Mirror image is a tricky thing, but it's pretty clear when you look at words in a reflection, that mirrors flip things horizontally rather than vertically. Or, at least it seems that that's the case.
For example, when you hold up a sign saying "Food" in the mirror", it flips reads a backwards "dooF", but the letters are still the right way up. And why, when you raise your right hand, your mirror-self raise its left hand, but it still moves it up rather than down. How does a mirror know the x direction from the y direction?
In the episode above, the Physics Girl explores what's going on. Because, you see, the reason that things appear to always be flipped horizontally in the mirror is because we flip them horizontally.
Try holding up that "Food" sign again, but then flip it vertically to face the mirror, and then you'll see an upside down, but forward facing, "Food" staring back at you.
The reality is that the mirror isn't flipping things horizontally or vertically at all, it's actually flipping them along the z direction - the one that points out in 3D from a traditional graph. You can see this illustrated in the video above.
That might sound confusing, because it's hard for us to visualise the z direction when we're so used to seeing things along horizontal or vertical axes - and because we're horizontally symmetrical, we don't notice that things aren't truly horizontally flipped when we look in the mirror.
But the Physics Girl does an amazing job of explaining it in the video above with a left-handed glove. To see how it would look in a reflection, you would need to peel that glove off and turn it inside-out. That's the true mirror image - no horizontal flipping required.
Of course, that doesn't make much sense until you see it for yourself. So watch the episode and be prepared to never look at your mirror-self the same way again.
Source: Physics Girl