Fun Science Idea

Mirror mystery --   This page is a correction of what has appeared here for the last few years. I admit it. What I had before was wrong. Not completely wrong. It did explain something. But it definitely missed the point. And it was misleading. Come to think of it, that's the same opinion I have of some government policies. Not most, mind you.

Stand in front of a mirror and raise your right hand. The image in the mirror seems to raise its left hand. Write a word on a piece of paper and hold the paper so that the word can be seen in the mirror. The word seems to be written backwards.

Why is it that a mirror appears to reverse left and right? And why isn't top and bottom reversed also?

How a mirror changes what you see depends on just where the object, the mirror and you are placed. There are a number of possibilities.
 
Look in the mirror and raise your right hand-
In this case the mirror doesn't really reverse left and right. What's over to your right side still appears to be over to your right side. Your right hand moves, the right side of the image moves. This situation seems tricky if you think of the image as a real person standing in front of you. But it is not a real person standing there facing toward you. It is the image of you facing toward the mirror.
 
Look in a mirror at paper with something written on it-
Here again the mirror doesn't reverse the image. Normally, to see what's written on the paper, the side of the paper with the writing on it is facing toward you. To see the writing in the mirror, you turn the paper around so that the writing is toward the mirror. So, it is you, not the mirror who 'turned the paper around' and reversed the writing.
 
The word AMBULANCE on the front of a vehicle-
Ever notice how the word ambulance is written backwards on the front of an ambulance. It's written that way so that drivers who look in their rear view mirror will see the word correctly.
 
Normally to read something behind you, you would need to turn around to look at it. When you turn around, everything that was to your left, is now to your right. Everything that was to your right, is to your left. You needed to do that change to be able to read what is behind you. The word AMBULANCE written backwards turns the word around without you turning around to look at it. The confusion is because the image in front of you is what you would see if you turned around and looked behind you.
 
The diagram below shows a different situation, what happens when the object is to one side of the mirror.

When you look at the arrow using the mirror, the arrow points to the left.

When light strikes the picture of the arrow, the light bounces away in all directions. A small part of that light travels the correct direction to reach an observer's eyes. In the diagram below, one dashed line shows the path that light takes going from the arrow's point to the observer. The other dashed line shows the path from the arrows tail to the observer. The arrow seems to point in the opposite direction because each path crosses over to the other's side.

If the picture of the arrow is placed below the mirror, the picture is reversed top to bottom.

To get to the observer's eyes, light from every point on the arrow must take a different path. Light from points highest on the arrow, end up lowest after being reflected by the mirror. Likewise, light from points lowest on the arrow, end up highest after being reflected.

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