Ripple mirror

A Ripple mirror design proposal by David Williams:
"I've been thinking about Brian White's project to make a reflector 
arrangement that will continuously reflect sunlight onto a cooking 
vessel for several hours, despite the sun's movement across (or maybe 
up or down) the sky. And I've had an idea... 
Suppose we have what I'll call a "ripple mirror". That's an 
approximately plane mirror, but with a set of parallel ripples in its 
reflective surface. The ripples are narrow and shallow, and may be 
essentially small sectors of cylinders, each subtending about 15 
degrees as seen from its central line. It doesn't matter whether the 
cylindrical surfaces are concave or convex. They might even be 
alternating, which would mean that the surface would not have any 
sharp angles.
"The ripples would be aligned perpendicular to the sun's movement in 
the sky. At around noon, as seen from temperate latitudes, the sun 
moves horizontally westward across the southern (or northern) sky, so 
the mirror would be set up with the ripples vertical. As seen from 
tropical latitudes, however, the sun often moves roughly vertically, 
climbing up the eastern sky in the morning, passing roughtly overhead 
at noon, then descending the western sky in the afternoon. In this 
situation, the ripples in the mirror would be aligned horizontally. 
Intermediate orientations would occur under different circumstances. 
Looking at the reflection of the sun in the mirror would show a line 
of small closely-spaced bright dots, aligned perpendicular to the 
ripples, i.e. parallel with the sun's motion. The length of the line 
would subtend twice the angle that is subtended by the cylindrical 
sections at their centre, so the line would subtend 30 degrees if the 
sections subtend 15 degrees. As the sun moves in the sky, the line of 
dots would move in the direction of its length, taking two hours to 
cover its own length, in this case.
"A cooker would have a second mirror, which is a simple paraboloid with 
the cooking pot at its focus. Sunlight would be reflected from the 
ripple mirror and fall on the paraboloid. Part of the line of dots 
would be focused by the paraboloid onto the pot, heating it. As the 
sun moves, the line would move along its own length. Since it takes 
two hours to cover its length, the pot would be heated continuously 
for that much time. 
At any time, part of the line of dots would miss the cooking pot, so 
that much sunlight would be wasted. Some inefficiency is an inevitable 
price that must be paid for having continuous heating with no moving 
mirrors.
"This design, with two mirrors, would allow off-the-shelf paraboloids 
to be used. The ripple mirror is not a "compound curve" so it would be 
easy to make from aluminum foil, for example. However, having two 
reflections would lead to a loss of light. It would be possible to 
make a single mirror, approximately paraboloidal but with ripples in 
its surface. This would do the whole job with only one reflection, but 
it would not be easy to make by hand. It could be stamped out in a 
factory."
Category:Reflective material