Some textbooks define the focal point as the point where the focal plane
intersects the lens axis. I have no problem with that. My problem is
the textbook definition of the focal plane.
Ignoring the fact that the focal plane is probably not exactly flat, I
would define the focal plane as that plane where the primary object of
interest is in focus. That's where you would put the film in the case
of a camera. Then the focal point definition given above still holds as
far as I am concerned.
I may be all wet, or conversing in the wrong circles, but the above is
the way I hear it and view it. Whatever our subject (object) is, we
want to get the film at the focal plane for that object, or equivalently
we want to move the focal plane onto the film plane. This usage means
the focal plane and focal point are dependent on the object distance.
When students are doing lab work with the optical bench, I think this
wording makes a lot more sense. Using the standard illuminated arrow,
the students try to verify 1/O + 1/I = 1/F F is the focal length of the
lens, O is the object distance (easily measured), and I is the image
distance when the image is in focus, which means when the screen is at
the focus (focal plane) for that particular circumstance.
Otherwise, you have to say that when you have the image sharply focused
that the screen is not really at the focus (focal plane) of the lens,
and that just won't seem right to the students (or to me). I say the
focal plane is where the image is in focus.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
_______________________________________________
Phys-L mailing list
Phys-L@electron.physics.buffalo.edu https://www.physics.buffalo.edu/mailman/listinfo/phys-l