ooh whilst you're here
if you teach physics can you explain rainbows to me please?
( Kitty v1.0 desperately naive, Thu 25 Mar 2010, 10:27, 2 replies, latest was 16 years ago)

I think dogs sick them up.

( girlinthehole, Thu 25 Mar 2010, 10:29, Reply)

hahaha

( Kitty v1.0 desperately naive, Thu 25 Mar 2010, 10:30, Reply)

you missed out the crucial bit
where they have to be kicked to make them sick
( Vipros. clever got me this far, then tricky got me in, Thu 25 Mar 2010, 10:31, Reply)

kicking dogs until they make a sick
is not where rainbows come from!!
( Kitty v1.0 desperately naive, Thu 25 Mar 2010, 10:35, Reply)

It must be true because DonkeyGums said so.

( girlinthehole, Thu 25 Mar 2010, 10:38, Reply)

^this

( Vipros. clever got me this far, then tricky got me in, Thu 25 Mar 2010, 10:49, Reply)

Sorry, I had to bugger off to vivafy them as you posted that
So, rainbows: as you are probably aware, the light that reaches us from the sun contains all the basic colours that are visible to us. If we think of light as a wave*, each colour is a different frequency. The sun emits all of these frequencies, and they blend together to give "white light."

Now, think back to the cover of Dark Side of the Moon. This shows a beam of white light passing into a prism, and emerging on the other side as a rainbow at a funny angle. This happens because, when light passes through a different material, its speed changes. So initially it is travelling through air, then it enters the glass and is forced to slow down. Also, because the glass is at an angle to the beam of light, the path of the light bends. This process is called refraction.

Now, different frequencies of light will refract more than others. Blue light has a much higher frequency than red light, and therefore its path is bent at a greater angle. Therefore, when the light emerges from the other side of the prism, the colours have all been refracted to different extents and therefore come out in separate beams, as opposed to the single beam of white light we started with.

In the case of rainbows, you see the same process, except in droplets of water rather than glass prisms.

Does that make sense? I hope I've managed to explain that reasonably well, but it did take a while to get my sister to understand this a few weeks ago so I'll be happy to take questions.

*<sub>It's more complicated than that, but I'll spare you the quantum stuff

PS Ergo is the Latin for "therefore," and HTML italics are achieved by putting an 'i' in triangular brackets (chevrons) to the left of the word and a '/i' in chevrons to the right of the word.</sub>
(LongJohnBaldry, Thu 25 Mar 2010, 12:08, Reply)

I knew the latin
but not the html, so thanks!

The rainbow was explained by you much better than by my GCSE phsyics teacher, who I think hated me.

Why does the rainbow always have a feinter twin? And why can you not find the end of it? Surely the lightbeams reach the earth at some point?
( Kitty v1.0 desperately naive, Thu 25 Mar 2010, 12:14, Reply)

I must confess I had to head to Wikipedia for that one
But basically in some cases you get a reflection inside the water droplet, so whilst most of the light is refracted through to form the first rainbow, a small amount of it is reflected back in a different direction. Obviously, in order to leave the raindrop, it must go through the same refractive process, and so you see a feinter secondary rainbow.

As for the end of the rainbow, this picture might partly answer your question.
(LongJohnBaldry, Thu 25 Mar 2010, 12:23, Reply)

oooh wow
I was hoping that would be the answer, now I can pursue my official career as a rainbow hunter!!

Thanks for that, I feel enlightened.
( Kitty v1.0 desperately naive, Thu 25 Mar 2010, 12:33, Reply)

You're very welcome
There's something very satisfying about imparting knowledge to someone else. Especially if they understand whatever I'm blithering about.
*is satisfied*
(LongJohnBaldry, Thu 25 Mar 2010, 12:36, Reply)