Through the No-Looking Glass

Frosted glass — you can see a picture of some here — is a special type of glass which is translucent (that is, light goes through it) but not transparent (objects on the other side can’t be seen clearly). While it is sometimes used for decorative purposes, it also has a practical function: it is designed to let light into an area without giving up privacy. But in many cases, that privacy can be undone with a single, readily-available tool: cellophane tape.

Don’t believe me? Watch!

The video is pretty straightforward. The person takes a piece of what is often called Scotch tape (due to the predominant brand of cellophane tape), sticks it on the frosted glass, rubs it flat, and like magic, the glass becomes almost transparent — as if it weren’t frosted in the first place. But it’s not magic. It’s science, and not all that complicated science at that.

Frosted glass starts off as regular, transparent glass. Via acid etching, sandblasting, or another similar process, one of the sides is riddled with tiny imperfections which are often barely detectable by touch. These bumps and ridges cause the light to scatter as it passed through the glass, leading to the frosted effect. If the bumps weren’t there — or if they were somehow filled in — the glass wouldn’t be frosted.

That’s precisely what the tape does. Not much happens when the tape is applied to the frosted glass, at least not immediately. But when the tape is rubbed flat, the adhesive glue fills in all the nooks and crannies, effectively restoring the glass to its original state. Because the tape is transparent (okay, maybe a bit foggy), the frosted glass becomes see-through. When the tape is removed, enough of the adhesive comes off with it such that the glass returns to its frosted state, usually. It may end up damaging the effect somewhat, though.

If you try this yourself it should work fine, but if it doesn’t, there’s a good reason why — you have to put the tape on the frosted side of the glass, and it’s sometimes hard to tell which one that is. And in some cases, the glass is frosted on both sides. In either case, you’ll need another piece of tape — one for the opposite side. (And in the case of the double-frosted glass, make sure the tape pieces are lined up!)

Bonus Fact: “Scotch” tape isn’t named for the brand — not exactly, at least. Wikipedia notes that the term was used in a not-so-nice sense at first. During an initial test of the product, a customer was upset with the lack of adhesive on the tape and believed that 3M, the manufacturer of the tape, was cutting corners to save a few cents. He used the term “Scotch” to mean “stingy” and the insult made it back to the bosses at 3M. But 3M decided to adopt the slur as a name, calling the brand “Scotch tape.”

From the Archives: Make Your Own X-Rays: Another trick with Scotch tape.

Related: A lot of Scotch tape. Frosted glass, you’ll have to find yourself.