Date: Fri Apr 13 13:04:19 2007

Author: Steve Wonnell

Subject: Re: [Phys-l] pinhole camera

Post:

Zeke:

> The reason for giving it a different name is that the image made by a
> pinhole is qualitatively different than the image made by either a
> converging lens or mirror.

No objection here.

> To be pedantic for a moment, in a converging lens or mirror the rays of
> light from one spot of the object are brought together and cross at
> another spot on the focal plane. These rays of light streaming off this
> spot in space are qualitatively the same as the original spot on the
> object. For nearly all optical intents and purposes, the object has been
> moved to the focal plane, even if a screen is not there.

What you say can be equally well used to argue that a virtual image is
not an image either.

> And here's the kicker, nearly anything that you could have done
> optically on the object at its original location you can now do on this
> image. For example, you can look at the image with a magnifying glass
> and make it larger. After all that's what a telescope does.

Again, a strike against the virtual image, too.

> On the other hand, the image made by a pinhole just doesn't do this --
> without a screen. With a screen you can take the ray and split it up,
> but it cannot be done otherwise. When I am feeling cruel, I assign my
> students the task of using a pinhole and a lens to make a telescope. A
> frequent test question asks if it is possible to use two pinholes to
> create an erect image.

Without a focusing lens such as your eye, a virtual image can't be seen
either.

> The pattern of light (to use Dewey Dykstra's term for it) of a pinhole
> may look superficially like the image made by a lens, but it behaves
> differently. Call it what you'd like, but they are really not that
> similar.

Okay, it's a "type III" image. :)


Steve


> Marc "Zeke" Kossover
>
> ----- Original Message ----
> From: Rick Tarara
> To: tap-l@lists.ncsu.edu
> Sent: Friday, April 13, 2007 8:41:47 AM
> Subject: Re: [tap-l] [Phys-l] pinhole camera
>
>
> Well to each his own. I doubt there is a 'standard' definition of image,
> but many here seem happy (as am I) with the notion that if the light forms a
> focused, viewable picture on a screen, then it has formed a real image.
> That the pinhole does so differently than a lens or mirror and that the
> 'image' is formed over an 'infinite' depth of parallel planes doesn't
> necessarily mean that it is not an image--unless you define it to not be!
> ;-)
>
> Rick
>
> ***************************
> Richard W. Tarara
> Professor of Physics
> Saint Mary's College
> Notre Dame, IN
> rtarara@saintmarys.edu
> ******************************
> Free Physics Software
> PC & Mac
> www.saintmarys.edu/~rtarara/software.html
> *******************************
>
>
> ----- Original Message -----
> From: "Richard Berg"
> To:
> Sent: Friday, April 13, 2007 11:17 AM
> Subject: Re: [tap-l] [Phys-l] pinhole camera
>
>
> >
> > I believe the logic is that there is no focusing so there is no image. A
> > pinhole produces a "negative shadow" in which the bright area is the
> > "shadow."
> >
> > You can certainly expose a film with it because the light is actually
> > there. You can also use it as an object for a lens. But if you select an
> > adjacent plane you can also expose a film with that or use it as an
> > object. If you produce a true "image" with a lens, it becomes defocused
> > if you view it on planes different from the image plane. Not so with a
> > pinhole image. All of the light from a point on the object is "focused"
> > onto the corresponding point on the "image" as these terms are usually
> > defined.
> >
> > I think "pinhole image" does not have the characteristics of an image as
> > it is normally defined.
> >
> > Dick
> >
> > On Fri, 13 Apr 2007, Steve Wonnell wrote:
> >
> >>
> >> Dick,
> >>
> >> Why not? The "image" can be projected on a screen. Photographs can be
> >> made with the pinhole. I understand that the pinhole is not a lens
> >> nor a mirror...but the "image" sure seems like an image.
> >>
> >> Steve W.
> >>
> >>
> >> On Fri, 13 Apr 2007, Richard Berg wrote:
> >>
> >>> Date: Fri, 13 Apr 2007 10:33:30 -0400 (EDT)
> >>> From: Richard Berg
> >>> Reply-To: tap-l@lists.ncsu.edu
> >>> To: tap-l@lists.ncsu.edu
> >>> Subject: Re: [tap-l] [Phys-l] pinhole camera
> >>>
> >>>
> >>> I presume that everyone knows that a "pinhole image" is NOT an image.
> >>>
> >>> Dick
> >>>
> >>> ******************************
> >>>
> >>> I am thinking it would be a good way to show students that light travels
> >>> in straight lines, and to introduce them to real images (rather than
> >>> begin
> >>> with virtual images in a plane mirror).
> >>>
> >>> ***********************************************************************
> >>> Dr. Richard E. Berg, Professor of the Practice
> >>> Director, Physics Lecture-Demonstration Facility
> >>> U.S. mail address:
> >>> Department of Physics
> >>> University of Maryland
> >>> College Park, MD 20742-4111
> >>> Phone: (301) 405-5994
> >>> FAX: (301) 314-9525
> >>> e-mail reberg@umd.edu
> >>> www.physics.umd.edu/lecdem
> >>> ***********************************************************************
> >>
> >
> > ***********************************************************************
> > Dr. Richard E. Berg, Professor of the Practice
> > Director, Physics Lecture-Demonstration Facility
> > U.S. mail address:
> > Department of Physics
> > University of Maryland
> > College Park, MD 20742-4111
> > Phone: (301) 405-5994
> > FAX: (301) 314-9525
> > e-mail reberg@umd.edu
> > www.physics.umd.edu/lecdem
> > ***********************************************************************
> >
>
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