Spy Through Frosted Glass with a Single Piece of Scotch Tape

Frosted glass windows are meant for privacy, plain and simple. Sometimes you see it used for aesthetic effect, but primarily it's used to let a little light in while keeping wandering eyes out—unless those wandering eyes have some Scotch tape in their pocket.

Yes, a mere piece of transparent tape is all you need to turn blurry images into recognizable shapes. Privacy into peep show. It's not exactly a secret these days, but if you haven't seen this classic trick yet, this is how it looks:

How This Trick Works

Frosted glass is made after the fact. You start with a normal piece of flat clear glass, then sandblast or acid etch until the smooth, transparent surface becomes rough and full of inclusions. Those surface imperfections diffuse light when it hits or passes through them, creating the warped, blurred "frost" on the glass.

When you place pressure-sensitive tapes made of clear cellophane (like packing tape) onto the glass, the clear glue fills the microscopic crevices in, while the cellophane helps flatten the glass again. The light is no longer diffused and images become clearly defined. It's still slightly blurry because of the etchings and glue, but a significant increase in visual perception is experienced.

How to Make It Work Even Better

To get this trick to work better, keep these tips in mind.

The thinner the abrasion of the frosting, the better this will work.
Most frosted glass has only one actual frosted side. As long as the tape is on the frost, you should be able to see in, no matter which side you're peering through.
If both sides are frosted, then you'll need to apply tape to both sides for maximum acuity.
In order to see anything clearly, your eyes need to be close to the tape—in true peeping Tom-fashion.
Other things produce similar effects, like water on frosted shower doors.