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Re: Virtual and Real Images



At 8:39 -0600 3/19/02, Tina Fanetti wrote:

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.

They are still confused and I am at a loss.

The difference is entirely in where the image is formed. What we
"see" are always real images, because they are formed on the retinas
of our eyes. It is important to separate the images formed by
external optical systems from the images formed in our eyes or on
camera film. When we "see": a virtual image, or take a picture of
one, what we have done is use our eye or camera lens to cause the
diverging rays of the virtual image to converge on the focal plane of
our eye or camera.

One rule that may help, is that the only way one can see those images
we call virtual (in lensed systems, that is--in mirrored systems
almost all images are virtual) is by looking back through the lens.
The virtual image results when the light rays from the object are
diverging after they pass through the lens. So we must use our eye to
*construct* an "object" at the point in space where these diverging
rays seem to be originating. Maybe we have given it an incorrect
name. Perhaps we should call it a "virtual object," rather than a
virtual image, since it is located at a point in space where it
appears to actually be, even though the real object is located
elsewhere.

On the other hand, a real image does not depend on where we are
standing in order for it to be seen, and it does not present us with
the appearance that the object is somewhere that it isn't. That a
real image can exist in space even without a surface for the image to
be realized on can be demonstrated in several ways. One is to darken
a room, project a slide into space (i.e., no screen at the location
of the image, and then rapidly move a broomstick across the light
beam at the point where the image should be. The viewers will see the
image appearing on the broomstick, and because of the persistence of
vision, it will appear to be an entire picture. There are other ways,
several of them have been well described in a book of optics
activities prepared by Dewey Dykstra some time back. I haven't the
reference now, but perhaps he can provide it.

The difficulty of distinguishing between virtual images and real
objects is at the heart of any number of illusions used by magicians
(Pepper's Ghost being one of the favorites, dating from the late
1800s, IIRC), among them the "floating coin" illusion, which can be
obtained from several science supply houses. I think Arbor Scientific
is one.

Don't despair. The concepts involved in understanding optical imaging
are not trivial. The best one may hope for in a first-year course may
well be that the students remember a few rules of thumb which allow
them to distinguish between the two.

As noted above, I have been talking about virtual images created by
lens systems, but the two illusions I mentioned are caused by
mirrors, and the differences between the two may well be at the heart
of the confusion. I waffled above, in saying that almost all mirror
images are virtual. I can think of no exceptions to this rule, but
there may be some that I have forgotten. In any event, if you examine
the arrangement of images formed by mirrors, it will inevitably be
seen that, unless you are looking into the mirror, no image will be
visible. In other words, the image is one constructed by the viewer
by looking back into the mirror. This is certainly the case in the
floating coin image. Some of the magical illusions that can be formed
by mixing real objects and virtual images can be seen in the floating
coin illusion, by placing a piece of straw in the region where the
image appears, and the straw will appear to have penetrated the coin.

As I think about it, I recall that it is possible to create a real
image in a concave mirror, but because of the geometry, it is
difficult to display the image.

The differences between real and virtual images are real, they are
often difficult to understand, and they need far more time than we
are ever able to devote to them, if the students are to get any real
understanding of them. Whether the issue is worth spending that much
time on is a matter for debate as we decide what is important in an
introductory course and what isn't.

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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