Re: [Phys-l] pinhole camera

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

I believe Jeff adequately addressed Herb's question (how can you be sure
that the image is there when you remove the screen?).  I'd like to add
that the image can more easily be observed if you place a wire hoop at
the location of the image (with the image contained within it).  That
way, the eye has something to focus on.

EYE . . . . Hoop (with image) . . . . Lens . . . . Object

This works best for real images.  Virtual images can also be seen this
way but the wire hoop must be larger than the lens, so that the observer
sees the hoop "outside" the lens and sees the image "inside" the lens.

----------------------------------------------------------
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 Jeffrey Schnick
> Sent: Friday, April 13, 2007 8:08 PM
> To: Forum for Physics Educators
> Subject: Re: [Phys-l] pinhole camera
>
> 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