TikTok Myth of the Week: The Fingernail Test Shows Whether Hotel Mirrors Are Spying on You
TikTok is full of people “discovering” spy mirrors in their hotel rooms and Airbnbs. Or, more often, they use a normal hotel mirror to demonstrate a trick that supposedly reveals such mirrors. (Phew—this one’s fine! We’re safe!) Unfortunately, these videos rely on a misunderstanding of how mirrors work. Read on and I’ll tell you why these tricks are bullshit, and an easy way of actually telling whether a mirror is two-way. (It doesn’t involve your fingernail.)
What is a spy mirror, anyway?
The TikToks all say they will show you a way to find out whether you’re being watched. The idea is that either there could be camera hidden behind the mirror, or the mirror might be the kind you see in interrogation rooms on TV, where the “mirror” is actually a window between two rooms. People on the other side can see right in.
These mirrors do exist. They are perhaps most properly known as “one-way mirrors” (since the mirror only works if you’re standing on one side of it) but you’ll also hear them called “two-way mirrors” (since they can act as either a mirror or a window). I’m going to call them “spy mirrors” to avoid confusion.
Now, do spy mirrors exist in hotels? I mean, I kind of doubt that Holiday Inn goes to the trouble and expense of installing a specialized form of glass in all their bathroom mirrors, when they can’t even keep the pancake machine working. But you never know. So let’s dive in to the problems with the TikToks—and then I’ll tell you how these mirrors really work, so you can spot them easily.
What the TikToks say
My favorite example of the genre is this one, a lengthy step-by-step involving Scotch tape, three colors of dry-erase marker, and one person patiently instructing another on how “a girl” can figure out whether they are “safe” when traveling. The video is three fricking minutes long, and it doesn’t demonstrate anything that putting your fingernail against the mirror wouldn’t do in seconds.
According to TikTok, the telltale sign of a spy mirror is that you cannot see a gap between your finger and its reflection. You may hear the rhyming rule: “If there’s no space, leave this place”—the “space” being that gap between your real finger and your mirror finger.
So why not just use your finger? Good news: people simply putting their fingers against mirrors are all over TikTok, as well. Someone puts their finger up to a hotel mirror, then declares it to be either a spy mirror, or a normal mirror. (Sometimes the camera angle lets us see the gap, too; many videos make it hard to see if the person has done the test correctly.)
What the TikToks get right
It’s true that you’ll see a gap between your fingernail and its reflection in most “real” mirrors.
The most common style of mirror you’ll see in home and hotel bathrooms is made of glass. It has a thin layer of reflective material (“silvering”) that is usually applied to the back surface of the glass. This way, when you put your fingernail against the glass, you’re not directly touching the reflective part. This is called a “back silvered” or “second surface” mirror. So, yes: normally you’ll be able to see a gap.
What the TikToks get wrong
According to the TikToks, if you don’t see a gap, then the mirror is watching you. That’s not true.
The gap just tells you that the mirror is back-silvered. Real mirrors can be back-silvered or front-silvered and still be legit, normal mirrors. What you’re looking for, if you’ve got your eye out for spy mirrors, is something that is half-silvered. That refers to the type of coating, not just where it’s applied.
The silvering on (half-silvered) spy mirrors is applied to the front, so a front-silvered mirror could be a spy mirror. But it could also be a front-silvered regular mirror. (These aren’t very common, but they exist.)
A mirror that appears to show no gap could also be a back-silvered mirror whose pane of glass is very thin. I tested some mirrors around my house, and saw a large gap on one bathroom mirror, a smaller gap on another, and a very thin, difficult-to-see gap on a pocket-sized makeup mirror. The makeup mirror just has thinner glass than the others.
Spy mirrors work like mirrored sunglasses
If you’ve never seen a one-way mirror in person, you might think it’s some super-secret spy technology that’s impossible to spot. In reality, if you own a pair of mirrored sunglasses, you can do your own tests.
Mirrored sunglasses and spy mirrors work the same way. Three things make up the effect:
Light from the front can pass to the back.
Light from the back can pass to the front.
Bright light from the front reflects off the half-silvering, showing you your own reflection instead of whatever is behind the glass.
The whole effect depends on lighting. If the room behind the glass is dark, and the room in front of the glass is bright, then people in the bright room will only see their own reflection. But if the people in the dark room were to turn on their lights, or if the people in the bright room were to turn theirs off, the effect would be lost. It’s like a more dramatic version of how your home’s windows appear at night. You can’t see outdoors because the reflections from indoor lights are brighter than whatever is shining in from outside. The half-silvering just enhances this effect.
How to actually spot a spy mirror
Forget the fingernail test. All you really need is to manipulate the lighting, and you can do that in a matter of seconds with your phone’s flashlight. All you need to do is:
Turn off the lights in the room, or cup your hands around your eyes as you peer in. (Same as you would do if you were trying to peek into a car window on a sunny day.)
Put a flashlight right up to the mirror, and shine it through.
If the mirror is fake, you’ll be able to see the inside of the room on the other side of the mirror (or whatever is behind there).
That’s all there is to it. This trick works because you’re shining light into the area behind the glass, and allowing it to bounce back through the glass at you. (You don’t want to shine the flashlight on the exact spot you’re looking through, but if you hold it next to your face instead of right in front of your face, that will do the trick.)
I don’t have a police interrogation room handy, but I do have a pair of mirrored sunglasses, so I took the above photos of them with normal lighting (left) and in the dark with a flashlight (right). With the flashlight trick, you can literally see right through them.
By the way, you’ll also want to check what’s behind the supposed spy mirror, if you think you’ve found one. A mirror on a wall that is shared with an “employees only” room? Yeah, I’d be suspicious. (In that case, the flashlight trick would let you see the peeping Tom’s face, so you’d get a quick confirmation. Somebody please put that on TikTok instead of yet another “look at my fingernail” video.)
But I’ve seen at least one TikTok where the supposed spy mirror looks like a shower door. Mirrored shower doors are a cute design gimmick: the idea is for the person taking the shower to be able to see out, without others being able to see in. (Here’s a public bathroom that uses the same trick.) Half-silvered mirrors are also what enable smart mirrors like this one to exist: when the screen behind the mirror lights up, you see what’s on the screen. Otherwise, you just see your reflection. And finally, polished metal can be reflective and would not show a gap. That’s not a spy mirror either.