At 10:05 AM 2/17/03 -0500, Dave wrote:
>On Mon, 17 Feb 2003, Bill Marshall wrote:
>
> > Here's a silly question.....
> >
> > Has anyone ever heard of those lenses causing a fire or damaging the
> interior
> > by focusing the sun's rays onto a small area?
>
>Hmm...unlikely, I think.  It's not a straight magnifying lens, where you
>can focus all the light to a point.  It takes light coming in from a wide
>angle and bends it into a beam that's about as wide as the lens itself.

Yes.  A fresnel lens used in a window to see a wide angle is actually a
concave lense. It is spreading the beam out, not magnifying it...   Here's
an ascii drawing that your Mozilla might turn into a yellow
face.  :)   (Sorry, I couldn't resist.)

                                           /\
Beam--------------------------------/   \----
Of------------------------------------/      \--------__ Focused
Light--------------------------------\      /--------     Beam
From---------------------------------\   /----
Source                                \/
                                  Convex Lens

Wide--------

Source----------------\   /   ----Narrow
                            |  |   ----Percieved
Field------------------- |  |   -----Field
                            |  |   -----Of
Of---------------------- /   \  ------Vision
                          /      \
Vision-------       /          \

                   Concave Lens    (Yeah, I'm not an artist, shut up)

So, All that mess sort of shows the Convex lens curving the beams of light
inward, and the concave would show how it curves them away from
center...  And I'm wishing now that I'd never started, because you're all
laughing at me for my incompetent ascii art.

What has struck me is that one could buy a cheap plastic fresnel lens and
cut it in half the long way. Then, install it with both halves extending
your downward vision and the side to sides so you can watch your bumper end
caps get knocked off.  No one ever really needs to watch out for hitting
things above the van unless backing into a garage where it's too dark to
benefit from the lenses anyway.