Ever wondered why the sky is blue? Here's an experiment that we hope might make it easier to explain
| A fish tank full of water. |
| A really good torch (i.e. a mag-light) or a slide projector |
| Some powdered milk |
This experiment works best in a dark room
Arrange your tank and torch so that the beam goes through the long side of the tank, as shown below.
Now view the tank from the end so you should see a 'sun like' light in the middle of the end tank wall.
Add the powdered milk a spoon at a time.
The water will start to go whitish. As you keep adding the milk the water around the beam will start to appear blue and your 'sun' will appear yellower. If you keep adding more milk the 'sun' will start to go orange and then red, just like a sunset
| The colour of the sky (and the water in out experiment) is determined by the size of the particles that it hits and then scatters off. In the sky it's the air and dust in the atmosphere that give it its blue colour. In the water it's the milk powder. As blue light is scattered off the beam, the resulting beam will appear more yellow (if you think about it in terms of lights primary colours, there is less blue which means more red and green which make yellow…see 'mixing colours of light'). As the real sun sets it has to go through more of the atmosphere and so more blue is scattered making the sun go redder. In our experiment adding yet more milk simulates this. You might have also noticed that at this point the beam near the torch is yellow, but at the far end of the tank it is red. This shows how the blue light is scattered more and more as the light passes through the tank. |  |
On exceptionally clear days when viewing a sun setting over the sea (you have to be at the coast) the sun gets so much light scattered that before it dips over the horizon it will turn green. This is called 'the green flash' and is something that is only normally known by physicists.