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

[Rick Tarara <rtarara@saintmarys.edu>]

For real understanding, I think you need to combine all four answers.  As 
the object approaches the focal point of a converging lens, a real, inverted 
and very large image is formed far away from the lens and on the side of the 
lens away from the object IF the object is approaching from p > f.  If the 
object is approaching the focal point from p < f then a virtual, upright, 
very large image is formed on the same side of the lens as the object, but 
far away.  _IF_ the lens is ideally extremely thin and the object has no 
width along the optic axis, then mathematically, when p = f an image of each 
type would form at +/- infinity, but practically speaking that means that no 
usable image forms.

Now since objects have width and lenses aren't infinitely thin, then the 
first two options above ) are the more meaningful  (3 and 4 below if 
considered limits for the ideal situation).

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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----- Original Message ----- 
From: <carmelo@pacific.net.sg>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Monday, April 03, 2006 6:52 AM
Subject: [Phys-l] Image of object placed at focal point


> Hi,
>
> 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?
>
>
> Best regards,
> Alphonsus
>
>
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