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Re: Virtual and Real Images



At 13:06 -0600 3/19/02, Paul O. Johnson wrote:

----- Original Message -----
From: "Hugh Haskell" <hhaskell@MINDSPRING.COM>
Sent: Tuesday, 19 March, 2002 10:48
Subject: Re: Virtual and Real Images

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.

Now hold on thar, Hugh. We have such a projection exhibit at The Science
Place in which explorers swish a white 3/8-inch dowel up and down. But there
is no "point where the image should be". Because of the depth of field of
the projector, they can swish the dowel at almost any distance from the
projector and see a focussed image on it. The closer to the projector they
stand, the smaller the image is.

Of course. I oversimplified because I was in a hurry to get an answer
out and head off on some errands. The image isn't a "point" as long
as the object is extended, and any lens system has a "depth of
field," or distance (sometimes large, sometimes small, depending on
the ratio of the focal length of the lens to its diameter) over which
the image is in more or less good focus. So the person who is
"revealing" the image by sweeping some object, like a paddle or
broomstick doesn't have to be right in the focal plane. All of which
is why this is a fairly easy demo to set up. Neverttheless, this is a
real image, and moving around the room will not change it's position
in space.

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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