alt.energy.homepower

-> > Sorry, forgot this bit. Why should it be a parabola and not just a
-> > simple arc?
-> > Jim, temporarily brain-dead (hopefully temp.!)
-> >

-> The idea is to reflect all the parallel rays from the sun to a single thin
-> collection tube. So as you get farther to the side of the tube, you need to
-> reflect the light rays to a different angle. That at least is why it is
-> curved.

-> A semi-circle always has the surface's 'normal' pointed directly at the
-> center of the arc. Since the angel of reflection equals the angle of
-> incidence, that means the only light rays that such a surface can reflect
-> towards the center of the arc are those that are incident to the surface at
-> the same angle (90 degrees). Light striking the surface from any other
-> angle will be reflected off to some point other than the center of the arc.

-> A parabola has the unique shape that if light shines in along the axis of
-> it, then no matter where on the surface it strikes, it will be reflected to
-> the same point (the focus).

-> daestrom

Yes. But a circular arc, or a small part of a hollow sphere, *does*
focus parallel light from "infinity" to a point which is not the
centre of the arc, but is half-way between the centre and the
circumference. It's a reasonably good approximate focus, provided the
arc is only a small fraction of the complete circle. Of course,
shifting the direction from which light arrives just shifts the focus
around in an arc with half the radius of the reflector.

I recall learning this in high school, some 50 years ago. Maybe it's
been forgotten now.

dow