Re: virtual images and convex lenses

["John S. Denker" <jsd@MONMOUTH.COM>]

Tim Forth wrote:
> I am wondering about the image I can see _in_ the lens. That is, I can
> see an upside down crystal clear image through the lens. Is this a real
> image? Apparently not since it appears to be on the other side of the glass.
> What causes this?

I suspect the question is not answerable.  That is, I suspect
the language of "real" and "virtual" images is not applicable
to this situation.

Consider the ultra-simple case of a _flat_ piece of glass, like
a window-pane.  We can consider this a trivial zero-diopter lens.
This is not an image-forming system.  There is no real or virtual
image (unless by some contortion of language you say that the
object is an "image" of itself, which would not be recommended).
Suppose you stand outside and look into the room through this
"lens".  You will see something "in" the window.
 -- I would say you see a _scene_ in the window.
 -- I'm not sure it makes sense to say you see an "image" in the
    window.

=====================

The physical basis for all this is quite simple:  When we speak
of an image, we usually think of rays coming to focus on some
surface.  The retina in your eye is not a particularly good
way to detect such a focal surface, because the retina is too
firmly attached to the lens of the eye.

If you want to "see" a real image, we can consider various
possibilities:
 1) Normally you put something at the focus:  ground glass,
photo film, CCD imager, et cetera.
 2) If you really want to use the retina directly, you would
need something like a -25 diopter contact lens, to cancel the
effect of the lens in your eye.  This would not be easy.
Perhaps you could surround the eye with a face mask full of
index-matching fluid.  Ugh.