Ever thought about rainbows?

I suppose that as an ex-physicist I look at rainbows in an untypical way.  I forget that I take for granted a few things that others do not notice about them, and this is at least partly due to having some friends who are better at physics than me.  Some people have a true knack for explaining physics and I only aspire to pass on some of the fun in an accessible way (without the maths which you can find in Wikepedia's article if you like)!

I contend that rainbows are more fascinating and beautiful to me than they are to a christian who endows them with other meanings about promises from god.

If the sun is shining and you are standing in the rain, there's almost certainly a rainbow to be spotted.  Lets look at a few of the most interesting features.

What shape is it?

Does that seem a silly question?  Actually the rainbow is circular.  Normally we don't see the whole circle because the ground gets in the way, but if you are on top of a mountain, on a high building, or in a plane, you can sometimes see the whole circle.  Sometimes you see two bows, both circular, both with their apparent centre at the same place but one is a larger diameter than the other.

Obviously this one is artificial, created with a garden sprinkler, but you see the whole circle and the shadow from the camera position is at the centre of the circle, leading me to ask . . .

Where is it

Now you know that the bow is part of a circle, imagine where its centre would be.  It is always opposite the sun (and therefore always below the horizon).  Stand with your back to the sun and the centre of the rainbow will be in front of you.  A low sun gives a high rainbow (because its centre is opposite the sun, and therefore high)  and high sun gives a low rainbow.

That's why a rainbow in the evening will always look taller than one at mid day, just as this one would be if you could see all of it.

Part bows

Sometimes, as above, you only see a small part of a bow, not even the whole arc of the circle that is above ground level.  That's because you only see the light scattered from places in the sky where there is rain.  No rain means no bow.  The following picture illustrates this beautifully.

Who's rainbow is it?

You may be standing next to a friend looking at what appears to be the same rainbow, but actually you have your own personal rainbow - mine is my own too.  That's because the rays of light that reach your eyes come from different raindrops than the rays seen by anyone else.

How many bows?

When you see a bright rainbow you almost always see a second bow at the same time.  Have you ever noticed that the order of the colours is reversed?  Some people look for a third bow outside the second but they will never see it where they instinctively expect it to be.  In the laboratory the third bow can be seen but it is actually behind you, very close to the sun, and almost nobody has ever seen one without photographing the effect and then enhancing the photos.  (Back to this newsworthy topic tomorrow).

Here you see a double bow and its reflection.  I did once see a photo with eight bows (multiple reflections), but the person who presented it was not prepared to give me a copy.  I'll continue to search the web and post it if I can. 

More bows?

Don't despair.  You actually can see more  bows quite often, even though the third order bow is not obvious.  Look underneath the brightest (inner) bow, somewhere near the top.  If its a good rainbow you can see the 'supernumary bows'.  These are not as bright, and again the colours are reversed and a bit harder to make out.  Often you don't see all the colours.  I leave the physics of supernumary bows to someone cleverer than me.  I more-or-less understand the optics of the ordinary bows but these are beyond me!

Scattered!

Sometimes you can spot a fake rainbow photo very easily.  Look at the background sky.  You will always find that it is brighter inside the bow than it is outside, as you can see on some of the pictures above.  Its because more white light is scattered from the raindrops in that area to where you are.  Not all producers of fake photos know about this, but now you do.

This one might not actually be a fake, but the bows seem not to be concentric and the sky is not brighter inside the inner bow.  Let's just say that it has signs of not being genuine.

Pot of gold?

Next time you see a good rainbow with clear ground in front of you you might see where it reaches the ground.  I remember seeing a rainbow in front of some very close trees.  Your 'pot of gold' is nearer than you think, but the downside of this discovery can be inferred from the above.  As soon as you move to reach it, your own personal rainbow moves with you and you have to run VERY fast to get there. 

Miracles

Numerical ray tracing by a programme called BowSim shows that departures from sphericity in the raindrops by a mere 1-2% noticeably distort the primary bow. We have to marvel that rainbows are ever seen!

That's all for now. I hope you enjoyed a brief tour of the fascinating facts behind rainbow.


Related posts:
Quaternary rainbow observed
Glory be! - not a rainbow but it looks like one.