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Re: [Phys-l] half lens



I've only quickly skimmed the posts on this but I seem to have missed
someone commenting on the main difference between these two demos. As
far as I can tell, the difference is this: In case (1), you are
considering the image that is formed by the lens. In case (2), you are
considering the part of the image that you see.

For example, consider a picture hanging on the wall. If you cover up
the top part of the picture, the whole picture is still there, you just
can't see it. In a similar way, if you cover up half the lens, the
whole image is still there, you just can't see it all (i.e., the same
"all" that you would've seen without the lens covered up). Simply move
your position and you'll see the other parts of it, so the entire image
has got to be there.

Looking at the image directly without the help of a (diffuse) reflecting
screen causes the apparent paradox, I think. With the reflecting
screen, all parts of the image are reflected toward your eyes, no matter
where you are standing (as long as you are on the correct side of the
screen, that is). This also is why the image is darker when observed in
this manner (rather than observed without the screen).

If this has already been mentioned and I missed it, I apologize.

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf
Of Anthony Lapinski
Sent: Tuesday, May 23, 2006 3:49 PM
To: tap-l@lists.ncsu.edu; tap-l@lists.ncsu.edu;
phys-l@carnot.physics.buffalo.edu
Subject: [Phys-l] half lens

Case (1)
If you cut a plastic lens in half (through the diameter), you
can still form real images. Compared to the original (full)
lens, the images are the same size (same focal length), but
appear half as bright.

Case (2)
So I asked my students a follow-up lab question: If you put
black tape over half a camera lens (the old SLR type), how
would the images be affected? Obviously, they would be intact
but half as bright, no matter which half of the lens was
covered. But a few students, who are also in a photography
class, said you would only see half the object/scene. They do
this as a project using a "photo divider," which attaches to
the front of the lens and can rotate. So one can do double
exposures of half a scene at a time. Wow!

I had never heard of this before. And it seems to contradict
the half lens demo I did in class. Can someone tell me how
these camera attachments work? Do the objects have to be
"very close" to the camera?