Re: [Phys-l] pinhole camera

["Jeffrey Schnick" <JSchnick@Anselm.Edu>]

We have, in the physics lab, this big converging lens, about 40 cm in
diameter, with about a 45 cm focal length.  An incandescent light bulb
is placed about 6m in front of the lens, on the principal axis of the
lens.  As part of a laboratory exercise, a student positions herself
about 90 cm behind the lens such that the lens is at eye level. The
student's lab partner positions a piece of plain white copier paper
between the student and the lens, at the location of the image of the
light bulb.  The image is bright enough so that one can see it from
either in front of or behind the paper.  The lab partner slowly raises
the paper while the student focuses her attention on the image of the
light bulb.  About 1 in 5 students report that the image is visible
right where it appeared when the paper was there.  It appears to be
hovering there in the air.  I used to be able to see it.  It is
fascinating.  Whether or not it looks like it is right there, one can
use parallax to establish the location of the image (with the paper
screen removed).  I put a piece of white tape on the top of the lens and
when the student moves her head to the right, relative to her, the image
moves farther to the left than the piece of tape does.  When she moves
her head to the left, relative to her, the image moves farther to the
right than the piece of tape does.  Based on parallax, the student sees
the image closer to herself than the lens is.

When I try this with a piece of paper with a pin hole in it, what I am
looking at appears to be the same angular size as the object, and, it is
back behind the pinhole at a distance, based on parallax, that I judge
to be at about the same distance from me as the object itself is.  I
conclude that I am looking at the object rather than an image of the
object.

When I use a pinhole camera to form an image of a scene on a screen, I
think the light coming from a particular point in the scene is diverging
from that point on its way to the screen.  So I agree with Robert Cohen,
with no screen, there is no image.

Jeff Schnick
Saint Anselm College


> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of Herb Gottlieb
> Sent: Friday, April 13, 2007 6:17 PM
> To: phys-l@carnot.physics.buffalo.edu
> Cc: phys-l@carnot.physics.buffalo.edu
> Subject: 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?????