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Great demo! You have to see it to believe it! I have clear plastic
biconvex lens cut in half along the edge, and then one of those halves cut
in half across the middle. So now each piece has the same focal length.
Students can clearly see that the ENTIRE image (of a bright bulb filament)
with the "half" lens is half as bright. Of course, I show this only AFTER
I ask them to predict what they will see. Most get this wrong the first
time around. It defies intuition, even for my brightest students. If you
don't have a half lens, you can simply cover the lens with black tape. You
can also break a lens into several pieces to show that you will always get
a full image with reduced brightness.
Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
I see that BC found my optics web site. This is a fun lab.
I took a couple of large surplus lenses (about 100mm in diameter) and
went to the Geoscience department and asked to use their rock saw to cut
the lenses in half. They already know that I'm nuts so they just said
"sure" and watched as I "destroyed" some perfectly good lenses. It did a
very nice job of quickly cutting the glass in two.
It is odd to take half of a lens and cast an image. What is weird the
first time you see this is that in many ways the image from half a lens
is better than from the original whole lens. (This is because you have
smaller angles for many of the off-axis rays and thus some of your
Seidel Aberrations are reduced.)
When students cast an image with half of a lens you can see their
facial expressions go, "what the heck?"
As BC notes, it is difficult to do this activity and still stay wedded
to the concept that there are only 3 rays.
This activity is not an original idea. I don't recall where I first
heard about it though, so here is a general "thank you" to whoever it
was.
John
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
John E. Sohl, Ph.D.
Professor of Physics
Weber State University
2508 University Circle
Ogden, UT 84408-2508
voice: (801) 626-7907, fax: (801) 626-7445
cell: (801) 476-0589
e-mail: jsohl@weber.edu
Philip!Bernard Cleyet <bernardcleyet@redshift.com> 5/25/2010 4:37 PM >>>
Evidently, they must do this (your below) before they can usefully use
the matrix method. Its beauty is one may specify any (with in the
paraxial approx. for the non-exact method) input vector and obtain the
out put vector. The vector being the angle and height of incidence WRT
the optical axis of the pencil. Not incidentally, students are now
rather computer lit., so good exercise there.
--------------------
[Part I: Light Rays and Imaging:
Obtain an image using an object (use the old lamp housing with the
crossed arrows), lens and
screen.
Use a card to block the object. In particular, slowly slide the opaque
card across the face of the object. Watch the image. It should do
exactly what you expect.
Now use the card to block the lens. Keeping the card very close to the
face of the lens, slowly slide the card around while watching the image.
Questions 1: Is this what you expected? What happened to the image as
the card blocked the lens? How much of the lens had to be unblocked for
an image to form? Did it matter what part of the lens was unblocked?
Next, take just a few minutes to explore the effect of the size of the
opening. Set the opening of the iris so that it is a few millimeters
across at most. Repeat what you just did with the card, that is, move
the small opening around in front of the object and up close to the lens
while watching the image. Try a few different sizes for the opening of
the iris.]
Questions 1: Is this what you expected? What happened to the image as
the card blocked the lens? How much of the lens had to be unblocked for
an image to form? Did it matter what part of the lens was unblocked?
Next, take just a few minutes to explore the effect of the size of the
opening. Set the opening of the iris so that it is a few millimeters
across at most. Repeat what you just did with the card, that is, move
the small opening around in front of the object and up close to the lens
while watching the image. Try a few different sizes for the opening of
the iris.
© 2009, John E. SohlPage 1 of 4PHYS 3190, Multiple Lenses
Think about question 2 before you do the next experimental step.
Question 2: What do you think will happen if you take a broken piece of
lens and try to image an object?
You have a lens that has been cut in pieces with a diamond saw. (The
cut edges have been painted black to reduce light scatter from the rough
glass.) Question 3: The results you just obtained with the card, iris
and lens shard were probably surprising. Using the concept of rays (a
drawing or two might help) explain how this works. Especially, explain
how an image is formed with most of the lens blocked or even missing.
Question 3: The results you just obtained with the card, iris and lens
shard were probably surprising. Using the concept of rays (a drawing or
two might help) explain how this works. Especially, explain how an image
is formed with most of the lens blocked or even missing.
http://planet.weber.edu/Optics/Handouts/Lab4-MultipleLenses.pdf
Note that John's lab. address all the questions and comments on this
part of the thread.
bc forever a Googler.
On 2010, May 25, , at 14:04, Philip Keller wrote:
Yes, the books cover ray tracing. But often, students focus so muchon what happens to the three or four rays that they have rules for that
they think that those are the only rays there are! I like to have them
draw those rays in one color, but then as an exercise, add in a bunch of
other rays that you can draw after you know where the image is. This is
a necessary trick if you are going to use ray tracing to locate the
image formed by a "virtual object" in two lens systems.
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l