Why Is The Sky Blue? – A Simple Guide To Light

How often in the summer have you looked up into the cloudless, clear sky and thought to yourself: ‘why is it blue? Why not red or yellow?’ The answer to this question may in fact be never, however if you’re interested in learning more about how exactly light and the atmosphere interact then this article could be for you.

The Key Ingredients

Part of the reason the sky looks blue is down to the atmosphere. This surrounds the earth and consists of a mixture of various gas molecules, mainly nitrogen and oxygen. It is what stops harmful radiation from the sun reaching us here on earth.

 The atmosphere’s thickness depends on how high up you are. So for example you would feel a lot different stood at the top of Everest than you would at sea level.

The other key component involved in the sky’s colour is, obviously, light. Light travels at 299,792 km per second, faster than anything else in existence. It moves in waves, as you may remember from school, and although it may look white when you see it from a light bulb, or the sun, is in fact made up of several colours.

These colours are all part of what is known as the electromagnetic spectrum. The best way to explain it is by using a rainbow. If you see one, the colours in it range from red and orange, at one end, all the way through to blues and violets at the other.

Now, each colour consists of a different type of wave which has its own kind of frequency and wavelength. Red light has a particularly low frequency, meaning there are less waves of it per second, and a higher wavelength, meaning there is a larger distance between wave crests. At the opposite end of the spectrum violet has a much higher frequency and smaller wavelength. Consequently it has a lot more energy when it travels.

Confused? Hopefully you’ll feel a little more clued up once we bring it all together

So, Why is the Sky Blue then?

Right, so when gas enters our atmosphere it travels in straight lines – until it hits something. Light will hit one of two things on its way to earth through the atmosphere: either dust particles or gas molecules.

If a dust particle is hit then, because light’s wavelengths are much larger than them, the light will be deflected. However if light hits a gas molecule it may be absorbed by it and then radiated by the molecule itself. This is known as ‘Rayleigh Scattering’ after the physicist who originally discovered it.

All colours can be absorbed yet it is usually those at the purple/blue end of the electromagnetic spectrum which are. This is because of their shorter wavelength, which makes it easier. As a result gas molecules tend to absorb and give out a lot more blue light than any other colour. Consequently this is the colour of light which reaches our eyes and makes the sky appear blue.

Ah, you may say, why then does the sky then change to red once the sun sets? Well that’s also down to the Rayleigh effect. As the sun drops behind the horizon light has to travel further to reach our eyes. Consequently more of it is scattered by dust particles, especially blue/purple light. More of the red light remains, so the sky looks red.