Rainbows

[SteveD]

What is the diameter of a rainbow?

[jim360]

Rainbows don't have an absolute size, per se -- they have an angular size instead. If you are standing with the Sun behind you, then the two ends of the rainbow will be 84 degrees apart, 42 degrees on either side of the line that would pass from you and the Sun. But it's not any particular distance away from you.

[SteveD]

Thank you. However, that now begs the question, "Why 84/42 degrees?".

I suppose that the distance between the observer and the rainbow depends on where the rain shower which is creating the rainbow is.

If you can see both ends of a rainbow then you can calculate the distance between the points where the left end and the right end "touch" the ground. That is the diameter (assuming that a rainbow is half a circle).

[jim360]

The 42 degrees number comes from the properties of water and the refraction of light through it. I can't say I remember the derivation, but there's a bit of trigonometry involved plus Snell's Law of refraction, so it's a fixed angle for all rainbows.

I think the thing about the diameter is that it's perceived, rather than actual. In one sense it can be right next to your face, in another it can be miles away, but any "physical" diameter is basically an illusion. The pattern is caused by all the rain you see on a cone with angle 42 degrees, rather than rain that is some particular distance away from you, and the "endpoints" will just be the last pieces of land that obscure what would be the rest of the circle. Of course, using trig, you could calculate the diameter of the rainbow you see, but it will be different for every observer.

[mibn2cweus]

If you know your distance from a rainbow you can determine its diameter with this formula using 84 for angle 'A' -
http://www.astronomynotes.com/solarsys/angsz2.gif

[UzZiBiKeR]

Let's keep this simple guys. The rainbow is a product of refraction of light from water falling to the ground. Why would you want to know the diameter when it would constantly change from point to point. My question to you is why do you need to know this when just looking at a rainbow is one of the most beautiful acts of nature. There is no need for science to remedy questions when the beauty itself is enough.

[EDDIE51]

Here is one I photographed 4 days ago

[IMG]http://i64.tinypic.com/14b175j.jpg[/IMG]
It was the best one I have seen in years.

[EDDIE51]

Link did not activate

[IMG]http://i64.tinypic.com/14b175j.jpg[/IMG]

[EDDIE51]

Still did not work, you will just have to copy and paste .

[jim360]

"My question to you is why do you need to know this when just looking at a rainbow is one of the most beautiful acts of nature."

Why not both?

[SteveD]

[Old_Geezer]

You don't need the img tags, Eddie. It's what's messing it up.

[Coppit]

Once I sheltered in some tall bushes from a shower and looking out through a gap into daylight there was a perfect small rainbow about 1m diameter.

[scooping]

That's the daftest question I've heard for a long time. Are prizes available?

[SteveD]

Although this question resulted in a couple of intelligent answers, it now appears to be deteriorating into insults so I am unsubscribing to it.