Re: Virtual and Real Images

[John Clement <clement@HAL-PC.ORG>]

The floating coin is a real image.  When you look at the floating coin it is
in front of the mirror.  This means that the light is actually coming from
the location of the image.  The light rays actually cross there, so it is
must be a real image.  Virtual images do not have the light rays cross, so
they must be behind the optical system.  The light rays only appear to have
come from a virtual image.  You can also see the real image in front of a
convex lens if you hold the lens at arms length.  Normally real images are
inverted, but because the image is produced by 2 consecutive mirrors, the
image appears right side up.  Note that applying the definition of real and
virtual in terms of ray crossing requires formal thinking, and can be
tricky.  However a simple rule like virtual images are behind the optical
system and are not inverted brings the idea down to the concrete level.  The
in front/behind rule is also not easily used, as most students can not
really tell where the image is actually located by just looking at it.  This
latter skill requires some time to develop.

Again I would point out that if the students do not have the basic ideas
behind what is going on they will not be able to understand optical systems.
I suspect that they do not really understand that light is emitted by
luminous objects, and that you "see" objects by looking at the light rays
coming from them.  While it is possible to make certain light rays visible
under some circumstances, the fact that light rays are not normally visible
puts light rays in the category of a theoretical concept.  Recent papers by
Anton Lawson highlight the difficulty students have with theoretical
concepts.  Mathematics will not help the students to understand if they do
not have the basic concepts.  From most of Tina's descriptions I suspect
that she is dealing with students who test at the concrete or possibly
transitional level of thinking.  Concrete thinkers generally can not easily
connect math with physical descriptions.

McDermott has published 6 tutorials starting with light and shadow that
relate to geometric optics.  I suspect that the full series would be needed
to help the students.  Just one or 2 labs would probably not do the trick.

Incidentally the perceptual problem of telling whether the image is in front
or behind a lens is actually quite common.  Since I take 3-d photographs, I
am very aware of 3-d effects.  It is possible to incorrectly mount the
slides so that the right and left are reversed.  The visual effect is to
turn everything into a base relief.  The foreground and background are
reversed.  While I can spot this immediately, at least 9 out of 10 people do
not.  They will sometimes not notice the problem, or they will say there is
something wrong.  They will not notice the incorrect location of objects.

John M. Clement
Houston, TX

> At 12:17 -0500 3/19/02, Robert Cohen wrote:
> > The image of the floating coin is a real image, is it not?
> I have to admit I didn't go back and check on the arrangement of the
> floating coin before I made those, possibly rash statements, but I
> always was under the impression that the image was virtual. I am
> prepared to be wrong, however, and I will go look it up and see what
> it is.
>
> What it all adds up to, I think, is that the idea of real and virtual
> images is not just a simple rule to memorize. If the students don't
> get even in the simplest cases, it probably means that there is a
> serious misconception about how a lens works, and maybe Tina should
> go back to the fundamentals of just what a lens is and does.
>
> Hugh
> --
>
> Hugh Haskell
> <mailto://haskell@ncssm.edu>
> <mailto://hhaskell@mindspring.com>
>
> (919) 467-7610
>
> Let's face it. People use a Mac because they want to, Windows because they
> have to..
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