here’s a picture of a man lying down; a horizontal man:

You’ll notice that he’s wearing a black shoe on the foot closest to you, and a brown shoe on the foot farthest away from you (I’m deliberately not using the words “right” and “left”). He is also wearing an eye patch on the eye farthest away from you, but you can’t see that. Now let’s put a full-length mirror behind him, and examine the reflection:

The first thing to notice is that the mirror has not reversed things horizontally: the feet are still on the left of the picture and the head is still on the right. The next thing to notice is that it hasn’t reversed the image vertically either. The most important thing to notice is that (as far as the virtual man in the mirror is concerned) the brown shoe is now on the foot closest to you, and the black shoe is on the foot farthest away from you. Also, the eye patch is on the eye closest to you as opposed to the eye you can’t see. Does it now become clear? The mirror doesn’t reverse things horizontally (it may seem to, but that’s an illusion). It makes things that were on the back of the object that’s being reflected appear to be on the front. And vice versa.

Why, if you stand upright and look in a full length mirror, do you have the impression that your image has been horizontally reversed? Well, if you were to stand in front of a person, and they had a wrist watch on their left wrist, you’d have to move your eyes rightwards and downwards to see it. If you were standing behind them, you’d have to move your eyes leftwards and downwards to see it. If the features on the front of their body were somehow visible from the back, you’d still have to move your eyes leftwards and downwards to see the watch. That’s what the mirror does for you – shows you what’s on the front of the body without turning it round.

Why, if you write some text on a piece of paper and hold it up in front of the mirror, does the image of the text look horizontally reversed? That’s the equivalent of writing the text on something transparent – a sheet of glass, say – and then looking at it through the back of the sheet. All the letters are the wrong way round, and the word and letter order is backwards. Once again, the mirror is showing you what’s on the front of the object, as if you were looking through it.

EDIT: Many many thanks to the 158 thousand people who’ve read this piece since I posted it three and a half months ago, and to the 600 people who’ve upvoted it. A couple of people have reprimanded me for concocting the pictures (which I did, of course). So, as an answer to that criticism, here’s a photo I took today. A doll which I purchased from a charity shop (male equivalent of a Barbie); brown & black shoes; eye patch; reflection in a mirror:

I think that answers the question “Would this work if it was a real photo?”

(Why didn’t I get hold of a male model, 2 pairs of shoes, a full-length mirror & an eye-patch? ‘Cos I’m not made of money).