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Ask HN: Why do mirrors reflect text horizontally but not vertically?

5 points by amichail 2y ago

Is there a simple explanation for this?

yuvadam 2y ago
The mirror doesn't reflect horizontally or vertically, it reflects front-to-back, along the axis perpendicular to the mirror. Richard Feinman [1] explains this the best. [1] - https://www.youtube.com/watch?v=6tuxLY94LXw&pp=ygUOZmVpbm1hbiBtaXJyb3I%3D https://www.youtube.com/watch?v=6tuxLY94LXw&pp=ygUOZmVpbm1hb...
russfink 2y ago
Think in terms of projections. Take a piece of paper. Poke holes in it to make the word “TEXT.” Face the front of it to the mirror - it’s reversed in the reflection, but in a way that matches the backside of the paper. Hole for hole, they are projecting onto the mirror surface. What you see is the result of that.
PaulHoule 2y ago
Because you rotated around the vertical axis to look at the mirror. If you did a headstand to face the mirror it would look upside down.
- pmdulaney 2y ago
  I'm commenting on your answer because you got the "right" answer first. This is what I was going to say: My explanation is that the reason a mirror appears to reflect text (or anything else) horizontally rather than vertically is because, whether physically or in our mind's eye, we conduct the comparison by rotating our point of view around a vertical axis. If instead you adjust your point of view by rotating about a horizontal axis (e.g., compare what the UCLA on your sweatshirt looks like in your bathroom mirror to what it looks like looking down at your own chest), you find that things are unchanged left-to-right but upside-down vertically.
dave4420 2y ago
The mirror reflects, but your brain interprets it as a 180° rotation around an axis in the plane of the mirror, perpendicular to the line between your eyes. The mismatch reverses the text.
Two4 2y ago
Because you are confusing relative and cardinal axes of direction. If you stand in front of a mirror and point up, your image points up. If you point left, your image points right. However, up is relative to the surface of the earth, and left is relative to you. If you were to point left as well as east, your image would point east too - now you are considering direction along a common two dimensional set of axes, relative to the earth. The real brainfuck occurs when you point toward or away from the mirror, because now your image points north when you point south, and vice versa - however, other comments here have a much better explanation for that than I can muster.

5 points by amichail 2y ago

Is there a simple explanation for this?

The mirror doesn't reflect horizontally or vertically, it reflects front-to-back, along the axis perpendicular to the mirror. Richard Feinman [1] explains this the best. [1] - https://www.youtube.com/watch?v=6tuxLY94LXw&pp=ygUOZmVpbm1hbiBtaXJyb3I%3D https://www.youtube.com/watch?v=6tuxLY94LXw&pp=ygUOZmVpbm1hb...

russfink 2y ago

Think in terms of projections. Take a piece of paper. Poke holes in it to make the word “TEXT.” Face the front of it to the mirror - it’s reversed in the reflection, but in a way that matches the backside of the paper. Hole for hole, they are projecting onto the mirror surface. What you see is the result of that.

PaulHoule 2y ago

Because you rotated around the vertical axis to look at the mirror. If you did a headstand to face the mirror it would look upside down.

pmdulaney 2y ago
I'm commenting on your answer because you got the "right" answer first. This is what I was going to say: My explanation is that the reason a mirror appears to reflect text (or anything else) horizontally rather than vertically is because, whether physically or in our mind's eye, we conduct the comparison by rotating our point of view around a vertical axis. If instead you adjust your point of view by rotating about a horizontal axis (e.g., compare what the UCLA on your sweatshirt looks like in your bathroom mirror to what it looks like looking down at your own chest), you find that things are unchanged left-to-right but upside-down vertically.

dave4420 2y ago

The mirror reflects, but your brain interprets it as a 180° rotation around an axis in the plane of the mirror, perpendicular to the line between your eyes. The mismatch reverses the text.

Two4 2y ago

Because you are confusing relative and cardinal axes of direction. If you stand in front of a mirror and point up, your image points up. If you point left, your image points right. However, up is relative to the surface of the earth, and left is relative to you. If you were to point left as well as east, your image would point east too - now you are considering direction along a common two dimensional set of axes, relative to the earth. The real brainfuck occurs when you point toward or away from the mirror, because now your image points north when you point south, and vice versa - however, other comments here have a much better explanation for that than I can muster.