Re: How exactly does a magnifying glass work?

When you look at something through a magnifying glass, you are seeing 
the same area you would without, but some parts are larger and some parts 
are smaller.  How does this work?  Imagine a magnifying glass, but it is 
two-dimensional.  Part of the area under the lens is the part you want to 
observe: say, a point on a map.  Now, imagine two lines coming from either 
end of this point you want to observe.  Now, without a magnifying lens 
these lines would travel straight up, or straight away.  The lens makes 
these angle towards each other.  This way, the two lines eventually meet 
in a point in the air.  The closer that point is to your eye, the better 
it is magnified.  That is why you sometimes have to move the lens closer 
or further away from the object you are viewing, it moves the focus of 
these lines.  Now, imagine again that there are an infinite number of 
these lines, each representing the borders of different areas under the 
field of view.  Each of these infinite points is the optimal viewing point 
for a certain area within the field of vision under the lens.  To turn 
this into a real situation, imagine these infinite number of lines, but 
three-dimensionally, so that their paths do not form a triangle, but form 
a cone.
     Now, it would logically follow that the further you are from this 
focal point, the worse your view of the area in question is.  As you know, 
a magnifying glass can make your view of a certain area worse, if you use 
it wrong.  On the edge of the field of view things are small and bent.  
This is the opposite of what you see on the inside.  Starting to see how 
this all works?  It can be demonstrated with the reflective side of a cd: 
just bend it one way or the other.  Bent towards yourself, the middle is 
smaller and the edges are bigger, and vice versa.  Try it, it's neat.
     Now, why, you are probably asking, do things turn upside down 
sometimes when looked at through a lens?  Well, remember that cone?  It 
doesn't just stop at the focus; light doesn't just stop.  It keeps going, 
each line going to the other side.  So, what you get is the same image, 
but upside down.  Also, the magnification gets very strong at this point, 
so that it's mostly impossible to see the area in question.  And that's 
beside the fact that it's upside down.
    I hope this cleared things up for you, or at least got things started 
moving in the right path.  If you're interested in learning more about the 
physics of light, ask more questions, or check out your library.  There's 
tons of information waiting to be sucked up out there.

[note added by MadSci Admin:  There are a fair number of previous
answers in our archives dealing with lenses and images.  It would
be worth your time to search for them using our search engine.]