Re: virtual images and convex lenses

[Robert Cohen <Robert.Cohen@PO-BOX.ESU.EDU>]

I agree with Michael Porter.

I'd like to just add a couple of comments:

1. Contrary to what John Denker wrote, I thought Tim's analysis addressed
his main problem from before (i.e., assuming the "scene" to be *in* the
lens).

2. As Mike wrote, I can easily "see" both real and virtual images without
the help of a screen.  This is just a matter of semantics.  I wear glasses
with diverging lens.  I'd say that *everything* I see is a virtual image.
If there is a problem with how I use "see" then is it possible to even "see"
objects?

--------------------------------------------
Robert Cohen           rcohen@po-box.esu.edu
570-422-3428         http://www.esu.edu/~bbq
Department of Physics
East Stroudsburg University
East Stroudsburg, PA  18301
--------------------------------------------

> -----Original Message-----
> From: Michael Porter [mailto:mbt@SYMPATICO.CA]
> Sent: Saturday, January 05, 2002 3:12 PM
> To: PHYS-L@lists.nau.edu
> Subject: Re: virtual images and convex lenses
>
>
> On 1/5/02 1:17 PM, John S. Denker wrote:
>
> > 2) The human eyeball is not and cannot be a suitable instrument
> > for "seeing" real images, let alone virtual images.  A real image
> > is formed upon the retina, but only after the light has been
> > subjected to +25 diopters (or more) of intraocular refraction.
> ...
>
> The fact that we use a real image on the retina to perceive
> both real and
> virtual images is a potential source of confusion to
> students. However, I'm
> not sure what you mean by your first sentence. Do you mean
> that the human
> eye cannot distinguish between real and virtual images
> because it's always
> trying to form a real image on the retina?
>
> > Again, the eye is unsuitable for "seeing" real images.
> > If you want to form a seeable real image, you need to build
> > a camera obscura...
> ...
> > There will be a real image hanging in space about one f
> > in front of the lens, but you have no easly way of knowing
> > that unless you interpose a screen.
>
> It's been a while since I've played with this but I think I
> disagree, unless
> you are saying that you could not detect the real image if
> you were standing
> off to one side. If your eye is colinear with the object and
> the lens, then
> the real image will be seen without the aid of a screen. To
> determine its
> location, I've had the students focus on the image, then hold
> their thumb up
> along the line of sight and move it back and forth until the
> thumb is in
> focus. The location of the thumb ends up being roughly the
> same distance
> from the lens as the image, verifiable by calculation. It's a
> nice exercise.
>
> We have done a similar thing to locate the virtual image
> formed by a mirror
> (plane, concave or convex, take your pick). I have the
> students focus on the
> tip of a finger, and then I tell them to move it as close to
> their eye as
> possible and still keep it in focus. This tells them the near
> limit of their
> ability to focus. Then I place the mirror between their eye
> and their finger
> (i.e. closer than they should be able to focus). They have no trouble
> focusing on their image in the mirror, suggesting that the
> image is farther
> away (behind) the mirror.
>
> Mike
> --
> Michael Porter
> Colonel By Secondary School
> Ottawa, Ontario, Canada