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Re: Cat's eyes

At 06:33 PM 9/3/00 -0500, brian whatcott wrote:

Not OK. The photosensitive surface is not planar.

>Hint: the mirror is in the FOCAL PLANE.

No it isn't. See above.

This is nitpicking in the worst tradition. It is the opposite of
helpful. It is also wrong.

1) It is the opposite of helpful because a retroreflector with a nonplanar
focal surface operates on the exact same principle as one with a planar
focal surface. Emphasizing the nonplanar aspects is a needless complication.

2a) It is implicitly wrong because the implication that the mirror is not
at the focus is wrong. Unless Ludwik's cat has bad eyesight, the mirror is
at the focal surface. I think Ludwik is smart enough that he can, if/when
necessary, generalize the concept of "focal plane" to the more general
concept of "focal surface".

2b) It is explicitly wrong because I am free to create a model system
consisting of an ideal lens and a mirror in the focal plane. If Brian
wants to discuss some other system, he can do that, but he shouldn't say
that my system -- planar by construction -- is nonplanar.

There are all sorts of good pedagogical reasons for introducing a
simplified model. Attacking the key simplification -- without offering
anything constructive in exchange -- is bad pedagogy.