How is the refractive strength of a lens measured?

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Offline hellogreen

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How to Measure magnification
How can I see how strong a magnifying glass is?

I bought this magnifying glass (Link removed; it could be an advert. Mod |) and I want

to know how to test it to see how strong it is. I hear a lot of people talk about

magnifying and how strong the magnification is, but I would like to know the true

magnification of my magnifying glass. I have a few of them and some seem stronger then

others. How can I rate these? How can I pin a correct number on mine? How do the companies

that make these come up with these numbers.
« Last Edit: 11/12/2008 09:57:30 by chris »



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Re: How is the refractive strength of a lens measured?
« Reply #1 on: 03/12/2008 14:38:36 »
There is a sensible answer to this question.
It all depends on what the 'near point' of your vision happens to be. 
A simple magnifying lens acts, more or less, like a very strong pair of reading glasses (convex lens) by taking a close up object and producing a virtual image far enough away so that you can actually focus on it.  As you get closer to an object, the angle it subtends at your eye increases - and it looks bigger but blurred. The magnifying glass simply manages to get rid of the 'blurredness'. Brilliant.
If you have become very long sighted in old age (near point of 1m, say), the same focal length of magnifying glass will give you better value than it will give  a young person with a near point of 25cm or, even 10cm. The younger person could get so much closer to the original object than you, they won't get 'the benefit' that the old one will.
A microscope has, basically, two lenses. The bottom one (the objective) produces a real, enlarged, image, inside the tube very near your eye and the eyepiece acts like a magnifying  glass, allowing you actually to focus on this image.
They don't 'do' ray diagrams at School any more - sad.
Take a look in Wickers. . .
« Last Edit: 03/12/2008 14:45:34 by sophiecentaur »