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| | | Subject: Why the sky is blue (and why we get rainbows)...
Posted Jun 4, 2003 by Researcher 108409
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 | | Let's start with what we know about light? What colour is light? What we normally think of as white light is actually a whole range of colours, from red right through to purple. You should be able to show this fairly easily, by recreating one of Isaac Newton's experiments.
All you need is a torch and a clear prism. If you can't find a glass or plastic prism, you can make something that should (!) work just as well - take a flat bottomed clear plastic (or glass) container, and fill about a quarter full with water. If you tilt the container, one end will be 'deeper' than the other, so you effectively have a prism.
Just shine the torch through the prism - you may need to move the torch around, trying different angles to get a good result. Also, it will be easier to see the result with a narrower beam of light.
So, 'white' light is made up of lots of different colours. What the above experiment also shows is that we can split the colours using a prism. The prism bends the different colours by different amounts - red gets bent the least, blue and purple gets bent the most.
We now need to think about how the light from the Sun is affected as it comes to us, on Earth. As it approaches the Earth, it enters the atmosphere, which acts like a prism, and bends the rays of light.
Since the red/orange/yellow light doesn't get bent very much, we see the Sun as being a yellowy orange colour. All the green, blue and purple light gets bent much more - so much that it appears (to us) not to come from the Sun at all, but from the rest of the sky. So when the Sun is high in the sky, we get a blue sky.
However, things are different when the sun is low in the sky - just after dawn and before dusk. If you look towards the Sun (but not straight at it, as this can damage your sight), the sky is orangey red. Again, the blue light has been bent more than the red - but because of the position of the Sun, the light is passing through more of the atmosphere, so the blue light gets bent *even more* than before. It gets bent so much that we don't see very much of it at all. The red light is also bent more then before, which (along with the absence of blue light) explains why more of the sky is orangey red.
So what about rainbows? These are caused by the same effect, but this time it is the rain which acts as a prism. Each drop of rain splits the light, just like the prism in the above experiment. As well as this, the light is actually reflected off the back of the raindrop (notice that whenever you are looking at a rainbow, the Sun is directly behind you)!
Let us concentrate on the middle part of the rainbow - where the colours are (mostly) horizontal stripes. When the light leaves a single raindrop, it is split so that the blue light is 'higher' than the red light. The effect of this is that you only see red light from a raindrop that is higher in the sky than one you see blue light from. So, for the central part of the rainbow, you see red at the top and blue/purple at the bottom. The same effect is true for the rest of the rainbow, although the 'directions' are slightly different.
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