Science Questions

Why does the rainbow sometimes seem to end?

Sun, 18th Dec 2011

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Bob, Althorne asked:

I have been fortunate on a number of occasions to be fairly close to the end of a rainbow, 300 or 400 meters away, where you can see the actual end of the rainbow striking the ground. If you look through it to see objects behind, you can see clearly that the coloured lines are on your side.


The actual rainbow seems to be close to half a circle, but why does it appear to end there? Is it going beyond or is it still in front of that tree, but doesnít appear to be so?


When you see a rainbow, every raindrop, when sunlight shines onto it, actually produces a cone of rainbow.  When you actually look at it projected, you see a cone projected from each individual raindrop.  What that means is if you look at each raindrop from different angles, it can look a range of different colours. 

So if you're looking over to your right, low down, some of them will look red at one angle.  If you see them see them at a different angle, they look green or blue or different colours.  And because itís a cone of light, you get the same effects up above you and over to the left.  So the whole rainbow ends up as this arch.

How close does it look?  Well essentially that will be to do with how dense those raindrops are.  If you're in a light rain, then the amount of light reflected back to you per meter is quite small.  It will look like the rainbow is a long way away because there would be hardly any coloured light coming to you from the nearest 100 meters.  This means you have to add up several hundred meters, maybe a kilometre, just for you to see the colour.  So it appears to be a long way away. 

For an incredibly heavy rainstorm, which I expect which is probably what you saw, then there might be so much light being reflected off the rain close to you that most of that light is coming from just 100 meters away.  This makes the rainbow appear in front of the building or the tree which is hanging behind it, even though the light comes from both in front of, and behind the object.


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