Rainbows

[paul m]

We've a good collection of rainbow pictures in the Weather gallery. They obviously have a special fascination for some of us!
I've just added a new one which was almost a triple bow
Unfortunately I wasn't carrying my camera and one arc had gone and the other was fading by the time I found it (Trust me!).

I've only ever seen a triple bow once (in Sheffield, we're good at rainbows up here). I suppose that the 'extra' arcs are due to a double diffraction of light from the initial one - the colours come out in reverse order to the primary rainbow. Anybody understand the physics?

[paul m]

Last of the great romantics. "Rainbow" always sounds a bit dull to me, arc en ciel sounds more romantic (just a curve in the sky, I know). Are there any more interesting names?

arc-en-ciel - Wiktionnaire

Originally Posted by paul m

We've a good collection of rainbow pictures in the Weather gallery. They obviously have a special fascination for some of us!

[frostfire]

Originally Posted by paul m

I suppose that the 'extra' arcs are due to a double diffraction of light from the initial one - the colours come out in reverse order to the primary rainbow. Anybody understand the physics?

Hi Paul,

The 'extra' arcs, as you put it, are formed by a different diffraction angle than the main arc.

The main one is formed by a dubble diffraction; one caused when entering the waterdrop and the second when the light leaves the drop. They are also reflected by the back of the raindrop and when the red light is diffracted it makes a angle of 42 degrees. Red travels the smallest distance while blue travels the furthest. This means that red stays closer to the sun; so that is why red is the outer colour in the primary rainbow.

The extra arcs that are formed under go a second reflection caused by the inside of the water drop. Each reflection causes a decrease in light, so that's is why the extra arcs are less bright than the main one.
The colours are in reverse order because the light is defracted at a different angle due to the extra reflection. The light now undergoes diffraction in an angle of 51 degrees. Red still has to stay closest to the sun and there for ends up under in the spectrum.

As I didn't think my explanation wasn't the clearest, I decided to draw it!



~*~Frostfire~*~

[frostfire]

Originally Posted by paul m

Last of the great romantics. "Rainbow" always sounds a bit dull to me, arc en ciel sounds more romantic (just a curve in the sky, I know). Are there any more interesting names?

The Dutch keep it nice and simple as well, 'regenboog' (rain bow).
But I thought I would mention what people thought of rainbows before we learnt the physics behind them. The main rainbow was thought to be a bridge between Heaven and earth. In the Middle Ages people thought that the reversed colour rainbow (as Paul says the extra arcs) was the work of the devil as it wasn't as light as the main rainbow and on top of that it wasn't even properly copied: the colours are in the wrong order.
The devil could even go a step further in acting unsuitable than copying a 'real' rainbow incorrectly; by creating a rainbow that is half an upside down arc he could make a slide into Hell!

~*~Frostfire~*~

[paul m]

Scarey, what! Thanks for the explanation - I think I understand!

Originally Posted by frostfire

The devil could even go a step further in acting unsuitable than copying a 'real' rainbow incorrectly; by creating a rainbow that is half an upside down arc he could make a slide into Hell!

~*~Frostfire~*~

[frostfire]

I had a look in my Italian dictionary today and they have a more inspiring word as well: arcobaleno (arco = arc, baleneo = fire/lightening).
The Romans called it 'Iris', a term that is now used for anything to do with many colours. I believe the Spanish have dubbled up on the word by calling it 'arc iris'.

Thanks to Pauls question I've become quite intrigued to know what more primitive tribes call them and extinct/older forms of languages, as it will probably say a lot about how the people using those languages thought about them. As the words most of us seem to be using, lay the connection with it being a natural phenomenon whilst people used to see it as something sent by the God(s). Anybody know any of those as well?

[paul m]

The Romans (as always) stole Iris from the Greeks - she was the personal messenger of the Great Goddess:
MYTHS AND LEGENDS OF ANCIENT GREECE AND ROME : IRIS (THE RAINBOW).

Haven't had time to look through all of these yet:
Rainbow