Monday, 10 October 2011

Quaternary rainbow observed

Only a few weeks ago I was caught in a heavy shower with a beautiful double rainbow visible over St Martins in the Field.  I explained to a friend some of the physics of rainbows and explained that the third order rainbow has never been seen, but that this was partly because people have been looking in the wrong place.  It is not (theoretically) where you might expect, just above the second bow, but actually it would be behind you, quite close to the sun.  What's more, the chances are that the scattered white light from that part of the sky would be brighter than the bow, making it invisible to the naked eye.

And now this!  This really is something surprising.  The third and fourth order bows have been captured in a photo (albeit an enhanced photo).  They still haven't been seen with the naked eye, but a quick google search shows that people have been successful in their search for the tertiary bow a few times using some legitimate photographic techniques.  (I hadn't realised that.)  This one is remarkable because the tertiary and quaternary bows can be made out, albeit faintly.

Note one difference between these and the first two bows.  Like them, these appear to be circular and their colours are reversed relative to each other.  But they are centred on the sun itself, not on the point opposite the sun.  I wasn't surprised by this, because it seems obvious but after I thought about it for a moment I couldn't quickly work out why.  I suppose the best anaolgy that I can draw is that all the bows are centred on the line in space that passes through the centre of the sun and through your eye (or camera lens) and beyond.

See the BBC article (probably not to be trusted, but then I'm biased against the government's propaganda wing, as you might know)

Optics Infobase (almost certainly to be trusted) has the original information.

Related posts:

See my introductory post about the basics of rainbows yesterday, in Ever thought about rainbows? 

and see 

Glory be! - not a rainbow but it looks like one.

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