Plane Mirror

[MedPR]

Can someone explain this to me please. My fool proof thin lens equation has failed me.

If I can only focus on images 300m away, no closer no farther, how far from a plane mirror must I stand to see my own image?

The answer is 150m, but why? The explanation says that the image is formed behind the mirror the same distance that I stand from the mirror. So I stand 150 in front of the mirror and the image appears 150 behind the mirror.

What?? Images don't appear behind the mirror, if they did you wouldn't see them?

Edit: So I just did a little experiment in which I wrote a tiny x on two post-its. Stuck one on the mirror and held one right next to my eye. The one I stuck on the mirror was much easier to read than the one I was holding in my hand (and trying to read through the mirror).

Is the concept that the image appears "behind" the mirror equivalent to knowing that when you look at something in the mirror you must look x distance to the mirror and the reflected image has to travel the same x distance back to your eye? So it is just like standing 2x away from the same object you are looking at in the mirror from x away?

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[BerkReviewTeach]

Can someone explain this to me please. My fool proof thin lens equation has failed me.

If I can only focus on images 300m away, no closer no farther, how far from a plane mirror must I stand to see my own image?

The answer is 150m, but why? The explanation says that the image is formed behind the mirror the same distance that I stand from the mirror. So I stand 150 in front of the mirror and the image appears 150 behind the mirror.

What?? Images don't appear behind the mirror, if they did you wouldn't see them?

Edit: So I just did a little experiment in which I wrote a tiny x on two post-its. Stuck one on the mirror and held one right next to my eye. The one I stuck on the mirror was much easier to read than the one I was holding in my hand (and trying to read through the mirror).

Is the concept that the image appears "behind" the mirror equivalent to knowing that when you look at something in the mirror you must look x distance to the mirror and the reflected image has to travel the same x distance back to your eye? So it is just like standing 2x away from the same object you are looking at in the mirror from x away?

Your there-and-back distance perspective is a great one.

The distance between you and the virtual image that is your reflection is twice the distance between you and the surface of the plane mirror, just like the distance you describe the light from the X on the stickie next to your eye is travelling before it gets back to you.

 

[MedPR]

Your there-and-back distance perspective is a great one.

The distance between you and the virtual image that is your reflection is twice the distance between you and the surface of the plane mirror, just like the distance you describe the light from the X on the stickie next to your eye is travelling before it gets back to you.

Ok thank you!

 

[BerkReviewTeach]

BTW, the thin lens equation holds with spherical mirrors (and thin lenses), but not with plane mirrors. Luckily, plane mirrors always give a virtual image that is the same size as the object as well as the same distance from the mirror as the object.

 

[davcro]

I missed this question too. Really bugs me because the answer is common sense. I asked the same question to my friend Tanner (an oilfield worker) at the bar last night. He knew the answer within seconds. FML

 

[MedPR]

I missed this question too. Really bugs me because the answer is common sense. I asked the same question to my friend Tanner (an oilfield worker) at the bar last night. He knew the answer within seconds. FML

Yea I really considered 150m as I was answering it, but I went with 300m instead. At least we won't get it wrong on the real thing!