Here's something strange. When you look into the mirror, how big is the image of your face? Now, as you back away from the mirror, how does the size of the image change? If you're like most people you'll say that the image is roughly the same size as your real face but as you back away, the image will become smaller. Makes sense, right? Except that's not what happens. The image of your face will be exactly half the size of your actual face, no matter how far you are from the mirror. Which means that as you back away from the mirror, the size of your face's image won't change. This seems so counterintuitive that you may have to try it for yourself to believe it. I had to. I used a couple of small pieces of tape, at the top and bottom of my face image in the mirror. As I walked away, the image of my face remained exactly within the tape marks. And then, I had to demonstrate to every single one of my children, all of whom were awestruck except for one 10 year old who had such strong convictions about what would happen that she actually denied the evidence of her senses.
The physics behind this is not very complicated and is explained well here. What's much more interesting is to reflect (sorry...) on the reason why what we see confounds our intuitions. It's because what we see, when we look into the mirror, is not a reflection. For all intents and purposes, we are acting as though we are seeing another person who looks just like us. As we back away from real people, their images do get smaller, so we expect it of our reflected images as well. No matter how hard we try, it seems, we just can't see the image as an image, rather than as the real thing.
Mirrors: the original virtual reality machines.
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