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Re: [Phys-l] optical center : real-life physics

So if I read the ASCII-art correctly, the optical center of the reading
sections of both lenses was near the upper-right boundary. That does
have to be wrong. But what would be "right"? Both at the center of
their respective sections?

Incidentally, two weeks ago I was outfitted for the first time with
bifocals myself. I opted for the varifocal or progressive addition
lenses, and have adjusted to them moderately well. (It's a great relief
to be able to read again, and also very convenient for computer work:
just tip your head to the angle that brings things into focus.) But the
optometrist's explanation of their function confused me, and the more
complete wikipedia explanation
(http://en.wikipedia.org/wiki/Progressive_lenses) still doesn't answer
my main question: Why must there be a blurry region to each side of the
central vertical "channel" where the lens power gradually changes? I
would have thought the sides could be revert (with some gradient, of
course) to the normal distance prescription. But perhaps the side
sections are specified by the boundary conditions of matching the edges
of the central channel and not exceeding some specified thickness at the
rim. (My new lenses are significantly thinner at the rim than my normal
distance prescription lenses were -- I'm quite near-sighted.)

Does anyone here know more about progressive lenses?

Ken Caviness
Physics @ Southern




-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
Sent: Monday, June 11, 2007 4:36 PM
To: Forum for Physics Educators
Subject: [Phys-l] optical center : real-life physics

Hi --

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.

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