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Re: terminology: period vs. wavelength

John makes some good remarks, but I don't completely agree (which I guess
is appropriate for this thread)!

Not exactly. If everybody agreed on what incidents should or should not
classified "grand theft auto", then we would have hardly any need for
trials or trial lawyers, and even less need for courts of appeal (which
usually deal with questions of law, not questions of fact).

I would say that if the FACTS were well known, then we would have hardly
any need for trials or trial lawyers.
The trial isn't generally about what constitutes "grand theft auto" but
whether there are enough facts to convict in a particular case. And the
appeal is usually about whether evidence was presented in the appropriate
manner, or perhaps there is new evidence.

And further, if it IS an appeal about a particular point of law, the
outcome becomes the precedent for all subsequent trials, so the same issue
doesn't have to be argued again. (At least until lawmakers rewrite the

In mature areas of physics, the facts are usually well known. We all know
what is happening in freefall. It is just a question of whether we like to
call that "weightless". Why should we intentionally confuse students by
having various definition of weight and weightless in different books?

I indicated (with dimension lines) a particular distance and asked the
students to give a name for that distance.

I can think of several non-ridiculous answers to that question.
-- one wavelength
-- one cycle
-- one period
-- the pitch (in the context of a CRT, or a halftone mask, or
a diffraction grating, et cetera)
-- the lattice constant (in crystallography)

Certainly there are different answers in different contexts, and sometimes
more then one could be correct. Or one is closer to correct. I'm guessing
that the context was an introductory level phyics introduction to waves,
and that the picture was a sinusoidal oscillation with the axis labeled

Assuming that to be the context, then
* "wavelength" gets full credit
* "Cycle" gets partial credit. A cycle is the set of points, but the
wavelength is the repetion distance. (Like a wing and a wingspan are
* "period" gets some credit, because with a different label on the axis, it
could be correct.
* "pitch" gets no credit, because in this context it means a perception of
a sound wave.
* "lattice constant" gets no credit, because I'd be too confused.

In different contexts, my grading would be different. And if a student
wants to come in and explain his context, I might give more credit.

Suppose the assignment is to write a sonnet. Do we expect all the
to turn in identical sonnets? Is one sonnet right and all others wrong?

But if they turn in a limeric, they won't get much credit :-) The answer
has to at least be in the right general ballpark.

Closer to physics: There are hundreds of known proofs of the Pythagorean
theorem. Is one of these right and all others wrong?
Why should we assume that every solution-set has only one element?

If the question is reasonably well-posed, then there should be a set of
answers that are fully correct, a set of answers that are partially
correct, and a set of answers that are completely wrong. (That said,
making well-posed questions is a real challenge.) The instructor has to
know the material and the context well enough to recognize into which
catagory the asnwer lies.