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  • I placed a box in front of a plane mirror
  • I looked at the image and smudged a bit of Vaseline on the mirror as a mark to indicate where I thought where the virtual image was.
  • After that, I repositioned myself at different places of my room. One thing I noticed was that I the image was changing position to.

I read on this website (http://www.physicsclassroom.com/class/refln/Lesson-1/The-Line-of-Sight)

"The precise image location of the object is the location where all lines of sight intersect regardless of where the eye is located"

This was quoted from the website. So, apparently, the position of the image does not change regardless of the position of my eye.

My QUESTION:

When I was trying this out, why did it seem to me that the image was also changing position as I changed my position?

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  • $\begingroup$ You were observong a parallax error because the smudge on the mirror was in a different plane than the virtual image. $\endgroup$
    – Floris
    Aug 23, 2014 at 17:33
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    $\begingroup$ If you move around as you look through a window at something stationary outside, does the object outside apparently move relative to smudges on the glass window? This is optically the same as your question... $\endgroup$
    – DJohnM
    Aug 24, 2014 at 1:06

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Try this experiment: take a small mirror (so you can look over the top of it) and put it vertically on a piece of paper. Looking in the mirror in one position, try drawing a dot behind the mirror where you "see" the spot. Because the mirror is small you should be able to see where your pen is pointing.

Now shift where you stand (without moving mirror or paper) and repeat.

Look behind the mirror. Your two dots will be in the same place (within the error of your accuracy).

But if you draw a dot on the mirror, this dot will move with respect to the image as you move your head. This is called the parallax error.

In the image below I try to illustrate this. Two points of view: in both cases the virtual image is in the same place, but the smudge on the mirror aligns in only one position.

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Do you mean that the spot on the mirror where you see the image changes?

That's not the location of the virtual image. The location of the virtual image is not on the mirror, it is behind the mirror, at the location the box would have if 1.) your mirror were clear transparent glass and 2.) the box were really behind the mirror instead of in front of it.

Another way to say it: It's the location (behind the mirror) where the box appears to be.

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  • $\begingroup$ I meant that the relative distance between the image of the box and the spot I made. Also, when I changed my position, it seemed as if the position of the box behind the mirror also changed $\endgroup$
    – Eliza
    Aug 23, 2014 at 16:11
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    $\begingroup$ That relative distance you mention is not related to the location of the virtual image. The key word in your other comment is "seemed". One way to convince yourself that the virtual image is not moving is to use a partially silvered mirror and placing an object at the position of the virtual image (behind the mirror). When you look at the mirror you will see both the image of the box, and the object you placed behind the mirror. As you move around, the two images will always be on top of each other. The object you placed is clearly not moving, so neither too is the virtual image. $\endgroup$
    – garyp
    Aug 23, 2014 at 16:24

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