect. Later it was realized that the molecules in air, rather than dust particles, were sufficient to cause the scattering that gives the sky its blue color. Einstein calculated the exact formula for molecular scattering of light in 1911, verifying this explanation. While shorter wavelengths are scattered most, the sky does not appear violet because the sun emits less violet light, it is absorbed more in the atmosphere, and our eyes are less sensitive to violet wavelengths.

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178 views1 page

Tyndal Effect

John Tyndall discovered in 1859 that blue light is scattered more than red light when passing through particles suspended in a clear fluid, known as the Tyndall effect. Later it was realized that the molecules in air, rather than dust particles, were sufficient to cause the scattering that gives the sky its blue color. Einstein calculated the exact formula for molecular scattering of light in 1911, verifying this explanation. While shorter wavelengths are scattered most, the sky does not appear violet because the sun emits less violet light, it is absorbed more in the atmosphere, and our eyes are less sensitive to violet wavelengths.

Uploaded by

Subhash Dhungel
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Tyndall Effect

The first steps towards correctly explaining the colour of the sky were taken by John Tyndall in 1859. He discovered that when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red. This can be demonstrated by shining a beam of white light through a tank of water with a little milk or soap mixed in. From the side, the beam can be seen by the blue light it scatters; but the light seen directly from the end is reddened after it has passed through the tank. The scattered light can also be shown to be polarised using a filter of polarised light, just as the sky appears a deeper blue through polaroid sun glasses. This is most correctly called the Tyndall effect, but it is more commonly known to physicists as Rayleigh scatteringafter Lord Rayleigh, who studied it in more detail a few years later. He showed that the amount of light scattered is inversely proportional to the fourth power of wavelength for sufficiently small particles. It follows that blue light is scattered more than red light by a factor of (700/400)4 ~= 10.

Dust or Molecules?
Tyndall and Rayleigh thought that the blue colour of the sky must be due to small particles of dust and droplets of water vapour in the atmosphere. Even today, people sometimes incorrectly say that this is the case. Later scientists realised that if this were true, there would be more variation of sky colour with humidity or haze conditions than was actually observed, so they supposed correctly that the molecules of oxygen and nitrogen in the air are sufficient to account for the scattering. The case was finally settled by Einstein in 1911, who calculated the detailed formula for the scattering of light from molecules; and this was found to be in agreement with experiment. He was even able to use the calculation as a further verification of Avogadro's number when compared with observation. The molecules are able to scatter light because the electromagnetic field of the light waves induces electric dipole moments in the molecules.

Why not violet?


If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength. The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high atmosphere, so there is less violet in the light. Our eyes are also less sensitive to violet. That's part of the answer; yet a rainbow shows that there remains a significant amount of visible light coloured indigo and violet beyond the blue. The rest of the answer to this puzzle lies in the way our vision works. We have three types of colour receptors, or cones, in our retina. They are called red, blue and green because they respond most strongly to light at those wavelengths. As they are stimulated in different proportions, our visual system constructs the colours we see.

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