Attempting to explain the operation of the simplest optical device known to man, was the practical demonstration in my Inaugural Lecture at the University of Southampton, which took place on May 18th 2005.
The “Eureka” moment as the mirror’s ability to laterally invert text was seen to be illusory caused quite a stir in the audience and was a talking point for colleagues for weeks afterwards.
I chose this particular experiment as amongst other things, batteries and power were not necessary. I had seen from my colleagues’ previous Inaugural Lectures that anything that can go wrong will go wrong, especially if electrical power is involved.
Everyone is familiar with the mirror’s apparent ability to laterally invert text which gives rise to so-called “mirror writing”. I had always been unhappy with discussions about lateral inversion in mirrors, especially since as far as I was concerned all a mirror did was reflect light and I could see no mechanism for laterally inverting objects as well. So a couple of years before giving the Inaugural Lecture I decided to apply the scientific method to lateral inversion in mirrors and resolve the “mirror problem” to my satisfaction.
|Figure 1: Showing expected lateral inversion|
Figure 1 shows the reflection of “The Matrix” in a mirror (I use the same words that I used in the Lecture as the underlying theme was questioning what we call “reality”). As you can clearly see, since the camera does not lie, “The Matrix” appears laterally inverted when you look at the words in the mirror – absolutely nothing strange there, we see this every day.
But a mirror simply sends light back to the object, assuming the object lies perpendicularly in front of the mirror, so where does the lateral inversion come from?
Write or print “The Matrix” on a piece of paper and hold the text up in front of your eyes, you can see “The Matrix” quite clearly – no lateral inversion at all. Now rotate the paper 180 degrees so it is facing a mirror and look into the mirror, you now see “The Matrix” laterally inverted, and you may also have seen where the inversion came from.
Did you catch it?
You rotated the text yourself when presenting it to the mirror, the mirror then simply send back what it “sees”, and it “sees” laterally inverted text, because you just inverted it.
Now, to be fair, I was not the first person to notice this, although I didn’t know so at the time. I was in fact preceded by Martin Gardner who wrote mathematical puzzles for Scientific American, and he discusses mirror inversion of writing in his book “The Ambidextrous Universe” ((“The Ambidextrous Universe”, Martin Gardner, Pelican books 1970, ISBN 0 14 02.1081 4)).
Gardner also provides three interesting references on this subject ((“The Difference Between Right and Left”, American Philosophical Quarterly, vol.7, July 1970)) ((“Why Do Mirrors Reverse Right/Left but Not Up/Down?”, Journal of Philosophy, vol.71, May 16, 1974)) ((“Through the Looking Glass”, Philosophical Review, vol.86, January 1977)), one which says the above description is correct ((ref:2)) , and two which think the description is totally incorrect ((ref:3)) ((ref:4)). However, I feel I am able to offer a little more practical explanation than was given by Gardner in his book and this is what caused the general hubbub in the audience on the day of the lecture.
You will see from Figure 1 that I printed “The Matrix” on clear acetate sheet. Why? Because if it is simply the case that I caused the lateral inversion by rotating the page towards the mirror, then I can rotate the page 180 degrees (and still see the writing as it is on acetate) before presenting it to the mirror, and if the above description of what is happening is correct, I will see “The Matrix” non-inverted in the mirror. Figure 2 shows the results of this part of the experiment.
|Figure 2: Showing non-inverted text|
The title of my Inaugural Lecture was “Indistinguishable from Magic” (with thanks to Arthur C Clarke who proposed that any sufficiently advanced technology is indistinguishable from magic). I thought the demonstration was particularly apt, as along with smoke, mirrors are often associated with magic. It’s very nice to see a problem such as this arising simply because we hadn’t understood the correct frame of reference.