Sci: Why the sky is blue

This is all about dispersion, which is what we call the separation of white light into its different elements (the colours or wave lengths). Dispersion happens because the refractive index of a material such as glass, dust or water is different for different wavelengths, so different colours are refracted through different angles. The why is Deep Physics, so we will dodge around it.

A colloidal suspension contains huge numbers of very tiny particles that are too large to be called dissolved, but too small to be seen by any ordinary microscope. When a beam of light shines through a colloidal suspension, you can see the beam because some of the light is scattered sideways by the particles. You can test this by shining a light through a mixture of milk and water. To explain what happens, we need to know something about the nature of waves, and about the wave nature of light.

To a physicist, light is made of waves, if that’s convenient. If not, then light is made up of little particles called photons. And if that won’t do either, then light is made up of little particles made up of waves called wavicles. Or light is made of green cheese, if that helps to explain what is going on. Is that clear now? If it isn’t, don’t worry: just lie back and think “spherical horses”.

For the purposes of this discussion, light is made of waves of different wave-lengths. When these light waves pass through milky water, they bounce around all over the place, but something strange happens. The blue end of the spectrum is scattered, and the red end of the spectrum is allowed to pass, at least comparatively speaking.

When a beam of white light shines through milky water, a person standing to one side will see a blueish beam glowing in the milky water, but somebody viewing the beam end-on will see a reddish beam coming at them. This variable scattering is called the Tyndall Effect, which causes the sky to be blue, and sunsets to be red.

When the sun shines through the atmosphere at dawn and at dusk, red light tunnels through to us, but the blue light is scattered in all directions: up into space, down to the Earth, all over the place. This was a telephoto shot at Negombo in Sri Lanka.

This, say the physicists, happens because the efficiency of the scattering of the light is inversely proportional to the fourth power of the wave-length. In simple terms, if you halve the wave-length, you get a sixteen-fold increase in the efficiency with which the light is scattered.

When it is dusk here, it is noon somewhere else. People close to midday are directly underneath the sun’s rays that reach us, and they see the blue light that is missing from our sunset, coming from all over their sky. At the same time, we see the sun’s white light, minus the blue part, so the sun appears red, and now you know why the sky is blue.

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