Why is the Sky Blue?

On a beautiful clear summers day the sky is a vibrant blue. But why?

Our planet, The Earth, is protected by the atmosphere. When I think of the atmosphere I think of it as though it is a protective force field that sweeps around the earth. The atmosphere is so important to our planet and we will explore this further in a separate “Questions Kids Ask”.

Our planet gets light from the sun. This light has to pass through the earth’s atmosphere to reach us and brighten up our day!

But what is light?

Let’s keep this simple – light is an energy that travels in waves. Light is both an electric and magnetic field.

Light is only a small part of what we call the “electromagnetic spectrum”. Different parts of the electromagnetic spectrum are characterised by their wavelengths. Lots of other types of energy travel in electromagnetic waves as shown below.

Did you know that light travels very, very fast. But how fast? Well that depends on what light is passing through. In a vacuum light is moving at approximately 300,000 kilometres per second! However, light has been measured more accurately than that. In 1975 light was measured as moving at 299,792,458 metres per second.

If you were driving, or a passenger in a car, that would be the same as traveling at 671,000,000 miles per hour – simply blink and you would be at your destination.

Visible light is the part of the electromagnetic spectrum that our eyes can see. Although light might look as though it is white, it is actually made up of lots of different colours.

You may have noticed, that sometimes, when it is raining and sunny at the same time a rainbow appears. This rainbow is magical because as the light strikes the rain drops it is split into it’s different colours. The different colours of light become visible due to reflection, refraction and dispersion of light in the water of the rain droplet.

The below picture shows the parts of the visible spectrum (measured in nanometres) that our eyes can see.

When light passes through our atmosphere it travels in a straight line until it bumps into something. When you look up at the sky, it might not look like there is much to bump into, but there are all of the gas molecules and particles of dirt and dust, even really, really high up in the atmosphere.

As light passes through the atmosphere it is scattered in all directions by these gas molecules and dust particles.

The blue colour of the sky is caused by something called Rayleigh scattering which was discovered by an English physicist called John William Strutt, 3rd Baron of Rayleigh. (note from the editor – his book The Theory of Sound is well worth a read and still referred to by acoustic engineers to this very day).

As red, orange and yellow have longer wavelengths they are not as affected by the gas molecules in the air and pass straight through. Blue however has a shorter wavelength and is absorbed by the gas molecules and scattered over and over again all over the sky.

That is why the sky appears to be blue.

As we look closer and closer to the horizon the blue gets paler and lighter.

This is due to the fact that by the time the light reaches the surface it has passed through more and more gas molecules in the air. All of this scattering of the blue light, as well as the reflection of the light from the surface of the earth, means that at the horizon the sky almost appears white.

Image Credits: [credit – almoond], [credit – normaals], [credit – normaals], [credit – phochi].