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

["Bob LaMontagne" <rlamont@postoffice.providence.edu>]

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Rick Tarara
Sent: Monday, April 03, 2006 11:16 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Image of object placed at focal point

----- Original Message ----- 
From: "John Denker" <jsd@av8n.com>


> Rick Tarara wrote:
> > when p = f an image of each
> > type would form at +/- infinity, but practically speaking that means that

> > no
> > usable image forms.
>
> Astronomers might have a different opinion about the practicality of
> focussing on infinity.

But they don't focus on a true, mathematical infinity to which my comments 
referred.  Billions of light years may be damn close to infinity and the 
width of the object may be damn near zero on a relative scale, but they are 
not mathematically infinity or zero.  ;-)

On the lab optical bench--when the object is really close to the focal 
point, you don't get an image formed anywhere in the room.  So in many 
'practical' situations, the no image answer can make sense.

Rick 

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