TikTok Myth of the Week: a Nail Test Will Tell You If 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 an ordinary hotel mirror to demonstrate the trick that such mirrors supposedly reveal. ( Ugh, that’s okay! We’re safe! ) Unfortunately, these videos are based on a misunderstanding of how mirrors work. Read on and I’ll tell you why these tricks are bullshit and why it’s an easy way to tell if a mirror is two-way. (This does not apply to the nail.)

What is a spy mirror?

All TikToks say they will show you a way to find out if you are being watched. The idea is that either there could be a camera hidden behind the mirror, or the mirror could 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 directly.

These mirrors exist. They’re perhaps most commonly called “one-way mirrors” (because the mirror only works if you’re standing on one side of it), but you’ll also hear them called “two-way mirrors” (because they can act as either a mirror, or window). I’ll call them “spy mirrors” to avoid confusion.

Are there spy mirrors in hotels ? I mean, I doubt Holiday Inn would spend the money to put specially shaped glass in all the bathroom mirrors when they can’t even keep the pancake machine running. But you never know. So let’s dive into the problems with TikToks and then I’ll tell you how these mirrors actually work so you can spot them easily.

What tiktoks say

My favorite example ofthis genre is a long, step-by-step story involving tape, three colors of dry erase marker, and one person patiently instructing another on how a “girl” can figure out if they are “safe.” during the journey. The video is three damn minutes long, and it doesn’t show anything that couldn’t be done in a matter of seconds by holding a fingernail to a mirror.

According to TikTok, the hallmark of a spy mirror is that you can’t see the gap between your finger and its reflection. You may hear the rhyming rule: “If there is no space, leave the place” – “space” is the space between your real finger and the mirror finger.

So why not just use your finger? Good news: there are people on TikTok just touching mirrors with their fingers, too. Someone puts their finger on a hotel mirror and then claims it’s either a spy mirror or a regular mirror. (Sometimes the camera angle allows us to see the gap; in many videos it is difficult to tell whether the person performed the test correctly.)

What TikTokers Get Right

It’s true that in most “real” mirrors you will see a gap between the nail and its reflection.

The mirror most commonly seen 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 of the glass. This way, when you place your fingernail on the glass, you are not directly touching the reflective part. This is called a “back-silvered” or “second surface” mirror. So, yes: you will usually be able to see the space.

What do tiktoks get wrong?

According to TikTok, if you can’t see the crack, it means the mirror is watching you. It is not true.

The gap simply indicates that the mirror is silvered. Real mirrors can be silver plated on the back or front and still be real regular mirrors. If you’re looking for spy mirrors, you’ll want something semi-silver . This applies to the type of coating, not just where it is applied.

The silver plating on (semi-silvered) spy mirrors is applied to the front, so a silver-plated front mirror may be a spy mirror. But it can also be a regular mirror with a silver-plated front. (They’re not very common, but they do exist.)

A mirror that appears to have no gap may also be a mirror with a back side, silver plated, with very thin glass. I checked several mirrors in my house and saw a large gap on one bathroom mirror, a smaller gap on another, and a very thin, hard to see gap on a pocket makeup mirror. The makeup mirror has thinner glass than others.

Spy mirrors work like mirrored sunglasses

If you’ve never seen a one-way mirror in person, you might think it’s some kind of top-secret spy technology that can’t be detected. In fact, if you have mirrored sunglasses, you can do your own tests.

Mirrored sunglasses and spy mirrors work the same way. Three things create the effect:

• Light from the front can pass to the rear.

• Light from behind can pass forward.

• The bright light from the front reflects off the half-silvering, showing you your own reflection rather than what’s behind the glass.

The entire effect depends on the lighting . If the room behind the glass is dark and the room in front of the glass is light, then people in the light room will only see their reflection. But if people in a dark room turned on the light, or if people in a light room turned off the light, the effect would be lost. This is a more dramatic version of how your home’s windows look at night. You can’t see outside because the reflections from the interior lights are brighter than what’s shining outside. Half-silvering only enhances this effect.

How to Actually Detect a Spy Mirror

Forget about the nail test. All you really need is to control the lighting, and you can do it in seconds using your phone’s flashlight. All you need to do is:

1. Turn off the lights in the room or cover your eyes with your palms and look inside. (The same thing you would do if you were trying to look out of a car window on a sunny day.)

2. Hold the flashlight directly to the mirror and shine it on it.

3. If the mirror is fake, you will be able to see the inside of the room on the other side of the mirror (or whatever is there).

That’s all. This trick works because you shine light into the area behind the glass and let it reflect through the glass back at you. (You don’t want to shine the flashlight at the exact spot you’re looking through, but holding it close to your face rather than directly in front of it will help.)

I don’t have a police interview room handy, but I do have a pair of mirrored sunglasses, so I took the photos above of them in normal light (left) and in the dark with a flashlight (right). With a flashlight you can literally see 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? Yes, I would suspect it. (In this case, the flashlight trick will allow you to see the Peeping Tom’s face and get a quick confirmation. Someone please post this on TikTok instead of another “look at my nail” 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 trick: the idea is to allow the person showering to see outside without others being able to see inside. (Here’s a public bathroom that uses the same trick .) Half-silvered mirrors are also what allows smart mirrors like this to exist: when the screen behind the mirror lights up, you see what’s on the screen. Otherwise you will just see your reflection. Lastly, polished metal can be reflective and leave no gaps. This is not a spy mirror either.

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