Why Do Mirrors Seem To Flip Things Horizontally?

YouTube screenshot, via Physics Girl

Have you ever wondered why, for example, if you held the word stressed in front of a mirror, it would read “desserts?” How come mirrors seem to only flip things horizontally, rather than vertically? Of course, the mirror doesn’t know the x-direction from the y-direction, so what’s going on?

In the video below, vlogger Physics Girl explains the reason, and although it may have never dawned on you before, it’s actually pretty simple. As Meg points out, it’s because we are presenting the images backwards, not the mirror. We flip them horizontally and not vertically. If you wrote the word desserts down on a piece of paper and then flipped it vertically to face the mirror, rather than horizontally, it would still read desserts, albeit upside down, of course.

Check it out here:


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