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Re: [Phys-l] pinhole camera


On Fri, 13 Apr 2007 14:38:18 -0400 "Robert Cohen"
<Robert.Cohen@po-box.esu.edu> writes:
My first reaction is to agree with Michael. If you remove the
screen
and look at the pinhole, you see the object (just a very small piece
of
it).

On the other hand, once you place the screen at a particular
location,
you produce an image there. By diffuse scattering, the rays
reflecting
off the screen are indeed (to a very close approximation) diverging
from
a point on that screen (which makes the image "real" in my view).

So, remove the screen - no image. With the screen, image.

With a lens, however, the image is there > regardless of whether the
screen is there
or not.

*** It certainly sounds reasonable ....but just how can you be sure that
the image is there when you remove the screen?????


This is not what I would have said prior to this thread, so I am
not
wedded to this and can be convinced otherwise.


----------------------------------------------------------
Robert A. Cohen, Department of Physics, East Stroudsburg University

570.422.3428 rcohen@po-box.esu.edu http://www.esu.edu/~bbq

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf
Of Michael Porter
Sent: Thursday, April 12, 2007 7:37 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] pinhole camera

On Apr 12, 2007, at 5:21 PM, Edmiston, Mike wrote:
My definition of real image would be more like... Light passing

through a particular point on the image came from a
particular point
on the object. That is, I see the object-image relationship as
a
one-to-one mapping of light from one point on the object to
one point
on the image.
A pinhole does that. A pinhole does not do that perfectly, but
as
John Denker said, a lens doesn't do that perfectly either.

I have to say, I'm a little surprised at what I see is a
somewhat fuzzy definition of an image that people are coming
up with (no pun intended...). Should we tighten it up a bit?
Aren't we all about nailing down the nit-picky details? ;-)

Yes, there is a recognizable "picture" on the screen of a
pinhole camera. But are we justified in calling it an image
and linking that picture to those created by lenses,
imperfect as they may be?

With a lens, the image is located where the
least-objectionable (most tightly focused bundle of light
rays) image is located.
What about a pinhole camera? No matter where you put the
screen, there will be a reasonably acceptable "image",
possibly as good as a lens. But the rays are diverging right
from the object. So where is the image? Everywhere? Or nowhere?

---
Michael Porter
Colonel By Secondary School
Ottawa, Ontario, Canada
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Forum for Physics Educators
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Herb Gottlieb from New York City
Where we live in peace and harmony