Science Experiments

Why is the sky blue?

Sat, 8th Apr 2006

Part of the show Forecasting Weather and Climate

What you Need

A lemonade Bottle or large glass

A large Jug  or a bowl and a funnel.


A little milk powder

A small powerful torch

A dark room

What to do

Fill the jug with water

Add the milk or milk powder - do this very slowly as you only need a tiny amount. 5-10 pinches will probably be sufficient

Turn the lights off

pour 1-2 cm of liquid into the bottom of the bottle.

Shine the torch up through the bottom of the bottle.

Look at the light coming straight through the bottle and the light coming out of the side, what happens to the colour?

Add some more liquid, and look thorough it again (repeat)

If nothing happens to the colour, you need to add a bit more milk powder, if the light doesn't get through the bottle, you need a bit less

What may happen

The more milk the light goes straight through, the redder the light should look. So with no milk the light bulb will probably look white, then go through yellow, orange, and if you are lucky end up looking red.

If you look at the side of the light beam, you may notice a blueish tinge to the light coming out of the side near to the torch and the scattered light will look more and more reddish the further the light travels through the liquid. You may have to shine the torch into the side of the bottle to make a clean enough beam to see this.

Sunset in a glass

Why does it happen?

Scattering in a bottleThe milk consists of tiny droplets of fat in the water. The white light coming from your torch  is made up of all the colours of the rainbow. When the light hits the tiny fat droplets some of it scatters (bounces randomly) off them.

The bluey colours are scattered more quickly than the redey colours (with green somewhere in between) this means that if you shine the light through a lot of mixture all that gets through is the red light.

Right at the bottom there is lots of blue light scattering out of the sides so it has a blue tinge, as you go through more milk the blue light runs out so there is just red and green scattering so it looks yellow and then orange.

How does this relate to the real world?

The air molecules and any dust in the atmosphere act in the same way as the fat globules in the milk scattering more blue light than red. The sun actually looks white from space but if the light goes though some atmosphere some of the blue scatters out leaving yellow, which is why the sun looks yellow.

At sunset or sunrise or sunset the light grazes though the atmosphere at a low angle and so travels through much more atmosphere, so the only light that gets though is red, hence sunsets are red.

If there is lots of dust in the atmosphere, because of fires or volcanic erruptions then there are more particles in the atmosphere to scatter the light so the sunset is even more intense.

A sunset

If you look up in any direction other than up at the sun the only light you can see is the light that has been scattered, this is mostly blue, so the skit is blue.

Blue Sky

Why does blue scatter more than other colours?

Light is a wave, it is part of the electromagnetic spectrum which includes radio waves, microwaves, X-rays, gamma rays etc. Like any other wave, light waves have a wavelength ( 400 - 700nm ) light with different wavelengths has different colours. The long wavelength is equivalent to red and the short, blue (with green in between).

Red light's wavelength is significantly longer than the size of the particles. If you imagine a post in a harbour and a long wavelength ocean swell coming in, the post will just be too small to make much of a reflection (the difference in wave height between the two sides of the post is tiny so there is nothing to reflect).

Long wavelength Scattering

however if there is a much shorter wave coming in (bluer) that is more similar to the size of the post, you will get much more of a reflection.

Shorter wavelengths are scattered more


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