Re: [Phys-L] lens magnification

[Roger Haar <haar@physics.arizona.edu>]

Hi,
    The simple magnifier is not simple.  One needs to
understand the optics of the eye.  The magnification of the
simple magnifier is angular magnification.  The size of the
image of an object is the angular size. (The classic
question is "Which is bigger a dime or the Moon." At arms
length a dime will be "larger" than the Moon.)  The angular 
size depends on both the size of the object and its distance.

   The human eye has a variable focal length which confuses 
things. The final image is formed on the retina.  There is 
no single combinations of object-to-lens distance and lens- 
to-eye distance the works with a magnifier.

    There  is a limit as to how close you can bring an 
object and still focus on it.  This limits the angular size 
of an object.  The closest that one can focus on is the near 
point.

   Normally when using a single lens magnifier one has the 
object-to-lens distance equal to about the focal length of 
the lens.  Then the image formed by the lens is at a + or - 
infinity and is infinitely large, but not infintely large in 
angular size.  (Sometimes it is easier to think about if one 
places the object a bit closer to the lens.)

Thanks,  Roger Haar

PS  I think diopters are in inverse meters.  IF one works in 
diopters the thin lense formula becomes as simple as a=b+c.
==========================================================
On 5/17/2013 12:30 PM, Trivilino, Herman wrote:
> > I've always wondered about this "simple" idea, and a
> > student just asked me. Magnifying glasses (biconvex
> > lenses) are sold with a power rating (3x, 4x, etc.).
> > How is this determined?
>
> As usual, it would help a physicist tremendously if
> everyone would simply attach the units when appropriate.
>
> I believe the units in this case are inverse
> centimeters.
>
> These numbers are called the diopter and are the inverse
> of the focal length, in centimeters.  Opticians use the
> same system when prescribing corrective lenses.
>
> Herman Trivilino College of the Mainland Texas City,
> Texas _______________________________________________
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