A seemingly simple but extremely puzzling mirror experiment has gone viral on TikTok and other video platforms. Place a sheet of paper on a mirror and place an object behind it. As you move, the object will appear in the mirror even though it is separated from the mirror by the piece of paper. TikTok user Baileycaviness12 is heard asking “How does the mirror know that is there?” And that’s just a fantastic excuse to talk about how weird mirrors are.
Despite being a common object, mirrors easily break our brains. The first straightforward mistake that we automatically make is assuming that mirrors flip left and right. We do that because we are a species that has evolved by looking at others. When we raise our right hand in the mirror, the mirrorverse counterpart appears to be lifting the left – but it's not really its left, it’s still the hand on the same side.
A mirror does not actually flip things around. Don’t believe what your brain suggests when you look at it. In reality, a mirror turns things inside out. Consider gloves that are designed specifically for each hand. If you turn them inside out, a left-hand glove would fit the right hand and vice versa. That’s what the mirror is doing: creating an inside-out version of the world.
So how is this inside-out flip work key to answering the mystery of “How does the mirror knows that is there?”
Imagine that when you look at an object straight lines go from your eyes to every bit of that object you can see. If you were looking at yourself, you would see those rays just bouncing back at you. It would be like wearing a perfect mask of your face and then taking it off and pushing it outward. Those are the same inside-out features.
But for the object behind the paper, the magic happens when you move, because you can see the law of reflection in clearer action.
The light is still moving in a straight light but once it encounters the mirror it's reflected towards our eyes with the same angle. This will create a virtual image inside the mirror. As long as there is a path between the object and our eyes that hits the mirror with the same angle, then the image of that object will appear “inside" the mirror.
Mirrors are fascinating objects and even though they are “simple” employ some pretty hardcore science.