A Minor Mystery

We have these interesting exit signs in the hallways at work.

Can you spot what makes this odd?

If you don’t see it immediately, focus on the reflection of the back of the sign. Look at the top of the sign itself, where you can see all of the mounting plaque.

Yes, the transparent sign has a reflection of the back that doesn’t match up with what we see on the front.

I tweeted this sign about a week ago and had several people very confused. Josh the SpokesGay accused me of hurting his head. (Sorry, Josh.) A few of us, including my husband, worked it through together.

First off, everyone trusted that I’d presented a fair puzzle. That was helpful. Then we started reverse-engineering the thing.

I got a couple of questions about silvering. I confirmed that, even up close, the edges of the sign showed what was behind them, at least within the tolerances for refraction as light passed through the plastic. I couldn’t say the same where the lettering was, though, with the light shining out. We talked about putting silvering behind the lettering.

Then I thought about magic tricks, particularly magic cabinets, and took a good look at where the sign was mounted on the ceiling grid. What is the major characteristic of a grid? It’s very regular. The part in front of the sign and the part behind the sign would look exactly the same. I decided I could no longer naively trust the evidence of my eyes that I was seeing refraction rather than reflection.

Thinking we might have the answer in hand, my husband asked whether I had access to a yardstick (meterstick for those using terrorist units). I didn’t.

While I was thinking about the resources I had in the office, both in terms of long things and people who would be willing to help me get a good picture, a friend presented us with the answer.

Two versions of this sign are made. One really is transparent. One has silvering sandwiched between two layers of sign. Mounted correctly on a grid, the reflections of the grid add to the perception that the sign is transparent. It’s a very neatly engineered piece of plastic.

Now, it’s also a mystery solved. I hope Josh’s head has stopped hurting.

A Minor Mystery

7 thoughts on “A Minor Mystery

  1. 3


    I didn’t know how much more I could stand walking around in that non-Euclidean geometry.

    You’re causing me sudden Shirley Jackson flashbacks… not cool!

  2. 7

    First saw those while I was working a thousand unit cubical install in VA. It helped that I was able to sneak over and pick up one of the uninstalled ones when the electrician wasn’t looking.

    Wait, what? On the sign, the arrow is next to the E, and in the “reflection”, it’s next to the T. So it couldn’t possibly be a reflection.

    It’s a reflection of the other side. Both sides are made to be readable, so the other side of the exit sign says EXIT> instead of <EXIT

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