In order to see something better, we need a larger image of it on the retina of our eyes. We can see something better--make a larger image on the retina--by bringing it closer to our eyes.
However, . . .
Our eyes are only able to focus a clear, sharp image from an object only so close. That distance is called the near point of your eye. As you move an object closer than the near point, the image formed on the retina become blurry and fuzzy.
A converging lens can produce a virtual image at infinity and then our eyes can easily and clearly focus on that image and produce a sharp image on the retina.
The magnification of such a simple magnifier is
M = (near point) / f
If we take a the near point of a "typical eye" to be 25 cm, then we can write the magnification as
M = 25 cm / f
A "jeweler's loupe" and the eyepieces of microscope and telescopes are simple magnifiers.
Return to Ch 26, Optical Instruments (c) Doug Davis, 2003; all rights reserved