One of my relatives recently had some bifocal eyeglasses made at Wal*Mart. The distant-vision lenses seemed OK, but she complained of big problems, including double vision, when she tried to use the reading lenses. The Wal*Mart guys said she’d get used to it in a few days.
The glasses looked OK to me at first glance, but on a hunch I used one of my dollar-store lasers to locate the optical center of the reading lenses. The results were as shown below, where the optical center is indicated by a “+” mark:
________________ ________________
/ \ / \
| | | |
| _________ | | _________ |
| / + \ | | / + \ |
\ | / \ | /
\ \ / \ / /
\____\_______/ \_______/____/
It was kinda fun to be able to do some physics that took only a minute or two, and was of immediate value to a real person (as opposed to long-term research that requires a big lab full of equipment, and is supposed to benefit “somebody” in the distant future).
I find it hard to imagine how anybody could mess up a pair of glasses so badly. I would have thought the grinding process would be highly automated. And FWIW this was their second bite at the apple; the first pair they made had a conspicuous seed right in the middle of one of the lenses. Sheesh. You can’t make this stuff up.
I don’t know how to make a classroom lesson out of this, but it’s fun to think about. It would have been nice to keep the darn things around for use as a “hands on” exam problem (“patient complains of double vision; is there something wrong with these glasses?”) ... but I didn’t think of that in time. I enjoy questions of that sort; they feel like “real world” questions, requiring sorting through an almost-unlimited set of hypotheses, which makes them very unlike the usual plug-and-chug questions.