Chasing Rainbows

I just re-read Richard Dawkins' book called " Unweaving the Rainbow" which, logically enough, gives a great explanation of how rain makes a rainbow appear under the right conditions.

 Rainbows are pleasing to look at,  and the way they come about is fascinating, so I wanted to share about it.

First, somewhat disappointingly, I have to report that there is no 'end of the rainbow'. Hence, there are no pots of gold, nor short drunk Irishman either. Wait a sec...What's that, Janet?  Oh, I'm sorry, apparently those little green people are called "Leprechauns" (not that there's anything wrong with that).

 But I digress.

OK, here we go. Because a rainbow depends on light being reflected back to the observer, it can only be seen when the sun is somewhere behind the observer, and not too high in the sky (otherwise, the angle that the light hit the water drops would send the reflections right into the ground, which would make them very hard to see).

A rain drop bends light in a similar fashion as a prism. When sunlight is passed through it, the light is diffracted (bent), and the different colors are bent at different angles.  The result is that some of the colors come out higher than the others, and some lower, and when they are all separated, we call it a  rainbow if its in the sky, and the 'visible light spectrum' when it's in the lab.).

Prisms differ from Raindrops in one important way, however. They hurt a lot more when they fall a few thousand feet and hit you on the head.

When light hits a raindrop, some of it passes right through, some of it is absorbed, and some of it goes through the front side of the raindrop, hits the curved inner-back surface, reflects off the back  and comes out the front, eventually reaching your eyes.  Depending on how tall you are (or where you are standing) only a single part of the color spectrum hits your eyes.  If a particular raindrop reflects orange to your eye, the ones just below it will reflect only yellow to you, the ones above it will reflect red, and so on.  Longer wavelengths of light (red) are bent the least, while shorter wavelengths (violet) are bent the most.This means a taller person standing next to you will see the colors at different places than you, your rainbow would have red where his has orange, your orange is at the same height as his yellow, and so on down to where you will see Violet when he hits Ultra-violet and sees noting

The visible light spectrum follow the acronym  ROY G BIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet).

By the way, the rest of the frequencies on the Electro-magnetic Spectrum will be familiar to you as well.  'Above' Ultra-violet are higher frequency X-rays and gamma-rays (which is what they used to zap my brain last time around), going 'south towards the shorter frequencies are Infra-red, Microwaves, and a wide swath of radio waves at the 'low frequency' end. Due to their extremely long wavelengths, radio waves can travel long distance on earth, as they tend to 'bounce' between the earth and the atmosphere as they work their way around the world. Can you say "a.m. radio"?

Back to rainbows.  You would have trouble seeing a color reflected from only a single raindrop, but because there are so many raindrops falling, together they reflect enough light that you can see wide bands of color.

Now, because a raindrop is falling, the particular color it reflects back to you will change as it falls. Remember, each raindrop actually reflects all of the colors, at slightly different angles due to their diffraction, but only one color frequency hits your eye from each raindrop at any given height. Rainbows are usually seen with the red at the top. The location of each color reflected off the back of the raindrop and striking your eye is always at a fixed angle measured from a line between your eye and the sun. This angle is approximately 42° measured to the top of the red band and approximately 40° to the bottom of the violet band. Say wha..?

When drops are higher in the sky, the suns rays are angled back well over the head of the ground dwelling-observer. As the raindrops fall and become level with  the sun, the suns' rays slap the water drops at flatter and flatter angles, until you see violet from the lower raindrops, up to red at the top, the full rainbow (it's interesting to consider that before the raindrop reflects red to you, it has already reflected infra-red. You just can't see it.).

So we have a 'sheet' of raindrops falling, and as each of them passes through the spectrum, you see a rainbow. It's a bit like the way a wave passes through water in the ocean, the energy moves but the water molecules stay where they were (except for 'local' motion). In this case, the particles move, and the 'energy' stays the same. Because the colors stay put, you see the rainbow as stable, when in reality the rainbow is a cauldron of intensity, millions of water molecules and other airborne particles providing a visual delight, truly one of natures' greatest wonders.

That's it!  A vast number of raindrops are falling, each one reflecting only a sliver of light to any one observer, yet together they create a beautiful streak of color in the sky.

By the way, a rainbow would be a full circle, except the ground gets in the way. 

In researching rainbows, I came across many online discussions about the possibility of standing inside the 'end of the rainbow', bathed in beautiful colors.  Without belaboring the point, the science behind how a rainbow is formed and viewed precludes this exciting possibility. That said, there are many people who swear they have done just that (although none of them claimed to have found any gold nor been accosted by any wayward leprechauns).

I highly recommend Dawkin's book Unweaving the Rainbow, which is full of easy to understand explanations of other miracles of nature, (and for that matter, any of Richard Dawkins' other books as well, especially the Blind Watchmaker).  

Some critics have complained that by 'Unweaving' the Rainbow,  Dawkins has ruined the beauty of it for them by explaining the science behind it. I don't agree with this at all, I find that understanding the physics behind this 'spectrum in the sky' makes it even more spectacular (choice of words intentional).  Honestly, I am not entirely convinced that 'spectrum' and 'spectacular' come from same root. But had I not pointed out the similarity, 96.3 % of my readers would have assumed they were, and that I must be smarter than I appear. (I suppose it would be pretty disappointing if I weren't!)