Google the title phrase, and you'll find a bunch of New Age twaddle. This is a physics blog, though, so here's a reliable scientific method for finding the location of a rainbow, such as this one seen over Chateau Steelypips after the thunderstorms that went through earlier this evening (it was much brighter half a minute before the picture was taken, but faded as the camera was fetched):
- Stand so you can see your shadow in front of you.
- Spread the fingers on both hands, and hold them so your thumbs just touch.
- Hold your hands so one pinky finger is just on the head of your shadow.
- Keeping that finger where it is, swing your hands so the other pinky finger traces out an arc. If there's a rainbow to be seen, it will appear more or less along that arc.
If you don't want to wait for a convenient thunderstorm to produce a rainbow up in the sky, you can always test this with a garden hose. This offers the additional advantage of not producing giant piles of hail:
The spread-finger trick works because the arc of a rainbow is determined by the optics involved-- the angle of the sun, the index of refraction of water-- and appears at around 40 degrees from the path of a straight ray from the sun. This happens to be just about the width of two spread hands held at arm's length, regardless of your size. My hands are considerably larger than Kate's, for example, but her arms are considerably shorter than mine, so the angular size of a spread hand at arm's length is roughly the same for both of us.
So, the next time you want to know where to look for a rainbow, you can use your hands to work it out.
(See also this video demonstration, which doesn't go into the angular details, but does show that you need the sun at your back, and you can probably estimate the angle from the video.)
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Comments
Good tip, I hadn't heard the fingers trick before.
And if the sun's too high in the sky (too close to noon), your fingers won't cross the horizon... so you won't see a rainbow. (Except with the garden hose.)
Also, a fist is about 10°. Count how many fists above the horizon the sun is; each fist will be roughly an hour of sunlight left.
(OK, 40 minutes, but the sun tracks at an angle... so an hour is a good rough estimate. It'll be less near the summer solstice and more near the winter solstice.)
Posted by: cisko | June 15, 2009 10:26 PM
Hey Chad, I've got a question for you related to rainbows.
I know why we get them, refraction of light from water droplets and all that.
That particular explanation has a couple of holes in it, and they've been there since I was a kid. I've never found a decent explanation for them.
The holes as far as I can see are how come it's a rain *bow*, i.e. how come it's curved and how come the light from all those little droplets ends up so neatly arranged into a rainbow?
No-one has been able to explain that satisfactorily to me, even at the PhD level I'm at now. Albeit, I'm a chemist.
Posted by: Elijah | June 16, 2009 1:10 AM
It ends up curved because the interference pattern appears at a fixed angle from the axis between the point of view and the light source. Rotate that angle around the axis and you have a circle. Cut part of the circle off with the horizon, you've got an arc.
Posted by: Dunc | June 16, 2009 6:01 AM
atoptics is my goto site for anything of this sort. The rainbow is not an interference effect, but simply due to refraction and reflection. You can work out all the possible angles from any one point in the sky and the only places that the light can reach you is the region you see as the rainbow.
I made a little gif of the ray diagram which can be found here.
Posted by: Jonathan | June 16, 2009 6:42 AM
Elijah: What Dunc said. Ask a pilot about a "rainbow". From a plane, they're not bows, but fully circular.
Posted by: Jamie | June 16, 2009 8:24 AM
Not being rude, but this is why I asked Chad. A man who can explain physics to his dog can explain to a poor, embattled chemist.
And the image on atoptics works a treat. I get that. A 25 year old mystery solved.
Posted by: Elijah | June 16, 2009 6:59 PM