Talking about the thunder storm in Pretoria last night, a co-worker of mine mentioned that she covered all the mirrors in her house because they can “catch and reflect” lightning… She said that lightning can behave like light and be reflected. I argued that lightning is not light, but a raw, electrical charge… I have never heard of this practice, and although it sounds like superstition to me, I was wondering if there is any truth in it? Please en”lighten” us!
Many years ago, I heard of an old lady who believed that it was very dangerous to have empty light sockets, because somebody might turn them on and then electricity would leak out and start a fire. I laughed at that story, just as I laugh at your colleague. Still, it’s worth remembering that not everybody enjoyed high-school science so here’s a recap on what lightning is, and what it can do.
When large turbulent clouds form, they build up a powerful electric charge through a mechanism that’s only partly understood, although the general principle is the same as when you make static electricity by rubbing a balloon with wool. As the charge builds, it creates an electrical field between the cloud and the ground. As that field gets stronger, the air begins to ionise until there are enough ions to provide a path for the electricity to discharge. Once current begins flowing down this path, the air in that path gets hotter and ionises even more, which lets more current flow to make the air even hotter and more ionised. Within microseconds, the amount of energy passing through that path heats the air so much that it begins glowing – not red hot, or even white hot, but ultraviolet hot. For the brief moment that the current is flowing, what we have is a lightning bolt which is basically air heated so much that it acts like a wire in an electric circuit. Once all that energy has been dumped to Earth, there’s nothing to sustain the electric field, the current stops flowing, and the lightning bolt disapears. Meanwhile, all that superheated air has expanded violently, in a loud explosion which we hear as thunder. Fun fact: This happens somewhere on Earth many times every single second!
So we’ve established that lightning is an extremely intense electrical discharge flowing along a temporary path of ionised air. Suddenly the original question, about lightning being reflected by mirrors, doesn’t even make sense. Although a mirror could possibly conduct electricity (they’re made by plating the back of a piece of glass with silver or aluminium, which are both highly conductive), it certainly couldn’t reflect it. If a bolt of lightning were to strike a mirror, it would simply blast through the mirror.
Now lightning is very bright, emitting a lot of light. Mirrors can reflect this light, if it happens to shine on the mirror, with ease. But the actual bolt itself won’t deflect from its original path towards the mirror simply because the light is being reflected.
That said, lightning can do some pretty weird things. There are records of lightning bolts striking and killing a man inside a movie theatre while leaving the building intact and his fellow patrons unharmed. Lightning bolts have been known to strike telephone wires and kill people using the phone kilometers away. The faint electrical fields around radio transmitters (like the ones we all carry around in our cellphones) have occasionally been known to attract lightning. So it certainly is possible for lightning to enter your home and strike your mirrors, if you’re spectacularly unlucky. But it could just as likely avoid them. Covering them up makes no sense, and certainly won’t change that particular roll of the dice.
What does all this have to do with astronomy? Well we can talk a bit more about mirrors. Most astronomical telescopes (and almost all home-made ones) use curved mirrors to bring light to a focus, instead of lenses. The curve is very subtle, similar to a shaving mirror, in that the actual curvature is hard to see, but the magnifying effect is obvious. Unlike a shaving mirror, the curve is also extremely precise — accurate to a a few millionths of a millimeter. Because the optical performance is so sensitive to the accuracy of the curve, telescope mirrors are silvered on the front of the glass to avoid the extra distortion and internal reflections that you get in regular mirrors. The cost of this feature is that the reflective surface (usually aluminium – silver is hardly ever used anymore because of the dangerous chemical processes needed to apply it) is exposed to the corrosive effects of air pollution. Depending on the cleanliness of the air, a mirror coating lasts anything from 6 months up to many years, and will eventually need recoating. And it is absolutely not lightning-proof!