alt.energy.homepower

"Jim" wrote in message
news:CtidnSgZ4rWixCvbnZ2dnUVZ_ruqnZ2d@ ...
>
> "David Williams" < @ > wrote in message
> news: @ ...
>>-> > Absolutely! I have *not* been trying to say that troughs are useless.
>> -> > Far from it. All I have been saying is that, under certain
>> -> > circumstances - early or late in the day, and not near an equinox -
>> -> > they do not perform properly.
>>
>>
>> -> I would suggest that praps what you meant was 'optimally' in place
>> of
>> -> properly.... Am I correct?
>> -> >
>> -> > dow
>>
>> No. Properly. In the early morning and late afternoon, sunlight that
>> is reflected from the parabola
>
> 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