Re: [Phys-l] pinhole camera

["Robert Cohen" <Robert.Cohen@po-box.esu.edu>]

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.

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