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Mirror Confusion

Are you talkin' to me?

Craboid selfie

It seems a common question about mirrors: "why is my image flipped side to side but not top to bottom?".

The inversion is actually front to back inversion, as if you are seeing your front from the inside through yourself from behind.


The reason we might think that our reflection is flipped from side to side but not from top to bottom is because we are bilaterally symmetrical. This is side to side or vertical symmetry, if we are standing. If we were symmetric from top to bottom (horizontal symmetry) like the Craboids of Beta Cancri D we would see that the image appears flipped top to bottom but not side to side. Our lower locomotion and tunnelling pseudopods would appear up in the air, and our upper manipulo-reproductive pseudopods would appear on the ground.


Similarly, if we humanoids look at our reflection while lying down, the image appears flipped top to bottom relative to gravity (up and down) but we consider that this is our left and right sides flipped.


Craboid wondering why it’s image if flipped top to bottom

but not side to side

Text reflection

You may think that text on a tee shirt is flipped left to right but not top to bottom when you check out your reflection. But it can be considered to be either of these.


This is my crude attempt at illustrating a room where the floor and side wall are mirrors while the back wall is painted turquoise with "42" painted on it in black. It takes a bit of imagination to discern the 3 surfaces.


The reflection on the floor has the 4 on the left and 2 on the right, as in the original image, but the numbers are upside down. The reflection on the side wall has the numbers the right way up but the 2 is on the left and the 4 is on the right, the opposite of the original image.

The two reflections however are identical but are rotated 180 degrees relative to each other. If you put your tee shirt on upside down (180 degrees rotation) the reflection will appear like the one on the mirrored floor in this image, flipped top to bottom but with the numbers reading left to right as in the original image.


This is because the mirror is actually flipping front to back so that it is as if you are seeing what an “observer” looking at you from behind but seeing through your transparent body to your front surface would see.


Another way to look at it is that you and the text on your tee shirt are both upright and the “observer” is also upright so the text appears upright. Flip your tee shirt or the “observer” and the text will appear flipped top to bottom and not side to side.