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What conditions would make it possible to see a naturally occurring fully 360° circular rainbow? Would it even be possible?

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wiki en.wikipedia.org/wiki/Rainbow#Visibility says, its possible from an airplane... –  Vineet Menon Apr 12 '12 at 10:15
    
Are you not familiar with "Full Double Rainbow" man? :) YouTube it. –  AdamRedwine Apr 12 '12 at 11:53
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@Adam if you've seen that video, then you may enjoy this parody:plus.google.com/114601143134471609087/posts/EjLB9zU7F23 –  Manishearth May 3 '12 at 18:35
    
i have seen one around the sun ... sun at the center. –  Santosh Linkha Aug 5 '12 at 19:36

6 Answers 6

up vote 10 down vote accepted

The centre of a rainbow is where your shadow would fall. When you're standing on level ground your shadow is obviously on the ground so you only see part of the arc. If you're in an aeroplane or on a mountaintop or somewhere where your shadow would be above ground, you may be able to see the whole circle.

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To make "somewhere where your shadow would be above ground" sensible you should probably mention that this is measured in the region where the refraction happens. Otherwise your shadow presumably falls all the way to the ground. –  dmckee Apr 12 '12 at 13:26
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I've actually seen it without leaving the planet. On many lakes there are fountains. If you swim to it, you'll get in the middle of the cloud and can nicely observe the full rainbow. Did it in Zurich, the fountain there suits the purpose well. –  texnic Apr 22 '12 at 10:38
    
I've seen a couple while skydiving...they are generally fairly small wrt "normal" rainbows. –  Ken Hiatt Jul 16 '12 at 7:04
    
Of course, this applies to first-order rainbows. There are higher-order ones (order = number of internal reflections in water droplet), some of which are centered on the Sun rather than the point opposite the Sun. Seeing them isn't easy, though, due to contrast. In fact, you can publish a paper for photographing such phenomena. –  Chris White Oct 25 '12 at 7:36

Glories come to mind. They are easily seen airborne as mentioned by Vineet.

One can also produce a full rainbow in the lab by reflecting light off tiny glass spheres. Frank Crawford (of Berkeley physics Waves) has described it beautifully in an AJP article. He called it "Rainbow dust".

A friend of mine reproduced this effect in the lab sometime back. Here is the picture.

Rainbow dust. Photo by H R Madhusudhan, Bangalore Association for Science Education, Jawaharlal Nehru Planetarium

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You can generate a circular rainbow using a garden sprinkler on a sunny day. The top of the rainbow is usually easy to spot - but, by spraying water drops near your feet, the bottom of the rainbow should also become visible.

Waterfalls are great places to see natural rainbows - and, if you are lucky, you might see a circular rainbow, as captured in the video at http://www.philiplaven.com/p5a.html

Most of the "circular rainbows" shown on the web are NOT rainbows! Remember that rainbows are huge: the red arc of the primary rainbow has an angular radius of 42 degrees, thus a full circle rainbow has a diameter of 84 degrees. To capture an entire circular rainbow, you need a camera with an ultra-wide-angle lens (e.g. a fisheye lens).

If you have a picture of a SMALL circular rainbow taken with a normal lens, it is almost certainly a glory. Glories typically appear as one or more coloured rings on clouds surrounding the shadow of an aircraft. Glories usually have an angular radius of less than 5 degrees.

On the other hand, if you see a LARGE circle around the sun, this is probably a halo caused by ice crystals in the atmosphere - see http://www.atoptics.co.uk/halo/circular.htm. Most halos have an angular radius of 22 degrees, but some have an angular radius of 46 degrees.

Good rainbow hunting .....

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Yes it's possible. Although small in size, I captured one during a flight.

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This is in fact a glory. –  Emilio Pisanty Jul 16 '12 at 16:08

Go to the Niagara falls on a sunny day, and you can see there a beautiful circular rainbow creeated by dispersion at the tiny drops of the hazy mist created by the waterfall. I have seen it and was very impressed.

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I have experienced a full circle rainbow near the city of Kolhapur, Maharashtra state, India.

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Hi Ravindra, can you support your answer by an image and a possible explanation as well. –  Ali Jul 30 '13 at 12:15

protected by dmckee Jul 30 '13 at 13:38

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