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Sitting on the barbershop, watching myself in the mirror which is parallel to another mirror (so that two mirrors are parallel to each other), I was seeing an infinite number of photocopies. After a moment, I guess that it's really not infinite, I can count the number of copies.

But we know from our elementary optics course, that the two parallel makes an infinite number of copies. This leads me to ask,

Why it is that parallel mirrors not make infinite images?


Here I think the finite speed of light would play a role, but I think not that significant. I don't if it's due to the fact if the mirror is not that enough parallel or they are not enough flat.

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    $\begingroup$ @Steeven That sounds like an answer! :) Should be relatively simple to figure out how "far away" the image in the mirror effectively is, and compare it to what you could imagine seeing. (EDIT: Whoops, you beat me to it! :) ) $\endgroup$ – Philip Mar 19 at 15:29
  • $\begingroup$ @Philip You are right, I made it into an answer. Yes, agreed, if you measure the down-scaling for a few of the first reflections, I guess you can calculate how many reflections it'll take before the reflected image is smaller than what is observable with the eyes' photoreceptors. $\endgroup$ – Steeven Mar 19 at 15:39
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There is an infinite number of reflections. The limitation that you notice comes from the "resolution" of your eye.*

You can't see further smaller reflections with the naked eye of the same reason that you can't see, say, a bacteria with the naked eye. It is too small. That doesn't mean it is not there.

Apart from size, for non-ideal mirrors the amount of light might also limit how many reflections you can see. A non-ideal mirror will not reflect 100% of the incoming light but will absorb some from each reflection. In such case the reflected image becomes dimmer and dimmer, until the image eventually is undistinguishable. At that point you might rightfully say that the reflection count stops; so, for non-ideal mirrors the reflection count is finite. But this might for most good mirrors be far beyond what the eye is able to see.


* This "resolution" is a combination of the amounts of light that the eye can receive, the eyes' photoreceptor differentiation, the fact that you have two eyes, the fact that eyes are in constant rapid micromotion, and the processing ability of the brian.

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For this question its most probably because the image keeps on going for long due to which our eyes can't differentiate between two images. Also for our brain can distinguish two different object from far differentiating two identical object from far away is very less possible.

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