Talk:One-way mirror

Discussion by page creator

• This discussion used to appear above the table of contents since it didn't have a header, so I added one. --Polm23 (talk) 06:20, 5 July 2016 (UTC)

Introduction -we hope this page doesn't violate any state secrets, but anyone who has watched detective shows or has tried on mirrored sunglasses knows something about all this. And any renter who wants to check out a suspiciously placed bathroom mirror should learn how to put their wife at ease that the landlord is not videotaping them in the shower for a future internet posting. Its quite common to see mirrored windows separating shopping areas from the manager' office. And mirrored ceiling tiles often hide attic video cameras. U.S. Post offices even have soundproof passages in the ceiling called Look Out Galleries, that have little mirrored bay windows to prevent theft or diversion of the mails by employees. Feel free to shorten or correct spelling.

The reference to disneyland may be out of date, as it was 1971 when i noticed this. Transparent ghosts were well lit reflections of ghost manakins located below the viewer but reflected against a glass wall to appear as semi transparent apparitions flying in a room next to the viewer. All the more impressive because they appeared in three dimensions. A similar effect was had by reflections in framed mirrors as the cars rotated and climbed out of the ride, and appeared in the mirror to be riding in the car with you. WonderWheeler 07:41, 26 February 2007 (UTC)

The effect referred to by WonderWheeler is called Pepper's ghost. — Paul G 17:13, 15 June 2007 (UTC)

Thanks for the addition of Peppers's ghost. I was unaware of the term, was only observing the effect. There is also a discussion at mirror under "one-way mirror". Will try to add a link there to this page, and thereby Pepper's ghost. WonderWheeler 01:05, 26 August 2007 (UTC)

urinal

I once used a urinal at a restaurant (the type where it is along a wall) and instead of a metal wall, they had a two-way mirror, that you could see all the patrons of the restaurant though. I don't understand how this type of glass could be fooled so easily (i don't think it was especially dark in the bathroom) as anyone wearing sunglasses or something could see you! there must be other types of mirrors. does anyone know? —The preceding unsigned comment was added by 202.156.66.110 (talk)

In case it's any comfort, I'm pretty sure sunglasses won't help you see through a two-way mirror from the bright side. The sunglasses will cut down the intensity of the reflected light from the bright room, but they'll dim the transmitted light from the darker room by the same factor. So the reflection will still overwhelm the transmitted image and you won't be able to see it. I admit I'm not completely sure that the mirror doesn't partially polarize the reflected image -- if it does then polarize sunglasses might have some effect. --Trovatore 01:54, 3 October 2007 (UTC)

this page is terrible

This page needs drastic revision. It is written like a how-to guide. It does not meet wikipedia's quality standards. Someone needs to completely rewrite it. --Rebent 17:20, 14 October 2007 (UTC)

It doesn't even say when two-way mirrors were invented. PolarisSLBM (talk) 01:27, 27 December 2007 (UTC)

Also, within the principles of operation section, there are only two citations. One is a blog and another has nothing to do with the science of how they operate. Petesims411 (talk) 06:59, 26 July 2021 (UTC)

Physics

As I understand it, a true passive one-way optical device is not possible—you can use a video camera and a screen, but you can't make optics that only let light through in one direction. Please correct me if I am wrong, but I think this is related to reciprocity. If this is in fact the case, the article should make it clear that so-called one-way mirrors are really a combination of coated glass with particular lighting. In the usual case, you use a semi-silvered mirror, so say 10% of the light passes through and 90% is reflected. If you put that between a dark room and a light room, where the dark room is 5% as bright as the light room, then on the light side, you will see 0.9 * 1 L + 0.1 * 0.05 D = 0.9 L + 0.005 D where L and D are the images of the light and dark rooms respectively if they were lit the same. On the dark side, you would see 0.05 * 0.9 D + 0.1 * 1 L = 0.045 D + .1 L which, adjusting for exposure is the same as L + .45D. So effectively, that's one-way glass, even though it really behaves symmetrically. —Ben FrantzDale (talk) 21:25, 26 May 2008 (UTC)

I think the confusion many people have with "one-way mirrors" is because what "one-way" refers to is not defined. There are two possible interpretations I see: transmission, and reflectivity. With regard to transmission, your argument is sensible, and I can't think of a counterexample, and so I'll put that issue aside. With regard to reflectivity, we have to decide what's being discussed very carefully, and so I'm going to talk about semi-silvered glass only.

To create an analogy, I'd like to reference a certain type of home security door. I can't find the name of the type of door I'm referencing, however, these are generally a sheet of metal from which many holes have been punched to make a thick metal screen. They are typically painted white, and installed like a screen door on a home's front door. On a bright day, it's usually brighter outside, and (by your argument) it's more easy to see out than in.

Now suppose we were to strip the paint from this door, rough up one side with a wire brush, and polish the other side to give it a mirror finish. One side of this now produces a specular reflection (like a mirror), while the other produces a diffuse reflection (like cement). Imagine that this surface is extended indefinitely, and the scale of the pattern of holes is shrunk to a very small size. What we now have is surface through which light may be transmitted that is reflective on one side only.

This is all to say that I believe it is a difference in the finish on either side of the metallic coating that causes the semi-silvered "one-way" mirrors used for psychological experiments, etc., to be perceived to be "mirrors" on one side, and "not" on the other. It's not that there's a violation of symmetry, it's that there's a difference in reflection type on either side. This, I suggest, paired with the phenomena you discussed, is a major contributor to the common misconception that reflection only happens on one side. PErdos (talk) 03:20, 3 August 2014 (UTC)

Regardless of optical laws it's impossible by reductio ad absurdum: a theoretical one-way mirror could be used to allow a cold body to transfer net thermal radiation to a hot body while preventing the hot body from heating the cold one, reversing entropy AkariAkaori (talk) 01:48, 12 April 2015 (UTC)

That’s the important point, and it seems to be missing from the article. Why can’t we build a “true” one-way mirror? Because it would violate a fundamental law of physics. If we had actual one-way mirrors, it would be straightforward to build a perpetuum mobile. --2001:16B8:174B:B700:E989:8521:9D4A:1F50 (talk) 13:19, 16 October 2021 (UTC)

One-way vs. Two-way mirror

I had never heard the term "two-way" mirror before seeing this article, but was quite familiar with the term "one-way" mirror. I did a comparison of Google search results to see which term is in more common used and it turns out one-way mirror is about 5 times as common: http://googlefight.com/index.php?lang=en_GB&word1=one-way+mirror&word2=two-way+mirror —Preceding unsigned comment added by 67.175.216.220 (talk) 05:40, 13 October 2008 (UTC)

Well, one-way mirror is just simply a misnomer — this sort of mirror works the same in both directions. Unlike a normal mirror, which works from only one direction, and would properly be called a "one-way mirror" if the term needed to be used. I don't think we can rely on the Google test here, as it's likely that many of the hits are from popular sources. We want to use the common name — but the common name at a certain level of discourse, I think. --Trovatore (talk) 08:06, 4 December 2008 (UTC)

Okay, but the claim in the first line (" . . . never called a one-way mirror . . . ") is incorrect and pretty much stupid. Someone needs to think up a better way to say that. Excalibre (talk) 18:57, 11 February 2009 (UTC)

I think that was a recent addition. I was assuming someone would revert it and I wouldn't have to bother. --Trovatore (talk) 19:02, 11 February 2009 (UTC)

Ok, maybe I missed something, but what is your source for the claim that "one-way mirror is just simply a misnomer"? MasterCKO (talk) 01:12, 7 March 2009 (UTC)

Try thinking about it this way. One-way mirrors are (mis)named because people can only properly see through one way. However, in the same direction a person can see through, the particular side of the mirror can also reflect. It is a misnomer because mirrors are for reflection, not for seeing through, and in a two-way mirror both sides have reflective surfaces but only one side allows a person to see through properly, so it is a two-way mirror with only a one-way vision-allowing side.125.238.169.31 (talk) 10:03, 22 April 2009 (UTC)

Ah, I hadn't really read this through before. No, that's not correct either. Both sides reflect, and you can see through both sides. The only difference is the illumination in the two rooms. The glass itself is not directional — if you were to turn the glass around the other way, but leave the relative brightness of the rooms the same, you would still be able to see from the dark room to the bright room, but not vice versa. --Trovatore (talk) 09:03, 11 May 2009 (UTC)

I find the claim that 'two-way mirror' is correct and 'one-way mirror' is not to be rather unusual. Another common name for this device is 'one-way glass'. That is, an object which is transparent like glass in only one direction. 'two-way glass' would seem to me to be normal window glass that works transparently both ways. Taken in this way, a 'one-way mirror' is a device that is a mirror in one direction. A 'two way mirror' would reflect from both sides, as a fully silvered piece of glass would. Another way to look at it is if two-way streets allow traffic in both directions, and one-way streets allow it in only one direction, then two-way mirrors are reflective in both directions, and one-way mirrors are reflective in one direction. This seems rather clear cut (to me, at least). —Preceding unsigned comment added by 71.254.26.105 (talk) 19:37, 10 May 2009 (UTC)

Actually, your comments illustrate pretty much exactly why two-way mirror is the correct name.

A two-way mirror does, as you say, reflect from both sides. It also transmits the image from both sides.

The reason it seems to be directional is that one room is much much much brighter than the other one. (You may not think it's much much much brighter; you might think it's only a little brighter. That's because of the nonlinear response of your vision.)

So just picking some numbers, but plausible ones, out of the air: Let's say the bright room is 100 times as bright, in terms of watts per square meter, as the dark one. And suppose that, of the light hitting it from either direction, the mirror transmits 40%, reflects 40%, and absorbs 10%.

Then you can do a quick calculation and see that, from the dark side, the transmitted signal is 100 times as bright as the reflected one, so its essentially all you see. On the other hand, from the bright side, the reflected signal is 100 times as bright as the transmitted one, so it's essentially all you see. --Trovatore (talk) 02:08, 11 May 2009 (UTC)

A one-way street allows traffic in only one direction because of arbitrary laws enforced by humans.  It's not a structural property of the road surface.  Likewise, a one-way mirror reflects light in only one direction because of arbitrary lighting conditions imposed by humans.  It is no different.  To say that it is "really" symmetrical "if only you change the conditions" is disingenuous.  The conditions are relevant.  Calling it a two-way mirror is wrong in exactly the same way that calling a street on which cars only travel one way a "two-way street" is wrong.  "Cars can travel both ways on it if only you let them!"  A two-way mirror is something that mirrors in two directions, such as a silver sheet or fully silvered glass.  This article is about one-way mirrors.  --99.240.223.41 (talk) 05:20, 5 December 2009 (UTC)

It does mirror in both directions. It's just that your sensory apparatus doesn't pick that up. The subject of the article is correctly called two-way mirror, and if you don't understand that, then you just don't understand what the things do. --Trovatore (talk) 08:11, 5 December 2009 (UTC)

To elaborate: I didn't say it's symmetrical if only you change the conditions. It's symmetrical, period. The proportion of light that it reflects from one side is the same as the proportion it reflects from the other, and the proportion it transmits from one side is the same as the proportion it transmits from the other. (Well, at least approximately -- I suppose one side could be dirtier than the other or something, but that's not an important part of the effect.) --Trovatore (talk) 08:51, 5 December 2009 (UTC)

I can't believe you're arguing about this.  I must be nice for you that your true opinions are so concrete.  It's just too bad (for everyone else) when they're wrong.  You accuse me of not understanding you.  "You disagree so you're stupid".  Thanks for dragging this into the gutter.  Well, as long as we're here, maybe try reading what I said again.  More importantly, try reading what everyone in this thread who is not you has said, which I'm sure is hard since your own words must be so compelling to you.  To simplify, it's not the proportion that matters;  it's the amount.  More light is passing from the bright side to the dark side than vice versa, so the situation is asymmetrical, both physically and perceptually.  Whether you want to admit you're wrong or not, you're still wrong;  reality doesn't care.  --99.240.223.41 (talk) 03:54, 2 January 2010 (UTC)

It is a two-way mirror, because it acts as a mirror, equally, from both sides. Period. --Trovatore (talk) 03:59, 2 January 2010 (UTC)

Your ego evidently trumps reality. This superpower can do more useful things. Stop wasting it here. (Jeremyhfht (talk) 04:07, 4 January 2010 (UTC))

I have normally heard and always used "one way" mirror. I have heard people say two-way mirror, but I have heard people say a lot of things. I have understood "one way" mirror in the following sense: That the glass is highly reflective, but far from opaque. As a result, under the circumstances in which it is used, glass acts as a mirror in "one way"/one "direction" and as a "window" in the other. Unlike mirrors which are designed so as to be a "mirror" in one way permanently and to be functionally opaque from the "back" side, a "one way" mirror is set up as a mirror on one side and an observation window on the other. The idea that "two way" mirror should be used here instead is grounded on the idea that since the "opacifying" treatments given to "regular" mirrors are absent, the mirrors in question can readily transmit light in both directions and reflect light in both directions--hence they are "two way" mirrors. I would submit that, by that logic, they are also "two way windows," for indeed, they are capable of being windows in both directions, just a surely as they are mirrors in "two ways." The problem, as well articulated above, is that they are not interesting when they are being mirrors in BOTH directions. When the glass is being a mirror in "two ways" it is also being a window in "two ways" and is therefore in "no way" useful for what everyone in world thinks "one way" mirrors are actually good for--being a mirror in "only" "one way" and a window in "only" "one way" simultaneously. That is the whole point of these mirrors. The closer they come to being "two way" mirrors, the less they are like, well, "one way" mirrors! When they ARE being mirrors in both directions and they are also being windows in both directions, they are aptly described as "two way" mirrors and "two way" windows and are consequently useless as "one way" mirrors. I would suggest to you that in the end we are talking about glass panes. They can be mirrors and/or windows depending on circumstances and use. "One way mirror" describes a use/situation involving a piece of glass acting as a mirror in one direction (principally) and as a window in only one direction (principally). It is most undesirable and ludicrous to name a thing based on a characteristic that is the antithesis of its reason for existing. Furthermore, the term "one way" mirror is mainly a popular one, as is "two way" mirror. When scholarly publications use either term, they use mainly the former. This includes "educational texts" like "Physics for scientists and engineers" by Shelden H. Radin, Robert T. Folk, 1982, p 739; and "scholarly journals" like "Comments on atomic and molecular physics," Volume 28, Page 8 (University of California, 1992). The term appears very often in scholarly journals of psychology and sociology, (rather more often that "two way"). In such contexts it is being used by scientists who actually use these mirrors for experiments. Of course, physicists don't use either term much because the term is a popular one and physicists would normally use use "one"/"two" way mirrors as an expression only to illustrate principles of physics to a lay person who has some notion of a thing commonly called a "one way"/"two way" mirror as a reference point. (In like manner a physicist might talk about reflections off the surface of a "pond." "Pond" is, however, not a physics term.) Incidentally, discussion of a "one-way mirror" is part of the official regents physics syllabus for high schools developed by the Board of Regents of the University of the State of New York. In any case, as stated, both terms are "popular" rather than "technical," and "one way" mirror is far and away the predominate term. "Two way" mirror is apparently a degeneration of this dominate term. It is also a synonym for "window." Neither technical users nor popular users prefer the minority expression "two way mirror." The article name should be changed accordingly.

Where to buy, how to install

I'm a land lord and I am interested in purchasing a one way mirror and installing it. Does anyone know where I can buy one? It would be best if it was about the size of a normal bathroom mirror. Thanks! 24.21.194.209 (talk) 08:49, 30 July 2010 (UTC)

Requested move

The following is a closed discussion of the proposal. Please do not modify it. Subsequent comments should be made in a new section on the talk page. No further edits should be made to this section.

The result of the proposal was move to One-way mirror which appears to be the more common name and the way the "general public" refers to the object in question (per the sole reference cited in the article). --rgpk (comment) 18:07, 18 January 2011 (UTC)

Two-way mirror → One-way mirror —Relisted. Vegaswikian (talk) 23:56, 19 December 2010 (UTC) The term "two-way mirror" is uncommon and not preferred by technical or non-technical users. Prothall (talk) 00:39, 10 December 2010 (UTC)

Survey

Feel free to state your position on the renaming proposal by beginning a new line in this section with *'''Support''' or *'''Oppose''', then sign your comment with ~~~~. Since polling is not a substitute for discussion, please explain your reasons, taking into account Wikipedia's policy on article titles.

• Support Prothall (talk) 00:42, 10 December 2010 (UTC)
• Oppose. Two-way mirror is not in fact uncommon, and it is more accurate. --Trovatore (talk) 00:47, 10 December 2010 (UTC)
• Strong support - More common on Google Books and Scholar. Marcus Qwertyus 09:49, 12 January 2011 (UTC)

Discussion

Any additional comments:

• Alternative. It might be acceptable to move to half-silvered mirror, or to merge into beam splitter. --Trovatore (talk) 00:48, 10 December 2010 (UTC)

• • I like the first option - it seems less confusing than the way things are now. Also, I should have said "less common relative to 'one-way mirror'". --Prothall (talk) 00:55, 10 December 2010 (UTC)
    • How about merging with beam splitter and putting the merged article at half-silvered mirror? --Trovatore (talk) 00:58, 10 December 2010 (UTC)
      • The physics may be related, but I think the use cases are too different. --Prothall (talk) 01:02, 10 December 2010 (UTC)
        • My problem with one-way mirror is that I think it actively misleads readers about the physics. I don't know which term is more used; my general sense is that two-way mirror is more common in the US and one-way mirror in the UK. But in this case I think the question of physical accuracy should break the tie. --Trovatore (talk) 01:14, 10 December 2010 (UTC)
          • Actually I was just talking about the difference between these mirrors and the beam splitter. First, I think moving this article to half-silvered mirror is fine, but I'd prefer one-way mirror. I'm American, and reading this article I first thought two-way mirror was a British term because I'd never heard it before and there's the mentions of Big Brother. Looking at the comments this past year, it seems like everyone else who's weighed in except you is also more used to this term. I can see why you might think it's a misleading name, but I think that owes as much or more to the way the mirrors are presented (since in, for example, an interrogation room the lighting conditions will always have them seem like a normal mirror on the same side). --Prothall (talk) 01:37, 10 December 2010 (UTC)
            • Well, but that isn't part of the mirror; that's part of the room. --Trovatore (talk) 01:47, 10 December 2010 (UTC)

I get both terms about equally common in Google searches [1] [2] which would suggest to me to leave things as they are. Andrewa (talk) 19:00, 10 December 2010 (UTC)

• Compare Google Scholar:[3] [4]. Also read check the notes in the last, large comment in the debate before the move request. --Prothall (talk) 19:41, 10 December 2010 (UTC)
• Comment I don't know which term is more used; my general sense is that two-way mirror is more common in the US and one-way mirror in the UK. No, the most common form in the United Kingdom is definitely Two-way mirror. Just done a very quick small survey here and nobody's ever even heard of the term one-way mirror before. Skinsmoke (talk) 09:45, 12 January 2011 (UTC)

Not half silvered

By far the most common form of one-/two- way mirror in my everyday experience is a sort we don't seem to mention at all... it consists of a conventional indirect mirror, with the "back" of the glass silvered and an opaque matt backing layer on top of the reflecting coating, and with narrow parallel lines at about one inch spacing that are transparent, with neither silvering nor backing material.

This allows people behind the mirror to see through quite easily, while those in front have difficulty seeing past the reflection. Such mirrors are particularly effective when the observer is closer to the mirror than the observed, as in supermarket supervisors' offices, or florists' back offices. They don't pretend to be secret, just the opposite, they just give privacy to the observer while letting the observed know that they are on show.

Where do these belong in our coverage? This may be relevant to what we call the article(s). Andrewa (talk) 19:03, 10 December 2010 (UTC)

This doesn't really ring a bell with me. Can you point me to anything that describes these more, or to an outfit that sells/installs/uses them? --Trovatore (talk) 19:15, 10 December 2010 (UTC)

Hmmmm... maybe they're an Australian phenomenon... I've tried Google with no luck, and the sorts of places they're used don't lend themselves to unannounced photography. Watch this space... Andrewa (talk) 00:01, 11 December 2010 (UTC)

The above discussion is preserved as an archive of the proposal. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

3 way mirror

has anyone ever heard these called a 3 way mirror or is that someone being silly with the common confusion between 1 and 2 sided mirrors? i have heard this type of glass be caleld both 1 and 2 but never 3 sided. who/what would even be a 3rd side on a plain? — Preceding unsigned comment added by 152.91.9.153 (talk) 04:44, 4 October 2012 (UTC)

Citation Quality

This page largely depends on citations to a self-published, unsourced website by a man named Jim Loy, who is not an expert in this area. (He has a BS in Mathematics and no professional experience in optics. Whether or not his information is correct, it's not a reliable source under Wikipedia's guidelines.) Furthermore, these pages no longer exist: the sources point to ten-year-old snapshots on the Wayback machine. I plan to remove all references to this site unless I hear an objection.

I'm also going to remove the claim that "a true one-way mirror does not, and cannot, exist." A form of this claim may be true for some definitions of "one way" for certain types of materials (i.e., passive optics) but one-way mirrors do exist in the general usage of the term, and the word "true" adds none of the required precision to support this statement. ("One-way mirror" is not a scientific term of art where one can say there's some technical meaning different from the common meaning.)

I have no objection to restoring this claim if someone can find a real source and explain the claim with more precision. As it stands it's not only unsourced but it's not literally true. Along with a source, it would need to be phrased in such a way that claims that a particular form of "one way"-ness is not possible, and for what category of materials. Gerweck (talk) 05:46, 26 June 2015 (UTC)

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Talk Page Archiving

While this talk page is only ~30k, given the number of old and settled discussions present I'd like to set up automatic archiving. If there's some reason I shouldn't do this please let me know. --Polm23 (talk) 07:06, 5 July 2016 (UTC)

Confusing statement about directionality

The intro to the article states: "This allows viewing from the side that is lightened but not by the darkened side." If this refers to viewing through the mirror, then it's the wrong way round -- you can see through the mirror from the darkened side and not from the lightened side.Blixton (talk) 18:17, 23 April 2019 (UTC)

Confusing one way mirrors for two way mirrors

This page says that one-way mirrors are the same as a two-way mirror, but that’s not true… Furthermore the page seems to be describing a two-way mirror instead of a one-way mirror, is there a way to move the contents of a page to a new one? Qubeman (talk) 06:34, 6 October 2024 (UTC)