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A PHYSICIST WRITES . . .
(September 2013)
When I get something wrong, I’m the first to apologise and to correct myself ... even if it was only slightly wrong, and eleven years ago! In the very first of these columns (July 2002) I described how a set of three mirrors, when angled as if in the upper or lower corner of a room, cleverly reflect light back in exactly the direction it came from. I said that this is how glass cat’s-eyes work (ie, having the three corner-reflecting surfaces at the back of the solid glass) and I implied that they had been made this way since being invented by Percy Shaw in the 1930s.
Here’s the correction: glass cat’s-eyes (as their name suggests) have always been simple spheres – maybe with the ‘side’ of the sphere removed all round to save glass and space, but anyway with a curved rear reflecting surface (not three flat ones). This works nearly as well as corner-reflectors in sending the light straight back. Or rather, it probably works better for drivers, because the light will spread out a little and so find your eyes, instead of (in theory) being reflected back only into your headlights.
It’s transparent plastic that is easily moulded to produce a corner-reflecting solid, or (more economically) a thin array of them: this is what you find in some road reflectors and the ones on cars and bikes. And while I think of it, I suppose I should be making an apology also to Reflecting Roadstuds Ltd (which was Percy Shaw’s original company) for not copying its proprietary name “Catseye” precisely!
So: why have I revisited cat’s-eyes now (and hence noticed my error)? It’s because although the basic item has hardly changed in 80 years, a reader has asked me about some recent new versions of it. These produce their own light instead of reflecting yours, and being significantly brighter too they show you the lie and the layout of the road much further ahead.
I haven’t knowingly seen the new cat’s-eyes myself: I’m sure I would have noticed the difference. They are not even bulbs, but light-emitting diodes (LEDs), powered in many cases by rapidly pulsed current. As with rear vehicle lights of the LED type (see my February 2009 column), you may be able to detect the pulsing by flicking your eyes across the field of view.
The electricity is supplied in one of two ways: either from wires laid under the road, or else from a rechargable battery which is charged up by a solar cell at the top of the road stud (this is a better name for them than cat’s-eye) during the daytime. The solar-powered studs are simple to install, and of course cost nothing to run. You will appreciate that they are no good for tunnels, however, and the A3 tunnel at Hindhead is one stretch where the wired type has been put in. Doing it this way is more complicated, but there is a particular advantage in having control over the illumination: the studs can be used in guiding and managing the traffic, for example when the flow needs to be reversed along a lane through a tunnel.
The next generation of the solar-powered studs could well be linked by two-way radio to a traffic controller – human or automatic – and be used not only similarly in traffic management, but also for supplying information on road conditions. For now though, what impresses me more than anything about self-illuminating road studs is that their lifetime under the onslaught of countless wheels (not to mention occasional snow-ploughs) is claimed to be several times that of traditional cat’s-eyes. What a triumph for tough electronic circuitry!
Early this month, a news item linked physics and motoring in a novel way. In case you missed it, a man parked his Jaguar on the shady side of Eastcheap in the City of London, and came back an hour later to find the plastic bits of the car in a sorry state. The joint culprits were the sun and a nearly completed skyscraper, known as the Walkie-Talkie for its top-heavy design (the idea being to increase the rental income from the more elevated offices). As a consequence of this design, the south-facing wall of glass formed a concave mirror which was known (even before this month) to focus the light and heat down into the street, where vehicles melted, shop frontages bubbled and smouldered, and eggs could be fried.
And the problem was entirely predictable, because it had occurred previously – in front of a hotel in Las Vegas designed by the same architect! Anyway, I’ve studied several photos of the Walkie-Talkie building in the sun, and I can report that the optical efficiency of its glass wall, as a focusing mirror aimed at Eastcheap, is in fact rather poor: the street might have become a great deal hotter if the curvature of the wall had been only slightly different. Though even as it is, there are probably locations on the roofs of nearby buildings where the reflection is better focused (depending on the time of day) and the pigeons are in considerable danger of vanishing in puffs of smoke.
Since writing in July about trading in a 16-year-old Corolla for a three-year-old Golf, I’ve got to know my new car a bit better. Its lesser features include nice ones that were a surprise to me (such as a rear-view mirror that dims automatically when bright headlights approach behind); others that are good to have again after a dozen or more years without them (knobs on the radio, for volume and for tuning – as well as preset station-buttons of course); some features that are totally pointless (a twin-exhaust outlet on the end of a single-pipe system); and a few that are definite steps backward, such as not displaying the total mileage while I’m on the move ... where’s the point in my wanting to take this car, like I did its two predecessors, to 100,000 miles (and beyond) if I won’t actually see this happen?
Peter Soul
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