Reflections on Reflections
----Mr. Ottewill
An old conundrum runs as follows. Suppose you stand in front of a full size mirror and hold both hands out horizontally to each side. Suppose you shake your left hand. Which hand of your ‘image’ shakes? If you picture yourself where your image is, then the answer is their right hand that shakes. Similarly, if you lean forward and move your hands in towards the mirror, eventually your left hand joins with ‘their’ right hand. This is not hard to understand, after all, they are your ‘mirror image’ and so it seems natural for things to be reversed. The question arises however: if everything is reversed, then why when you shake your head don’t the image’s feet shake? In other words, why are left and right swapped but not top and bottom? The beauty of this question is that it is very simple to state, yet actually quite hard to pin down exactly what the answer is. I believe that there are a surprisingly large number of ‘wrong’ answers which are often given. I will look at what I think are two possible wrong answers before giving what I believe is the correct answer. The first wrong answer is that left and right change, but not up and down, because we have two eyes side by side, not one above the other. I think that this answer is most easily seen to be incorrect simply by shutting one eye and noticing that this does not change anything – left and right still swap but not up and down. The reply could come that we are so used to having two eyes side by side that we still think like this when one is shut. A simple reply is that someone unfortunate enough to have only one eye from birth presumably sees things the same as us, i.e. their feet still don’t shake when they look in a mirror and shake their head. (Another alternative answer is to turn one’s head on one side, i.e. so that one’s eyes are now above each other – nothing odd happens, i.e. the image’s left hand doesn’t suddenly morph into their right hand, or the image’s head to their feet.) A second possible wrong answer, one given to me a few years ago by a well- respected Biology teacher (no longer at the College I should add) is that the effect is due to the image on a retina being upside down, our brains turning it the ‘right way up’ when we process the image. This sounds plausible until one realises that the reason for the image on the retina being upside down, namely that light coming in from above passes through the lens at the front of the eye and ends up towards the bottom of the retina, applies equally well to light coming in from the left, i.e. such light ends up on the right hand side of the retina and vice versa for light coming in from the right. The image being upside down on the retina
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