On Tuesday, November 23, 2010 at 5:11:40 PM UTC-7, Optical wrote:
I was having this argument at work.
Some claim that reading glasses work because they magnify small print.
They claim that reading glasses sold at drug stores are labelled in
terms of their "magnification power" or strength.
I was arguing that the goal of reading glasses is not to magnify, but to
move the focal point, and that they don't really perform any
magnification.
Who's right?
Do many people use the term "magnification" incorrectly when talking
about reading glasses?
Well, reading glasses contain convex lenses. So a strong pair of reading glasses
could be used like a magnifying glass.
But you're still right too.
For print held a certain distance from the eye, reading glasses don't increase the angular size of the letters.
Instead, they take the small letters close to the eye, and form virtual images of those letters, which are bigger in size, at a distance far away from the eye,
with the same *angular* size at the eye, so that they eye can bring the images to a focus.
So they let you read the printing as you hold the book closer to your face.
Since the virtual image does have a larger linear size at its new position, magnification does still take place, even though it isn't really what makes the print more readable (unlike, say, looking at something with the aid of an opaque
projector, or through a microscope).
Thus, the distinction is complicated, and explaining it is tantamount to an introductory course in at least Gaussian optics!
John Savard
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