come_to_think

As is well known, if you put three mirrors at right angles to each other, a ray of light entering the corner will be reflected back in the direction it came from.  The people we sent to the moon left some devices of that kind there, with the result that we can do lidar on the moon and measure its motions exquisitely.

I have wanted for a long time to put such a thing in an upper corner of my room, so that if you look at it from anywhere, you'll see your face upside down.  1-ft mirror tiles, available on the Web from various suppliers, are just the thing, but I worried about how to mount them.  Then it occurred to me that they wouldn't have to be mounted on the ceiling; they could rest on one of my tall bookcases.  So I ordered some, and put them together with duct tape, in such a way that the bottoms & tops of the vertical mirrors are flush.  The top mirror, being unsupported at the outer corner, sags a little, so that it looks as if you had four eyes; but an inconspicuous strut, consisting of a 12-in. stretch cut from a coathanger, takes care of that.

The tiles come in boxes of six, so I have three left over.  Anyone in the vicinity who wants to make a retroreflector is welcome to them.