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Re: [Phys-l] Image of object placed at focal point



Isn't it a bit confusing to talk about an image being formed at a "point" ?
Perhaps at a "plane" .....but certainly not a dimensionless point!.
That is ... unless it is not a real object at the source ???
.... or as Confucius once said..."He who sits on the point of a tack, is better off."


Herb Gottlieb from New York City
(Where nationals from all over the world
cab live in peace and harmony)

-- "John Clement" <clement@hal-pc.org> wrote:


There are four textbook answers on the "properties" of image of an object
placed at the focal point in front of a converging lens.

1. No image is formed.
2. 2 images are formed, one real and one virtual located at infinity.
3. A virtual image located at infinity, upright and magnified.
4. A real image located at infinity, inverted and magnified.

Should there be only one correct answer? Or is there a better answer?

Well this question has not been actually answered, despite a number of
answers being given.

One can say, all of the answers are correct, but what you really need to
know is which answers should be taught?

The students should experience the formation of images, and then should be
asked to come up with their own answer here. So in reality there can be
several answers all of which are equally valid.
1. takes the laboratory point of view, and is one that studens can
understand.

2. might be proposed by an honors class. Actually this might come out of a
class where some students take 3 or 4. Then getting the students to debate
the answers, they might realize that each of these answers requires taking
the limit from different sides of the problem.

3&4 are both very difficult for students because they are true in the limit,
and that is a cognitively challenging idea for the average students. Indeed
2, 3, and 4 should not be pushed with regular students because if they don't
see it on their own, they will probably just memorize it.

If one tests for these ideas, always ask the student to justify the answer,
so that the reasoning can be assessed rather than just the memorized book
answer.

So I would say there can be multiple correct answers depending. And just
because the text says it, doesn't make it either comprehensible to true.
The best answer is the one students can understand.

John M. Clement
Houston, TX


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