I'm always intrigued when I stumble across a cool physics phenomenon that most people would tend to pass by without a second thought. One of my favorite overlooked physics effects is scratch holograms. They can turn up just about anywhere you have a polished surface that gets scratched in patterns that are surprisingly easy to produce. In fact, most of the scratch holograms I've seen have been accidental.
While I was at the annual APS April Meeting in Baltimore last weekend, I found these patterns on just about all the tables scattered around the hotel where we were gathered to talk about black holes, gravitational waves, cosmic rays and other hardcore physics topics.
In the video, you can see a coin I set on the table to show you where the surface is. As I moved the camera back and forth, there appeared to be a shifting reflection. And it is indeed a type of reflection, but the pattern isn't a reflection of something above the table - it's a 3D pattern that appears to be several inches inside the table. That is, it's a hologram image encoded in the scratches on the table surface.
It's clear that someone made the scratches as a kind of decoration. In the process, they created a hologram that is effectively an image of the device used to scratch the table. In case it's not clear what I mean, here's an image of a scratch hologram made by accident on a car hood with a dirty polishing rag.
|William J. Beaty found this hologram on a car hood in 1995 and figured out how to make custom versions.|
If you have some time, I highly recommend making a few custom scratch holograms of your own. Someday, I plan to decorate my whole car with scratch holograms. It'll be the coolest (I hope).