One of my relatives recently had some bifocal eyeglass 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.