[indrajit]

indrajit asked the Naked Scientists:
   
my question - why rainbow is always forms in a bow shape?

What do you think?

[RD]

Coz they are parts of a circle ...

 [ Invalid Attachment ]
http://www.atoptics.co.uk/fz303.htm

[John Chapman]

They are not bow shaped. They are cone shaped and, if the viewer is standing within the 'wet' air zone that created the rainbow, the cone may travel all the way to the viewer's eyes. The more wet air a rainbow passes through the longer the visible part of the cone is and the more solid the colours are. If you look at a rainbow in the distance which has a nearby row of houses in front of it you may sometimes see that although the rainbow appears to clearly be distant behind the houses a faint ghost of the rainbow may also continue down in front of the houses. For that reason, when people ask how far away a rainbow is it could be anywhere between the eye of the viewer and the far edge of the atmosphere or indeed the whole depth of the distance between.

The reason they are arc shaped is because, since the cone travels from the viewer with the eye at the cones focal point and the rainbow effectively forms the wall of the cone, then a full rainbow does in fact appear to form a complete circle, although the part which is below the horizon tends to travel through so little wet air that it seems to disappear into the ground. If the rainbow is very close to the viewer then there may be enough space for the complete circle to form, as can be seen in RD's photo.
 
I'll be honest, I don't think I have ever read this anywhere but it seems intuitive to me. I would like to hear if any of you fizzycists and otherwise clever naked sciency types agree.
 

[yor_on]

RD, or whatever you now call yourself on Earth? That is your portal, right? I've seen them before, in the telly, no way you can fool me.

Your secret is out.
===

I have double checked my suspicion against the other rainbows and they all agreed with me..

And how the he* are anyone supposed to find a pot of gold inside one of yours alien ones, heh? Let me tell you Sir, Irish they are not..

[daveshorts]

This may be of use, the make your own rainbow kitchen science
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/diy-rainbow/