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Fresnel lenses again

Now that I think to understand the Fresnel lens, it isn't clear to me what
the advantage to us is.

In normal live these lenses are sometime more useful because they are flat
and are lighter. A disadvantage is that you can see all the rings, which
makes the image less clear.

But another way to make a lens is to use a curved mirror. This way the lens
has no interior and thus the advantage for using a Fresnel lens is only the
flatness and not the weight. Since we really don't need flatness I wonder
why some (Brian?) are suggesting to use such a lens.

Ah wait, I do know, Fresnel lenses are transparant, while a mirror isn't.

The advantage for a mirror is that is less dependant on the wavelength,
lenses do bend EM-waves different depending on their wavelength.
So since we are worring about doppler shifts a mirror may do it's work
better than a lens.

A final question is can we make a material that is transparant for
micro-waves and that can bend them enoug?

Another problem that we haven't thought of when using mirrors or lenses is
that they absorb some of the energy of the beam, this changes the density of
the lens/mirror. The result is that unwanted bumps arise and that the
focus-distance changes.