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A PHYSICIST WRITES . . .

(March 2007)

As everyone knows, light travels in straight lines. If it didnít, driving (not to mention life) would be difficult, even dangerous. Straight roads might appear curved and vice versa. A vehicle could be approaching you from a different direction than the one in which you can see it coming. As for your headlights, they might seem to be reflecting off things straight ahead but in reality could be lighting up the road round a bend. (A benefit or not? I canít quite decide!)

But enough of this nightmare vision ó letís think about light when itís behaving normally. This does sometimes include changing its direction from one straight line to another. What I have in mind is reflections. When you view a mirror at home, you can see yourself and whatís behind you in the room, all matching up of course with whatís there when you turn around.

Now Iíve tried to describe before (July 2003) the vast amount of high-speed processing that the subconscious brain needs to do, as it takes in the rough images supplied by your eyes and then forms the 3D picture of the world around you that you Ďseeí in your head. And Iíd say that if you are also looking at a mirror, your brain is having to work even harder in order to make instant sense of the reflection and of where the things in it really are.

So imagine the hidden mental effort thatís required when you are on the road and regularly looking in your rear-view mirror! Yet if there are vehicles in view, you seem to have no difficulty at all in judging how far behind you they are and how fast they are catching up, from just a couple of glances. My car has a total of five rear mirrors (counting the blind-spot ones stuck on the two side mirrors) and a quick scan across them gives me a complete sense of whatís behind and alongside. How on earth does the brain assemble it all, in no time at all?

I do remember, though, that not long ago I was in the outer lane of a dual-carriageway, pulling away from traffic lights. I looked in my centre rear mirror and noticed a white van in the left lane, just far enough back for me to move across safely. I checked the left side mirror and there it was too. I was about to signal and move over, when an alarm bell rang (inside my head): how could I be seeing so much of the van in each mirror? Youíve guessed it ó there were two white vans! You learn something new about hazards every day...

You also learn to use reflecting surfaces that arenít meant to be mirrors: shop fronts, house windows, even the surface of another car (if itís dark enough and clean enough). From these you can get warning of other vehicles approaching round a bend, or a view of the gap as you reverse your car toward the one parked behind, or confirmation that all your lights are working. You can also check the alignment of your dipped headlights from how they reflect off the car in front.

But back to light and the straight lines that it travels in: sometimes the lines become visible themselves. Iím thinking of projection lamps in a smoky atmosphere or, more naturally, the rays of sunlight that emerge sometimes from clouds (again requiring a hazy atmosphere). When you do observe a special cloud effect or a sunset itís more Ďlocalí to you, and to people nearby, than you might think. From only a short distance away it would probably look quite different. So stop the car (if you are driving), take a rest and enjoy the sight in safety!

And while youíre relaxing, consider this: when rays of sunlight become visible, they give you the strong impression that they are diverging from each other ó but in reality theyíre approaching you almost exactly parallel to each other. How could they not be, after all, when they point back to a sun thatís millions of miles away? Itís rather as if you were standing inside a long tube or tunnel that has lines drawn on it from one end to the other. The lines appear to be radiating out as they get nearer to you, but really of course they are parallel.

Wait a minute: if you turned around (in the tunnel) you would see the lines meeting again in the distance. But the sunís rays canít do that behind you, surely? Well it seems unlikely, but parallel means parallel, so it must be possible. You will need just the right sort of clouds and haze on the opposite side of the sky from the sun for the rays to be visible there. Also, they will all still be pointing in a downward direction, in fact converging on a point below the horizon ó the same angle down below it that the sun is up above it.

Did I read about how rays of sunlight could converge again like this (opposite to the sun), or did I work it out for myself? I canít remember. It was a long time back anyway. And then a couple of years ago I actually saw it! The effect was even more weird than I would have expected, because the rays were only visible in the distance and they seemed to be coming up from another Ďsuní below the horizon (rather than going down and away). I suppose I was on a motorway at the time, or I would have stopped the car and taken a photo of this once-in-a-lifetime(?) sight.

While writing this piece Iíve also thought of a few ways (not all unconnected with driving) in which light doesnít travel in straight lines. Next time, maybe.

Peter Soul

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