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Re: Virtual and Real Images

Unfortunately this is an extremely difficult problem which is not easily
solved by just a simple lab or explanation. The first part of the problem
is perceptual. Many students do not have strong binaural vision, and some
have none at all. This means that they judge distances mainly from motion
and size clues, and do not use the clues generated by 2 eyes. As a result
they will tell you the picture on a TV screen is inches behind the glass.
They can not perceive that a virtual image formed by a convex lens is in
front of the lens, rather than behind. Along with this comes the weak 3-D
visualization that most students have. Farm children, and students who do
mechanical work or sewing may actually develop good spatial perception and
measurement skills.

To solve this problem takes time. First they have to learn how to perceive
where an image is apparently located. This requires that they note how
images move when they shift their heads. They should also work with the
idea of shadows to be able to understand how light goes in straight lines.
The distinction between light being reflected and emitted also needs to made
first. The lab portion where they use lenses and observe images should come
before any explanations. They should be able to then come up with some
ideas about the images they observe. Then once they have made a number of
observations, they need to find out where the light came from and where it
is going.

Finally the distinction between virtual and real images can be made. At
this point they may have enough concrete preparation to begin to understand
the distinction. Of course you may decide that if they can tell you how the
image was formed and how it is "seen" is sufficient, so that being able to
categorize the image as real or virtual may turn out to be a fine
distinction not worth testing.

Part of the problem is that the light can not be "seen" as it travels. This
puts the idea in the classification as being "theoretical", so it is
extremely difficult to understand by students who test as low level
thinkers. Besides playing with lenses and mirrors students should also have
some experience with seeing the actual trajectory that light travels. Cheap
laser pointers and lenses can allow students to trace the path of light. 2
laser pointers tip to tip, but aimed in different directions can allow
students to find the actual path that light takes. If you put a cylindrical
spreader in front of a pointer students can do "flat" experiments with
lenses on a surface. They can actually see the path that the light takes.
Some $5 laser pointers come with diffraction spreaders that produce patterns
such as hearts, animals, and a straight line.

There are some McDermott tutorials which may be helpful, but I think that
they work better for students at higher levels of thinking. Some of them
may be too difficult for your students. I have put links to some good
simulations that might be helpful at
http://www.hal-pc.org/~clement/science.htm In particular the light and
shadow might be helpful in establishing the basic ideas that are necessary
before lenses.

It may not be possible to repair the problem after it has been taught, so
you may just have to move on. If your students are very low level, then
just getting understanding of some basic ideas such as the distinction
between emitted and reflected light, as well as the ideas behind light and
shadow may be all you can realistically hope for along with a few simple
facts about lenses. This is a case where optics can not be realistically
taught in just a couple of weeks. The typical physics survey course has
sacrificed understanding for coverage. Rensselaer Polytechnic Institute had
the right idea when they used to cover intro. physics in 4 semesters.

A quick fix for the virtual vs real image might be to note that you have to
look at the lens or mirror to see a virtual image, but you can see a real
image without being able to see the lens or mirror. While this does not
get them to understand the distinction, it gives them a concrete definition
that you can test.

John M. Clement
Houston, TX


-----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