Rainbow science

Rainbow science

A rainbow appears when drops of rain split the Sun’s white light in Red / Orange / Yellow / Green / Blue / Indigo / Violet (order from the top down). There are always two rainbows: the primary and the much fainter secondary (43% less light spread over 1.8x the angular distance of the primary). The secondary is turned around, so that the rainbows’ two red bands face each other. The distance between them is 9°.

I’ll spare you the math, but the diffraction angle for violet is around 40° and that of red is 42° - this tells us that the thickness of a rainbow is roughly 2 degrees (2.36° to be more precise). The center of the rainbow is the antisolar point – the point exactly opposite the Sun. It is always below the horizon and goes as far below as the Sun is high in the sky. This means that larger rainbows will appear when the sun is low, after sunrise or in the evening. When this particular photo was taken last Friday, the sun was about 18° in the sky. The center of the rainbow was 18° under the horizon and the angular distance between the center and the top of the rainbow is about 42° (diffraction for red light)… that means the red band of the primary was 42-18= 42 degrees high. As I’ve said before, the secondary is some 9° away and almost twice as thick, spanning on this particular occasion between 33 and 37 degrees above the horizon.

Three zones of luminosity can be observed: the brightest under the primary, the darkest in between the two rainbows, and the third above the secondary.

Under the right conditions, the primary bow displays several other greenish-purple bands of color below its lowest, purple fringe. They are called “supernumeraries” and you can only see one (barely) on my picture.

Leaving math and optics aside, this was probably the brightest I’ve ever seen… the photo I’ve posted is a .jpeg with minor enhancements: contrast and exposure. White balance is correct and no hue / shades /saturation changes have been made. I’ve actually shot a hundred or so photos in hope of stacking them to maybe see more supernumeraries and a brighter secondary, but after waiting 8 hours for DSS to align and stack them, the results were catastrophic. I may try again, but not much hope of getting anything worth showing.

There you go.

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Thanks for the informative post, complete with stunning illustrations.
Have a crack at stacking them with Registax or something else suitable for planetary/lunar images. It may be better suited to aligning on the hard edges in your image, rather than looking for stars. Just a thought.

Steve.

Cheers for your answer NZ Steve,

I've stupidly discarded all images taken without having tried Registax.. but seriously when the stacking program tells you estimated waiting time is hours and hours you start verbally abusing the CPU, then progress in general form making the machine obsolete, and so forth

I've tried DSS as I thought the diffuse rainbow would be more of a nebula type object rather than a planetary one - major brain short circuit as what matters is how the software aligns. Thank you for the enlightenment, now I have to wait for another rainbow.

On a different note, I've seen some impressive pictures where the arc was almost a full circle... taken from mountains mainly..

The dark zone in between the rainbows is called Alexander's Dark Band.

Yes. more info. moooore info.

Add your knowledge, it's most welcome.

Does anyone have a capture of a triple? I saw one once, but was not 'with camera' at the time.
I'd love to see a full circle rainbow. I always keep an eye out on airplanes, but alas I almost always nod off lol

they're never triple, the third is rather a reflectING or reflectED bow... either way, very rare and a treat if you catch it.

For extensive info, check the "atmospheric optics" section on spaceweather.com

A rainbow sparking interest among ultra-tech photo guys, this is something lol

DOUBLE RAINBOW OMG ALL THE WAY!!! OMG OMG OMG
http://www.youtube.com​/watch?v=OQSNhk5ICTI for those who havent seen it

on a serious note thanks for the info, learned something new

Great shot and thank you for the info. This is the only double I ever caught with camera. Was taken after a shower during the Penn Relays track meet at Franklin Field in Philadelphia around 5pm EDT on 4/22/10.

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Hi there,

Very nice, bright rainbow.. the second one is well defined as well - great catch.

Anyone have a moonbow?

Thanks for the nice comment. What the heck is a moonbow?

It's the same as a rainbow, just that it happens at night and the sun's light is reflected by the moon.

it's much fainter, barely any colours in it and thus a nice prize for those who get it on camera.

let's start an atmospheric optics contest - rarest phenomenon wins. then winner gets to pick the topic.

or take a photo of a cloud formation that somewhat resembles the shape of your country.

Awesome thread!
Rainbows are getting a lot of attention these days thanks to youtube, but they're always amazing.

footdoc66 wrote in post #10631052
Thanks for the nice comment. What the heck is a moonbow?

The elusive moonbow:

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!!!

That's impressive.