Re: Virtual and Real Images

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

> -----Original Message-----
> From: Tina Fanetti [mailto:FanettT@QUEST.WITCC.CC.IA.US]
> Sent: Tuesday, March 19, 2002 9:40 AM
>
> No matter how I try to explain it to my students, I can't seem to get=
>  across the differences between a real and virtual image.
>
> I told them that a real image could be formed on a screen, a virtual =
> could not.  A real image is where light converges.  There is no conve=
> rgences of light rays at a virtual image.

I'm not sure I understand the problem.  Yes, real images will have several
properties that differ from virtual images (e.g., can be formed on a screen
if the object is illuminated sufficiently).  Is the problem that the
students cannot remember this?  Yes, real images are those where light
converges.  Is the problem that the students cannot remember this
definition?

I'm not being critical - it is just that how you go about solving the
problem depends on what the problem is and what your objective is.

My guess is that the problem is that students are unable to understand what
is meant by "where light converges" and, as a consequence, they don't really
understand how lenses work and thus cannot predict the properties of the
images that are produced.  Objects/images is a difficult concept because we
can't "see" the light rays converging and diverging.

I recommend you start out having students *explore* the properties of
images.  For example, set up two small balls hanging from strings.  In front
of one, place a converging lens such that a real image is produced - it
should look like a "floating" ball.  Place a pencil at that location so that
students can see the ball floating above the pencil when they look through
the lens.  Do not call this a "real" image yet.  Ask students to identify
where the image is.

In front of the other, place a diverging lens such that a virtual image is
produced.  Ask them to identify where the image is.  They should be able to
see that the image is on the same side of the lens as the object.  [Chances
are they will also say the image is far from the lens - have them place
their eyes close to the lens - the image will get blurry because it is too
close to their eyes.]

In both cases, the image is seen at the point from where the light appears
to diverge (you probably need to explain what this means).  However, for the
real image, the light actually does diverge from that point (having
converged to that point by the lens) whereas in the virtual image it only
appears to do so.  You can then give the two the names "real" and "virtual"
and ask them to identify the properties of each by exploring different
set-ups.

____________________________________________
Robert Cohen; rcohen@po-box.esu.edu; http://www.esu.edu/~bbq
Physics, East Stroudsburg Univ., E. Stroudsburg, PA 18301