Subject: Re: Magnifying glass (Fresnel lens)
From: [email-masked] (Kevin Bjorke)
To: [email-masked]
Date: Wed, 27 Jul 94 17:38 PDT

Fresnels are a manufacturing strategem that allow large lenses to be
made.

Light is bent according to the incident angle. To focus light in a region,
a continuous surface arcs outward. For the greatest effect, both sides arc
outward. This works well when the weight of the material is small enough,
but as the lens grows the material becomes larger and larger until it
distorts under its own weight (this is why the largest telescopes are
reflectors, too).

If we ignore the requirement of a CONTINUOUS surface (as would be required for
imaging optics like eyeglasses, cameras, etc), and are interested in over
quantity of light, then a fresnel becomes practical. Fresnel lenses simply
collapse the internal lens volume, at the cost of some light scattering at the
borders between rings. Not great for optical focusing, but a 1-meter fresnel
1 mm thick can collect a lot more light than a 1 cm lens of the same thickness.
This makes fresnels ideal for solar heating, illumination, low-quality
lenses (security doors) and so forth. Light focused by a fresnel lens, however,
is focused by refraction, the same process as used by my own nerdy
little spectacles :)

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  Kevin Bjorke         |  If you draw a picture, and then draw more and
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