Rosemary Gant asked:
What determines actual shape of rainbow? I watched one the other night and it was almost flat along the horizon
Dave - Okay. The rainbow is actually always the same shape. Itís always a circle. The way a rainbow is formed is that when the light goes into a raindrop, it kind of reflects around the back and as it goes in and goes out, it refracts. It bends because light goes slower in water than it does in air. Different colours refract slightly different amounts, so the light comes out of the raindrop in different directions depending on its colour. A raindrop is circularly symmetric so the refraction means that you get a cone of different colours of light coming out of the drop.
So, if you look at the raindrop from certain different directions, it looks different colours. If you have a whole sky full of raindrops at different angles you see different colours depending on the direction you are looking in.
You get a circular rainbow exactly opposite the sun. If the sun is high in the sky, you just see the top of the circle and the rainbow looks flat. If the sun is low, you can see a full semicircle, and if you are in a plane you can see a full circular rainbow.
Chris - That makes sense and when you see a second rainbow thatís presumably where itís gone into the raindrop bounced off the front of the raindrop, gone to the back again and come out front. So, there are two journeys and thatís why you get a second rainbow around the first?
Dave - Itís doing more exciting things inside the raindrop.
Rosemarie Gant asked the Naked Scientists: Hi Chris One thing I was wondering as I drove home through the storms last night: what is it that sets the curve of a rainbow?† I've previously thought that rainbows have sometimes smoother curves than others.† But last night I saw the most extreme example I have seen: the rainbow was practically flat along the horizon with just the tiniest bit of sky showing in the centre.† So I thought I'd ask :-) Best wishes, Rosie. What do you think? Rosemarie, Sat, 11th Jul 2009
All rainbows have the same curvature: that of a circle.
You can get some funny shaped rainbows by spraying a fine mist in different patterns i.e. by sticking your finger over the end of a hose-pipe. These are 'artificial' though, of course. LeeE, Sat, 11th Jul 2009
Does the shape of the water particles affect the appearance of the rainbow? Herman Melville, Sat, 11th Jul 2009
It's not the shape of the water particles; they're all spherical. No, it's the shape of the mist/spray; with a bit of practice you can make a 'U' shaped spray, for example, and with special nozzles you can make different shaped ones. LeeE, Sun, 12th Jul 2009
Well I'm just going by what I've observed while messing around, sticking my finger/thumb over the end of a hose pipe. LeeE, Tue, 14th Jul 2009
I'm not disputing the mechanism of the optics, which I do understand, and what you've said is perfectly true for a uniform mist. The only point I was trying to make is that by making a non-uniform shaped mist you can get some strange shaped rainbows. I don't need to perform the 'experiment' again (I was washing a car using a hose without a nozzle and used my finger/thumb to make a jet in an effort to dislodge some crud from beneath the wheel arches) as I was so intrigued by the effect that I was distracted for a good five minutes, trying different spray shapes and orientations to see how the appearance and shape of the rainbows changed. LeeE, Wed, 15th Jul 2009
Thatsounds fun Lee.. I will try it sometime.. something kids may find great fun also... Karen W., Wed, 15th Jul 2009