A recent query from a friend reminded me that some topics are both very easy and very hard to grasp at the same time. The key is often in the way we look at the it. He asked “Why does a mirror NOT invert images top-to-bottom?” Or better yet, why DOES a mirror invert images left-to-right? I used to ask this question to my undergraduate students in my Modern Optics class at UT when teaching them basic imaging equations. None of them ever answered the question correctly. But the answer, it turns out, is simple. Mirrors don’t invert anything. You do, and not with your brain.
Try this experiment. Go up to a mirror with an open book in your hand. Look at the printing – obviously it is perfectly readable. Now turn the book to face the mirror – you see that all of the writing in the mirror is inverted, going from right to left instead of left to right. What caused that? You did, when you TURNED the book. Don’t believe me? Repeat the experiment with an overhead transparency (yes, I am old enough to know what those are). Write a word on the overhead transparency and hold it up to the mirror. It is not inverted, when looking at it either directly or in the mirror. When you turn the transparency to face the mirror, the word is inverted when viewed both directly and in the mirror.
When you look at yourself in the mirror and wave your right hand, it looks like your mirror image is waving their left hand. But that is because we imagine ourselves turned around and facing the other way. Again, it the turning (or imaging that we turned) that does the inverting, not the mirror.