Why is the Sky Blue?

The sky is made of air, which is obviously not blue. So, why is the sky blue? This seems like a tricky question which children ask a lot. But in two easy steps you can see for yourself the answer to this age-old question.

Step 1 – Making Rainbows

Most light sources give out white light, including the Sun. White light is a mixture of all the colours of the rainbow. When you mix all the colours of paint you get usually get a dark brown colour, but light is the opposite, mix all the colours of light and you get white.

You can show that white light is a mixture of colours a few different ways. Shine a light through a triangular prism shaped piece of glass of perspex and you’ll see a rainbow projected from it just like Pink Floyd. Hold a CD or DVD at the right angle and you’ll see a rainbow reflected off it. In our first science kits you can make rainbow glasses from diffraction grating which split up any light into a rainbow. Of course, you should never look at the Sun but any white light can substitute for the Sun in this experiment.

Rainbows produced from diffraction grating

So, we’ve seen that the Sun is shining out all the colours. The next question is why, when we look at the sky we only see the blue light? What’s happening to the rest of it?

Step 2 – Make a Milk Sky

To find out what’s happening you’re going to need a transparent container and a little milk. This will work OK with a large glass but the bigger the container the better it will work. Fill the container with water and put just a little dribble of milk into it, not too much.

Shine a light through the container. When you look directly at the light through the container it should look yellow/orange, just like the Sun does. The orange light is able to go straight through the milk and out the other side. But the blue light doesn’t fare so well and is bounced of the little droplets of milk, leaving behind the yellow, orange and red light to go through the milk and into your eyes.

image of a white shining through milk appearing orange

So, what’s happened to the blue light? If you look at the container from the side, you should find it! The container now looks blue. The blue light is being bounced off the milk droplets and coming out the sides of the container.

using milk to show why the sky is blue

The same thing happens with the sky. The sky does not contain milk(!) but the gasses and dust particles in the air do the same thing as the milk droplets in the water. Only if we look at the Sun (which we won’t do) do we see the yellow, orange and red light. When we look to the side of the Sun (otherwise known as the sky) it’s the same as looking through the side of the container – you see all the blue light that’s come from the Sun but been scattered sideways by the air into our eyes.

Happy experimenting

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