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A PHYSICIST WRITES . . .
(July 2002)
No, this is not a medical column – I am not a physician! A little knowledge of physics can help you understand better why some of the things around you look and behave the way they do. It certainly makes driving more interesting, so I want to try to illuminate your journeys with examples of physics “on the road” (without of course wishing to distract you from driving safely).
Let’s look first at cat’s-eyes, which made a fortune for their inventor Percy Shaw. How do they reflect your headlamps so brightly? If they were just tiny flat mirrors they would send the light back in the wrong direction, more often than not.
Perhaps you have noticed what happens when you move around in front of a pair of mirrors mounted at right-angles in the corner of a room: your reflection seems to stay fixed in the corner. The effect is even more strange with three mirrors all at right-angles. Imagine the third mirror fixed to the ceiling – wherever you happened to be standing, you would look up to see your face (upside down) apparently fixed behind the point where the mirrors all meet.
What the mirrors are doing is cleverly reflecting every ray of light exactly back in the direction that it came from. This is how cat’s-eyes work, though they are actually made from a solid piece of glass with the three reflecting surfaces at the back. On your car the red rear reflectors (if you look at one closely) are panels of tiny plastic cat’s-eyes, again with the angled surfaces at the back.
[Correction: glass cat’s-eyes are (effectively at least) spheres and don’t have flat surfaces at the back – as I eventually admitted in September 2013.]
Number plates, road signs and reflective clothing have coatings on them which contain either these mirror triangles (in microscopic size) or else tiny round glass beads, which can also do the reflecting-back-to-you trick.
If you experience a sudden glare from a road sign on a day when the sun is high, it probably happened because sunlight reflected off your windscreen towards the sign which then sent a reflection straight back, through the windscreen and into your eyes. Don’t complain, though – at least it means you noticed the sign!
Peter Soul
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