# Is There Any Reliable Method To Identify A Two-Way Mirror From The Reflective Side? **URL:** \ **Category:** Factual Questions\ **Created:** [April 17, 2006, 3:32am UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939 "2006-04-17T03:32:04Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Surreal](https://avatars.discourse-cdn.com/v4/letter/s/eada6e/32.png) [@Surreal](https://boards.straightdope.com/u/Surreal)\ **Post date:** [April 17, 2006, 3:32am UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/1 "2006-04-17T03:32:04Z")
Suppose you would like to determine if a mirror is just an ordinary opaque mirror or if it is one of those ‘two-way’ mirrors that has a reflective surface on one side but is transparent when viewed from the opposite side. Furthermore, assume that you do not want to risk tipping off any potential observers that you might be aware of them while investigating this matter. Is there any reliable way to determine if it’s a two-way mirror without removing or breaking it? How?? Thanks. ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [April 17, 2006, 3:44am UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/2 "2006-04-17T03:44:18Z")
The usual term is “one way mirror”. It’s actually a half silvered surface which reflects about half the light, and the effect comes from the fact that the room with the “mirror” is very brightly lit, while the room with the “window” is dark. If you could turn off the room lights it would help you detect it, but it would also obviously tip off any observer that you were suspicious. ---
**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\ **Post date:** [April 17, 2006, 3:58am UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/3 "2006-04-17T03:58:15Z")
I take that back. A lot of people seem to use “two way mirror” as a term interchangeable with “one way mirror”. It still works as I described, however. Some background: > **[How do one-way mirrors work?](https://science.howstuffworks.com/question421.htm)** > > Just about everyone has seen a television show or movie in which a criminal suspect is questioned while detectives watch from behind a one-way mirror. How does a piece of glass manage to reflect light from one side while remaining clear on the other? ---
**Author:** ![Electronic\_Chaos](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@Electronic\_Chaos](https://boards.straightdope.com/u/Electronic_Chaos)\ **Post date:** [April 17, 2006, 4:06am UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/4 "2006-04-17T04:06:29Z")
Yeah, the only way you’re going to be able to tell is if you reduce the light on one side, by either dimming the lights, turning the lights out, or cupping your hands around your eyes and putting them to the glass. Any way you cut it, you’re probably going to tip someone on the other side off. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [April 17, 2006, 4:40am UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/5 "2006-04-17T04:40:25Z")
Being in a police interrogation room that has a large mirror on one wall is a pretty good tipoff. ---
**Author:** ![commasense](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/commasense/32/3017_2.png) [@commasense](https://boards.straightdope.com/u/commasense)\ **Post date:** [April 17, 2006, 4:45am UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/6 "2006-04-17T04:45:20Z")
It seems to me that context counts for a lot. If you’re in police custody or in a focus group, yeah, that huge mirror on the wall is _definitely_ a one-way glass. Likewise, if it’s on the wall outside the office of a store manager. In a less obvious setting, any really large mirror in a window-like frame that appears permanently affixed to the wall is a pretty likely candidate. But if you’re wondering if your slimy landord has a peephole behind a mirror in your bathroom, well that’s a different matter. As others have said, it requires dimming or extinguishing the lights on your side and hoping to see some light coming from the other side to know for sure. Someone sneaky enough to set up a one-way mirror is _probably_ smart enough to make sure there’s little or no light coming from the other side to give the secret away. Although this might tend to tip the fact that you’re suspicious, ISTM that if you were to shine bright light into the mirror at an angle, you might be able to illuminate objects behind the glass enough to make them visible. I haven’t done the experiment, though. ---
**Author:** ![Joe\_Mahma](https://avatars.discourse-cdn.com/v4/letter/j/5daacb/32.png) [@Joe\_Mahma](https://boards.straightdope.com/u/Joe_Mahma)\ **Post date:** [April 17, 2006, 2:39pm UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/7 "2006-04-17T14:39:11Z")
The reflective surface on a regular mirror is on the surface of the glass that is farthest from you. Thus, if you touch an oblect, such as a pencil point, to the glass, its reflection will be visibly separated from the object (by the thickness of the glass). With a one-way mirror, the (parially) reflective surface is on the side closest to you, and there will be no gap between an object and its reflection. Or so I’ve been told - I’ve never tried this. ---
**Author:** ![Joe\_Mahma](https://avatars.discourse-cdn.com/v4/letter/j/5daacb/32.png) [@Joe\_Mahma](https://boards.straightdope.com/u/Joe_Mahma)\ **Post date:** [April 17, 2006, 2:40pm UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/8 "2006-04-17T14:40:20Z")
oblect = object. Wish we had spellcheck. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [April 17, 2006, 2:46pm UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/9 "2006-04-17T14:46:51Z")
> [@yabob](#): > > It’s actually a half silvered surface which reflects about half the light If it’s only reflecting half the light, your reflection in it should be noticeably dimmer than in a regular mirror which reflects 90-95% of the light. There’s probably some threshold, say 73% reflectance, above which that dimming isn’t obvious. ---
**Author:** ![Cheesesteak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cheesesteak/32/3694_2.png) [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)\ **Post date:** [April 17, 2006, 3:30pm UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/10 "2006-04-17T15:30:39Z")
If you darken the room, then hold up a flashlight flush against the mirror, that light will go through into the room behind the mirror. Since the flashlight is flush, you won’t get much direct or reflected light from it, and you’ll probably be able to see what’s back there. ---
**Author:** ![mazinger\_z](https://avatars.discourse-cdn.com/v4/letter/m/5f9b8f/32.png) [@mazinger\_z](https://boards.straightdope.com/u/mazinger_z)\ **Post date:** [April 17, 2006, 3:48pm UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/11 "2006-04-17T15:48:00Z")
When I was working crim law in DC, one of the jailhouses I went to (Virginia, I think) was undergoing some reconstruction. Apparently, the interrogation rooms were using flourescent lights. Apparently, flourescent lighting makes it easier to detect two-way mirrors. I’ve seen what they were talking about, and it was mostly at the top edge, but after reading the responses, it could have easily been for lack of lighting as these rooms were not lit very well and had a lot of natural sunlight interference. I assume that there could also be manufacturing issues as well. ---
**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\ **Post date:** [April 17, 2006, 4:54pm UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/12 "2006-04-17T16:54:56Z")
> [@Joe Mahma](#): > > The reflective surface on a regular mirror is on the surface of the glass that is farthest from you. Thus, if you touch an oblect, such as a pencil point, to the glass, its reflection will be visibly separated from the object (by the thickness of the glass). With a one-way mirror, the (parially) reflective surface is on the side closest to you, and there will be no gap between an object and its reflection. Or so I’ve been told - I’ve never tried this. [Snopes says](http://www.snopes.com/crime/warnings/mirror.asp) that’s not a good test. They have some tips that might help though. ---
**Author:** ![spingears](https://avatars.discourse-cdn.com/v4/letter/s/ebca7d/32.png) [@spingears](https://boards.straightdope.com/u/spingears)\ **Post date:** [April 17, 2006, 5:06pm UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/13 "2006-04-17T17:06:00Z")
Is There Any Reliable Method To Identify A Two-Way Mirror From The Reflective Side? YES > [@Surreal](#): > > Furthermore, assume that you do not want to risk tipping off any potential observers that you might be aware of them while investigating this matter. NO. not that I know of. > [@](#): > > Is there any reliable way to determine if it’s a two-way mirror without removing or breaking it? How?? Shine the beam of a high power flash light through the suspect mirror to see the far wall of the room on the other side! ---
**Author:** ![MrSquishy](https://avatars.discourse-cdn.com/v4/letter/m/b9e5f3/32.png) [@MrSquishy](https://boards.straightdope.com/u/MrSquishy)\ **Post date:** [April 17, 2006, 6:41pm UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/14 "2006-04-17T18:41:31Z")
> [@mazinger\_z](#): > > When I was working crim law in DC, one of the jailhouses I went to (Virginia, I think) was undergoing some reconstruction. Apparently, the interrogation rooms were using flourescent lights. Apparently, flourescent lighting makes it easier to detect two-way mirrors. I’ve seen what they were talking about, and it was mostly at the top edge, but after reading the responses, it could have easily been for lack of lighting as these rooms were not lit very well and had a lot of natural sunlight interference. I assume that there could also be manufacturing issues as well. So it was supposed to be some kind of secret that these were two-way mirrors? In an interrogation room? Maybe, just maybe, you could trick somebody who had **never** watched television. ---
**Author:** ![mazinger\_z](https://avatars.discourse-cdn.com/v4/letter/m/5f9b8f/32.png) [@mazinger\_z](https://boards.straightdope.com/u/mazinger_z)\ **Post date:** [April 17, 2006, 6:55pm UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/15 "2006-04-17T18:55:39Z")
> [@MrSquishy](#): > > So it was supposed to be some kind of secret that these were two-way mirrors? In an interrogation room? Maybe, just maybe, you could trick somebody who had **never** watched television. The point was that they were see through, with flourescent light settings. I noticed it from the entrance of the room, and I wasn’t really trying. I didn’t put up my face to the mirror because that was discouraged – I was actually told not to do so beforehand. They were in the process of re-doing the lighting last time I was there. I suspect that if you have a suspect in the middle of an interrogation and a witness comes by to ID, you don’t want to have that witness made. ---
**Author:** ![Punoqllads](https://avatars.discourse-cdn.com/v4/letter/p/d2c977/32.png) [@Punoqllads](https://boards.straightdope.com/u/Punoqllads)\ **Post date:** [April 17, 2006, 7:44pm UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/16 "2006-04-17T19:44:18Z")
One possiblility: bring a handheld mirror into the room with you. Look at your image in each mirror and compare the brightness of the two reflections. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 17, 2006, 7:58pm UTC](https://boards.straightdope.com/t/is-there-any-reliable-method-to-identify-a-two-way-mirror-from-the-reflective-side/352939/17 "2006-04-17T19:58:59Z")
> [@](#): > > Snopes says that’s not a good test. I love the advice given in the e-mail they quote… If you’re still not sure, throw a chair at it!