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Not sure if this should be in the physics section or here in philosophy. I think the topic may fit in both domains. What has lead me to inquire about this particular effect is the description of it as given on Wikipedia. I wonder if what I am reading is an accurate description of the circumstances or if there is an aspect or two of it that I am not able to comprehend. Here it is:

The 3D illusion mirror effect is produced whenever there are two parallel reflective surfaces which can bounce a beam of light back and forth an indefinite (theoretically infinite) number of times. The reflections appear to recede into the distance because the light actually is traversing the distance it appears to be traveling.

For example, in a two-centimeter-thick infinity mirror, with the light sources halfway between, light from the source initially travels one centimeter. The first reflection travels one centimeter to the rear mirror and then two centimeters to, and through the front mirror, a total of three centimeters. The second reflection travels two centimeters from front mirror to back mirror, and again two centimeters from the back mirror to, and through the front mirror, totaling four centimeters, plus the first reflection (three centimeters) making the second reflection seven centimeters away from the front mirror.

Each successive reflection adds four more centimeters to the total (the third reflection appears 11 centimeters deep, fourth 15 centimeters deep, and so on). Each additional reflection adds length to the path the light must travel before exiting the mirror and reaching the viewer. Each reflection and the additional distance traveled by the light reduces the brightness of the image, which also fades into the distance.

I suppose what I take from this description, in sum, is that there is a distance that light ("photons") is indeed traversing in the form of a reflection that is cast between two mirrors. When I think of "distance" I think of something that I or another object is capable of traversing, e.g. Walking from Point A to Point B. However, this kind of "distance" that is being alluded to in the article is not one that I nor any object are capable of traversing...not at least without shattering the mirror into pieces.

This effect can only occur via visual-photo transduction so without an observer it is not something that could happen on its own. Like a mirage. According to the description given what is being assessed is not an "illusion of distance" and if it were it would be the first one, that I know of at least, that can be measured in centimeters.

How is light capable of traversing a distance between two mirrors that lack the depth necessary for anything to travel through in such a way that the effect is being measured (as though there is a depth of some sort)? How am I to interpret the meaning of "distance" in this scenario?

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    In the study of electromagnetism it can happen that if you have a perfectly reflecting surface (or some other possibility like a perfectly conductive surface), it's provable that the electromagnetic field outside it is exactly equivalent to the field that would hypothetically be there if you removed the surface and put some mirror-image objects on the other side--see the method of images. So the light you see from a mirror can be equiv. to the light you would see if the mirror were replaced by glass and a mirror-world was on the other side.
    – Hypnosifl
    Jul 9, 2021 at 22:11
  • This actually helps very much. Thank you for the insight!
    – Tri Sat Nava
    Jul 9, 2021 at 22:50
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    It is the same kind of distance that you cover when walking from one side of the room to the other and back 10 times, it will be 20 times the length of the room. Nothing particularly subtle about interpreting it. The light does the same thing when bouncing back and forth between the mirrors. And it would do it on its own without any observers too.
    – Conifold
    Jul 9, 2021 at 23:13
  • @Conifold Yes, after thinking about it more it would occur without an observer. Just as it would between dew drops upon a spider's web. So, distance can only be interpreted in one way regardless if one is capable of walking through it or not. Like Hypnosifl pointed out there are spaces that are nonetheless there but that we cannot directly access like one would a room in a home, etc.
    – Tri Sat Nava
    Jul 10, 2021 at 0:56

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I think the comments have already ansered the question. But nevertheless.

The reference to a 3D infinite space is to the experience of the experience you have when you place yourself between the parallel mirrors and see the the endless row of lights stretching out in front of you and behind you. The row doesn't actually extends to infinity. If the speed of light were very small you would see the light go on one after another. The first would appear at the smallest distance and the others behind.

If the mirrors are perfectly reflecting it is the question if you would survive. If you step between the mirrors when a parallel laser bundle has been traveling for a while there would be enough photons to harm you. If the light from the source is emitted spherically then you have nothing to fear though. All photons are diverging away from the source.

In the case of a parallel bundle the source has to be outside the two mirrors though. If not, the source itself would obstruct. Unless you put two parallel mirrors on the side of emission... Which only lets photons pass coming out of the device and reflect photons coming from the mirror.

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