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or, Why the Sky is Blue!

Visible light is made up of all colours: red, orange, yellow, green, blue, violet, and everything in between. In space, light from the sun and stars would look almost white, as it contains all these colours. On the surface of the Earth, however, light has passed through the atmosphere, and the Sun appears very yellow. Let’s look at the reason why, and along the way we’ll explain why the sky appears blue, and why blue Christmas lights on your house don’t seem very bright from a distance.

The questions about blue light first arose when we were looking at how to demonstrate that white light is made up of different colours, using glow tubes.

We had previously noticed that the blue Christmas lights on our home, while nice and bright in the yard, were invisible from the road. The question we asked was ‘Why does blue light seem to fade out with distance, while red and yellow don’t?’

The answer has to do with air … the molecules that light must pass through to get to your eye. In order to explain what happens to the light, you’ll need to remember that ordinary white light contains all the colours of the spectrum.

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The fact that white light is made up of all the colours of the rainbow was demonstrated by Isaac Newton using a prism. White light passed into the prism came out in a spectrum, because the glass caused the different colours to slow down by different amounts, changing their angles of exit. This phenomenon is called refraction.

As white light from the sun enters our atmosphere, it must pass through this gas, which is composed mostly of nitrogen and oxygen molecules, before it can reach the surface. As it passes through, each of the different colours in the light interact with the air molecules.

A simple way to describe what happens would be to say that the air molecules, which are about 0.0004 millimeters in diameter, are very close in size to the waves of blue light. Other colours have bigger or smaller waves, and mostly pass right through, but the blue light waves hit the air molecules and scatter in all directions. As the sunlight comes down through the atmosphere, the blue light in it gets scattered all over the place. Some of this scattered light (there’s a lot of it) reaches our eyes, making the sky seem blue. The sunlight that’s left that finally reaches the ground has lost most of its blue light, leaving it yellowish in colour, so the sun itself appears yellow.

In space, there is no air to scatter the different colours in light, so the sun would look white and the ‘sky’ would be black.

This also explains why blue lights get dimmer with distance. If you’re far enough away from the blue Christmas lights decorating your house, much of the blue light from them gets scattered every which way, and there wasn’t very much of it to start with, so very little of it gets to your eye. You can’t see the bulbs.

A more detailed explanation of the scattering phenomenon goes something like this. Molecules of air, in particular nitrogen and oxygen, are surrounded by the electron ‘clouds’ of their individual atoms. The outermost of these electron clouds can vibrate at a certain frequency, called the resonant vibration frequency. Any outside light energy hitting them will cause additional vibrations, and that light will then scatter off in a different direction. This is called Rayleigh scattering.
All light energy striking these molecules scatters a little bit in this way, but the energy of blue light more closely matches the resonant vibration pattern of the molecules themselves, so more of it is used to vibrate the electron clouds, and more of it gets scattered. Thus when white light hits air molecules, more of the blue light in it is scattered to the side than the other colours.

Incidentally, the energy in ultraviolet light exactly matches the resonance frequency of nitrogen and oxygen molecules. As a result, almost all of the energy from ultraviolet light in sunlight is absorbed by the vibrating electron clouds, and almost none makes it to the surface of the Earth.

This leaves us with the question as to why lights on police and some emergency vehicles are blue. Wouldn’t it make sense  not  to use blue, since at a large distance the blue light would scatter and make the lights hard to see?

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Description automatically generated In fact, police and emergency vehicles use all sorts of lights. Blue lights continue to be used because they are restricted to these vehicles by law, so they are uncommon. When you do see a blue light you are immediately alerted.
But these vehicles also use red lights, which don’t dim as much with distance, and powerful flashing white strobe lights which, because of their intensity, will carry over great distances regardless of their colour. Flashing lights ahead of you on the road can make it difficult for you to estimate how far ahead the emergency vehicle is, so they also use non-flashing lights.