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Re: Test Writing



On Tue, 30 Apr 2002 22:54:29 -0700 Bernard Cleyet <anngeorg@PACBELL.NET>
writes:
"1. You cannot always see a real image. The real images that are
formed by a convex lens or a concave mirror are usually invisible. To
see
such an image it is necessary to place a screen at the position that the

image forms . If a screen is not placed at the image position , the
image
is still formed there .... but it is invisible."


Please give me a specific example (excluding those which cannot be
seen because ones head is in the way -- off axis and a diagonal mirror
will? solve that problem.) of an invisible real image.

*** OK...Let's consider this example..
A physics teacher attempts using a slide projector to display
a diagram of a series circuit containing a battery, two resisors , a
switch and connecting wires.

Because the lens of the projector is too far from his 2 X2 slide
inside,
no image is observed on the screen at the front of the classroom.
However, one of the students seated near the center
of the classroom, does a quick calculation and predicts that there
is a real image somewhere in the classroom. To confirm his
prediction he raises his notebook to his calculated location and
(as if by magic) the image of the series circuit comes to life
and is seen on his notebook,


"2. You can see a virtual image. Just .........."

I think this is an analytic statement

*** Yes.... It is a statement but I do not believe that it is an
"analytic"
statement because it does not contain any analysis. I would
prefer calling it a declarative statement.

Herb Gottlieb from New York City
(Where we sometimes see virtual images and can't see real images)

bc

Herbert H Gottlieb wrote:

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On Tue, 30 Apr 2002 07:53:03 -0500 Tina Fanetti
<FanettT@QUEST.WITCC.CC.IA.US> writes:
I would prefer not to give multiple choice tests, however, there
has
been some friction over the essay/short-answer questions.
Example:
On the last test I asked students to explain the difference
between a real and a virtual image.

Student answered: You can see a real image. You can't see a
virtual image. I took off points for this because it is not
fully correct.

****************************************

Perhaps Tina should have taken of more points for the student's
answer because his answer was absolutely wrong for the following
reasons:

1. You cannot always see a real image. The real images that are
formed
by
a convex lens or a concave mirror are usually invisible. To see such
an
image it is necessary to place a screen at the position that the
image
forms . If a screen is not placed at the image position , the
image is

still formed there .... but it is invisible.

2. You can see a virtual image. Just look into a plane mirror and
there
it is!

Herb Gottlieb from New York City
( Where we can see virtual images but cannot always see real
images)

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