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Brain wrote,

>"To focus or to produce a large beam of parallel light rays, a sizable 
>converging lens is necessary.  The large mass of glass necessary to form 
>such a lens is bulky and heavy; moreover, the thick lens absorbs some of the 
>light, and is likely to show aberrations . . . .  Fresnel recognized that 
>the refraction of light takes place at the surfaces of a lens.  Hence, a 
>lens could be made thinner -- even flat -- by removing glass from the 
>interior as long as this was done without changing the refracting properties 
>of the surfaces."
>Note they are "easily molded from plastic."

Ah, yes, I assume you are right, the problem is that I have not a single
physics book explaining fresnel lenses. I only have 2 pages explaining
Fresnel diffraction. These pages made me confused I guess. Now that you made
me rethink this, I see that I was wrong.