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Mirror Image: Newsletter Challenge (04/12/05)

Posted April 12, 2005 7:00 AM

The question as it appears in the 04/12 edition of Specs & Techs from GlobalSpec:

Why is an image in a mirror inverted left-to-right but not top-to-bottom?

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Anonymous Poster
#1

Not sure

04/12/2005 8:49 AM

Obviously it has to do with the properties of the mirror - how it was designed. If you go in a fun house, you find mirrors that distort the returned image in all kinds of ways.

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Participant

Join Date: Apr 2005
Posts: 2
#2
In reply to #1

Here's looking at you!

04/12/2005 10:36 AM

No, it has to do with the properties of the head. Because my two eyes are displaced horizontally, the image is direct horizontally. If my eyes were displaced vertically, the direct (no inversion) would be from top to bottom.

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Anonymous Poster
#3
In reply to #1

The mirror question

04/12/2005 11:02 AM

Excellent explanation here: http://www.mathpages.com/home/kmath441.htm

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The Architect
Engineering Fields - Software Engineering - S/W Architect Popular Science - Evolution - Fascinating! Fans of Old Computers - TRS-80 - A fine computer United States - US - Statue of Liberty - NY

Join Date: Dec 2004
Location: GlobalSpec, Troy NY
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#5
In reply to #3

Re:The mirror question

04/12/2005 3:56 PM

Whew! That's a long read (if you, unlike me, stick with it to the end).

I liked this "angle" (pun intended):
"From the context of the original question we can infer that it refers to the image in the mirror being reversed relative to the image we would see if we turned around and looked directly [behind us]. If so, then the answer depends on HOW we would turn around. Most likely we would rotate our field of view about a vertical axis, so the image we see will be reversed laterally (left-to-right). Of course, if we turn about a horizontal axis (difficult to do, but not impossible), the image we see would be reversed top-to-bottom."

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Mark Gaulin
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#6
In reply to #3

Re:The mirror question

04/13/2005 10:29 AM

I looked at the discussion on the mathpages site and was disappointed; the explanations given presumed a set of re-interpretations of the question, then tried to decide which re-interpretation was the most satisfying (to the person who wrote it). The correct answer is that the image created by a single mirror gives an inversion of the "handedness" of the object viewed in the mirror, and the plane about which this inversion takes place is the mirror surface. So, if you hold a mirror in front of you, the image is a front-to-back inversion of the object. The images of your ears appear farther in front of you that the image of the tip of your nose. The wall behind you appears even farther in front of you than your ears. Place the mirror face-up on the desk in front of you, and the image of the top edge of your computer monitor appears below the image of the bottom edge of your monitor; the image of the ceiling appears to be below the floor that your chair is sitting on. I.e., now the image in the mirror is a top-to-bottom inversion of the object viewed in it. Hang a mirror on the right-hand wall beside you, and the images of objects toward your left appear to be toward your right. In this case, the image in the mirror is a left-to-right inversion of the object. The impression of a front-to-back image reversal is why those convex side mirrors have a label that says "Object may be closer than it appears." The convexity of the mirror changes your impression of the distance of the object viewed

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Posts: 71
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#4
In reply to #1

Re:Not sure

04/12/2005 12:51 PM

The answer is a "trick answer" (similar to a trick question); the real answer is that a mirror inverts images front-to-back.

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Join Date: Dec 2004
Posts: 1995
Good Answers: 35
#7

And the answer is....

05/12/2005 2:56 PM

As reported in the 04/19 edition of Specs & Techs from GlobalSpec:

The image in a flat plane mirror is a virtual image — a type of image created by diverging light rays. While it may seem that a mirror reverses an image left-to-right, the image is actually reversed front-to-back. Its front and back are the opposite to you. For instance, if you write a word on a piece of paper and hold it up to a mirror, it appears as if the letters are reversed. But this is because you have actually rotated the piece of paper and are presenting it to the mirror reversed. It is the rotation which inverts the image, not the mirror.

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