h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Does anyone have any idea?

I know it's something to do with light being refracted in water droplets but I feel that's a bit of a simplistic explanation.

Rama

Light being refracted through water droplets is a simple explanation but is exactly what happens. However, as "white" light is made up of the range of 7 visible colours and each of these colours has a different wavelength and frequency (see any A-level, Undergrad physics book) they spread or refract differently. Hence we have the rainbow created by a spread of the refracted colours that make up "white" light.

Hope this sheds some more light....

Yep rainbows are created through refraction of water droplets. Usually (possibly always) there is a rain storm between the sun & the rainbow. I remember seeing an interesting episode of Open University one Saturday morning on the subject. If memory serves me right the rainbow occurs at about 47 degrees from the horizon. But the interesting bit is that it's possible to get a double rainbow, one that occurs 47 degrees below the horizon (I think you need to be on the top of a cliff). I guess, in theory, you should be getting a circle i.e. top half rainbow is normal, bottom half you don't usually see.

Remember ROYGBIV - Richard in York Gave Battle in Vain.

BBS

Apparently you can see the full circle from aeroplanes.

Perhaps what might help is understanding what "refraction" is. A technical definition of the term would go something like: "the change of propogation of a wave when it passes from one medium into another in which the wave velocity is different." (For simplicity's sake, I'm considering light as an electromagnetic wave since it behaves like a wave in this respect, but refraction also happens with sound waves as well as waves on the surfaces of fluids).

To understand refraction a little better, one might consider the analogy of an army formation marching due north across a nice flat field. They're perfectly in step with each other, a real show of discipline and control. Imagine that these soldiers only know one thing: march north, and march at a quick-march speed. Since they're all doing this and since we'll pretend that they all march at the same speed as each other, something strange is about to happen when they come to a large river that they must ford. Let's say a soldier's speed through waist-deep water is about 2/3 that over dry land. Let's also say that the river is flowing southeast, or in other words at a diagonal to the soldiers' northward march. So, the soldiers on the closest column will hit the river first and slow down, whilst the other soldiers in the next column still have more land to cover until they reach the water, and so on for each consecutive column. If one were flying above the formation watching them, an odd thing would be noticed: as the entire formation approaches and goes through the river, they get "scattered" or "spread out" by the river interfering with the progress through the open fields. When they reform up on the other side, the columns will no longer be lined up in the same rows as they were before, and the whole thing will be skewed.

This a bit like what happens to light waves when they hit water droplets, except its a little more complex because of the geometry of the water droplets (they're more or less spherical).

Hope that helped a bit.

I have often seen a double rainbow. The second one is about 50% less bright, twice as 'thick' as the primary and about 10 to 15 degrees higher, I was hoping that one of the poetic types was going to give a far less prosaic description of rainbow building. Perhaps something to do with armies of leprachauns building a bridge to their pots of gold?

I don't know how other Rainbows are created, but I think I was the result of a drunken night in Norwich!!

That's basically it, but rainbows only occur when you have your back to the sun, because the light actually goes through the raindrop and bounces off the inner rear surface, coming out the front again and into your eyes in its refracted form. So it's not quite that simple... not that it's not possible to make rainbows without this - anyone playing with a light and a triangular prism in a school science lesson will know that - but that's how they occur in nature.

The rainbow you see is the result of light being refracted - split or spread into its constituent wavelengths - by countless millions of water droplets. The shape is circular because the angle for each colour is at exactly the same distance across and around your field of view for each colour, and semi-circular if you're standing on the ground because there is no line of sight below ground level to complete the circle.

Traditionally we say there are seven colours, but it's hard to see more than five or six (red, orange, yellow, green, blue, but forget indigo and sometimes violet). In fact, of course, it's an uninterrupted continuum of colours, since each of the millions of water droplets gives you a slightly different part of the spectrum.

What really stops me when I think about all this is that the rainbow you see at one moment is made up from refractions through a different set of droplets from that of the moment before. They're falling through the sky, after all; it's just their position relative to the source of light and your eye which determines what colour they send in your direction.

Even more humbling is the thought that - in one sense - the person next to you doesn't see the same rainbow at all. Same light source, same colours, but a completely different set of droplets. And the person behind you, looking over your shoulder so that her angle is almost exactly the same as yours might see red from the same set of droplets that are sending you green.

1. You can make your own mini-rainbow with a garden hose on a sunny day - just experiment with a fine mist and spray it around at different angles with the sun behind you. Plant misters work well too, especially the big ones that you pressurise with a handpump. There's no pot of gold, though.

2. A colleague who owns a plane (why is there no smiley for jealous) confirms that rainbows do indeed appear as a complete circle from the air - and that you can even get a double one, which must be impressive.

3. Poetic bit - I'm told that it's said in the Highlands of Scotland that when the sun shines while it's raining there's a wolf's wedding taking place. To another wolf, I presume.

I used to be a bridesmaid, but I'm all right no-o-o-ow!
Now why did I say that?

If it sunny and raining and you think there should be rainbow but can't see one, try this. You will look incredibly stupid but you might find a faint rainbow!
Stand with your back to the sun with your arms out staight in front of you. Splay all of your fingers out as wide as you can and put one hand above the other-thumb touching little finger. Line up the bottom hand so that the thumb is over the shadow of your head on the ground.
Still there?
The rainbow should be roughly where your top little finger is.

Somebody turn the sun on. I want to try it!

This may be a bit quibbly but I thought the droplets *diffracted* the light not *refracted*. at least that's what I was taught at A level - I remember being marked wrong because I said it was refraction.

For those who don't know refraction occurs when a wave encounter a different medium to pass through and its direction changes, different wavelengths are affected differenlty and so the white light spreads out into its spectrum. Diffraction is when a wave passes through a narrow opening of the same order of magnitude as its wavelength and is spread out in a semi-circular pattern - again this has the effect of spreading out the wavelenghts of white light so you wouldn't see much difference.

Hmm. I hope you didn't get marked wrong for saying that rainbows were created by refraction instead of diffraction. Refraction is the phenomena that causes rainbows (that is, the light is refracted by the water droplets), not diffraction. Diffraction causes scattering of light, but not dispersion. Diffraction is any redistribution in space of the INTENSITY of waves that results from the prescence of of some object that causes variations in either the AMPLITUDE or the PHASE of the wave. (I capitalized the key words just to make them stand out, not to shout at anyone ) So, diffraction is indeed responsible for certain "scattering" effects on light, but not for "bending" it or splitting it up into its separate colors (the term for that is "dispersion" which might have caused the confusion to begin with). It's hard to imagine what's going on with light waves, since it's hard to mimic certain things that happens to light on a scaled-up level (as in waves on water).

To help define the difference between diffraction and refraction, its crucial to understand that wavelength and wave velocity are affected by refraction and amplitude and phase are affected by diffraction. Color is a function of the wavelength of light, as was pointed out, not of amplitude or phase. Diffraction is not responsible for changes in the velocity of waves (either speed or direction) through a medium.

Examples of diffraction can be seen by making a slit in a piece of paper and holding it close to an intense light source to see several "slits of light" shining on a dark wall, or it can be seen by making ripples in smooth water with a very small, stationary island in it.

It can be seen why I chose a simplistic explanation for my original response. Things can get very complicated very quickly. To make it very simple, I thought I'd leave out the geometry of the water droplets and all the interaction between refraction, double refraction, and internal reflection. Since this isn't a Physics course, I'll just happily refer the reader to a physics book (any good college-level one should do, I guess that's what was meant by "A-level") or a handbook on the fundamentals of optics.

I'd encourage anyone to look up what diffraction of light has done for science (of both the very very small and the very very far away), as well as how refraction has allowed us to with bad eyesight to see clearly again and even internal reflection as the basis for fiber optics. Not to mention sunglasses and the whole science behind polarization. Light is fantastic.

Hmmmm. OK, so UV and IR spectrometers don't use diffraction gratings to disperse light into various wavelength so you can scan a spectrum then?

In fact they do.

Diffraction would most certainly explain why rainbows are circular. Sorry but I'm not convinced by the above. Idefinitely did get marked wrong for saying the rainbow effect was caused by refraction

Also, you won't get light to diffract by putting a pinhole in a piece of paper - for a wave to diffract it has to either pass through a slit (or around and object - say a water droplet ) that is of EQUIVALENT size to its wavelength - therefore to get light of say 600nm (green light) to diffract you need a slit of width 600nm - not easy with a pin.
Also, anyone who has ever studied diffraction to a reasonable level knows that the interference pattern you get when a single wavelength is allowed to diffract through multiple slits is dependent on the wavelenght of the light - I can't remember the exact equations but blue light (short wavelength) gives a narrower interference pattern than red light (long wavelength) so if light is passed through a diffraction grating of appropriate slit width you do indeed spread the light into a spectrum - often far more effectively than with a prism. As I said, many infrared and ultraviolet spectrometers (and I use these in my job) often use diffraction gratings to split the light source into its spectrum (although admittedly the most modern ones simply created a double beam interferogram and fourier transform the result into a spectrum).

Why do you get a circular rainbow if its caused by refraction? If a beam of light is diffracted by a water droplet you will most certainly get a circular diffraction. Also how does one get a double (or even triple as I have once seen) rainbow with refraction. Easy with diffraction, its simply the second constructive interference band. The atmosphere definitely does refract light, that is why the sun appears red when its low in the atmosphere, if refraction was the answer its hard to imagine why you don't get a rainbow every time there is a clud in the sky. If diffraction is the answer then you must get a fine mist with water droplets of diameter 200-800nm (roughly the wavelength range of visible light) capable of diffracting the light. A much rarer ocurrence - hence the relative rarity of a rainbow.

I *am* prepared to be wrong here BTW, but I have most definitely always been taught that rainbows are caused by diffraction. I think they usually tell people its refraction as that's less complicated than diffraction and therefore easier for your layman to understand.

I look forward to further discussion

Thanks for all the replies so far.

The diffraction versus refraction debate seems to be going strong. So I would like to add this into the mix.

Do Rainbows only occur when the sun is behind you?

If this is the case then they must be made by refraction as the light will be 'bent' as it passes through the different medium of the raindrop.

If however the sun is in front of you diffraction would be taking place as diffraction will not be able to bend the light beam.

Opinions please.

Rama

Can't say as I've ever noticed where the sun is when there's a rainbow (too busy gaping at the rainbow ) but no, the light can be diffracted whether its coming from behind, in front or sideways to you.

I would imagine that the sun has to be behind you simply because the intensity of its light would overpower out any scattering effect whether by diffraction or refraction.