Re: virtual images and convex lenses

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

Michael Porter wrote:
> ... 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.

Yes, this is nice.  Under favorable conditions it can
even work with virtual images, if the lens is small
enough that you can hold you thumb in a suitable place
(on the far side).

My main point remains that you can't perceive distance
just by the "feeling" associated with accommodation.

> move it as close to their eye as
> possible and still keep it in focus.

OK, the near limit of accommodation is another good trick.

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

What I should have said in my previous message is something
like this:

The eye normally accommodates freely over a wide range,
subconsciously.  Therefore when peering into an optical
system, you can't tell where the image is unless you
interpose something (a screen, or a thumb, or some such)
or somehow defeat the accommodation mechanism.

Perhaps more to the point, it is often exceedingly unimportant
to say anything about "where the image is".  In a telescope,
the image could be 1000m away, or 100m, or 10m -- it doesn't
matter.  If the image gets to be 1m away, your eyes will might
tired eventually -- but even then you might not know why.

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

Robert Cohen wrote:
> I can easily "see" both real and virtual images without
> the help of a screen.

But you can't so easily tell with any precision _where_ they
are formed.  The natural human perception of distance depends
on many things, almost _not_ including accommodation.

> This is just a matter of semantics.

Perhaps.  But I doubt it.  I think that misunderstanding the
nature of monocular depth perception was a key part of the
bundle of misunderstandings that provoked this thread.

> 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?

I don't object to this use of the word "see".

But I think it is an abuse of the highly specific technical
term "virtual image".

Suppose you take off your glasses and look through a piece
of window-glass.  Window glass is a completely non-image-forming
system.  Does that mean you can't see anything?  I doubt it!

To my way of thinking, the only image that really matters
is the real image formed upon the retina.  In ordinary situations
it is formed by the lens etc. of the eye, with perhaps a few percent
of help from eyeglasses.