[Phys-L] Re: optics terminology

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

Michael Edmiston wrote:

> When I first responded to John's question, I was not in my office and
> didn't have a supply of textbooks to examine.  Now that I'm in my
> office, I find the same thing John said.  Every book I examined says the
> focal point is the place where paraxial rays come to a focus.  That
> makes the focal length always equal to the distance between the lens and
> the focal point, just like John said he read.
>
> I am surprised I have just now noticed this, thanks to John, because I
> do not agree with this.  It appears to me that the textbook convention
> is not consistent with everyday use of the word by practicing physicists
> as well as the general public.

As I wrote earlier in response to John Denker's original post, it was
my impression that practicing physicists (if not the general public)
almost universally understood a "focal point" to be a place on the
optic axis one focal length away from the lens.  Now that Mike
confirms John's impressions to the contrary, I suppose I will have to
become a little less cavalier about that assumption.  Still, I can't
help but wonder how common John's and Michael's alternate usage of
the term really is for the same reason others have expressed:  What
ARE we to call that important point that ies one focal length away
from the lens on the optic axis that is SO useful in the construction
of ray diagrams?

Michael goes on to write:

> For example, the textbooks also state that the focal plane is one focal
> length behind the lens, and that the focal point is where the focal
> plane intersects the lens axis.

and takes issue with that definition.  Here I would agree.  It seems
to me that the term "focal plane" refers to a locus of points (not
necessarily a plane and not necessarily including what *I* would call
the "focal point" of the lens or lens system) on which a desired
image is formed.  I do now, for the first time, appreciate the
apparent illogic of having a "focal plane" that does not include "the
focal point."  Nevertheless, and despite John's and Michael's
testimony to the contrary, it seems to me that that IS standard usage
at least among physics educators and, I still think but am less
confident than I used to be, among physicists in general.

--
John "Slo" Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://www.csupomona.edu/~hsleff/OoPs.html>
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