Hot Spot; big convex mirrors

From: Nicholas Bodley (
Date: Wed Mar 28 2001 - 13:55:07 PST


How efficient is the mirror as a reflector of infrared? I recall
seeing about gold-coated mirrors as being better. Shouldn't be too
hard to coat the surface with gold leaf, but you do need to learn
how, and obtain at least a brush for it. (Gold leaf isn't terribly
expensive.) Better to make a replica surface, but for such an
experiment, surface accuracy is truly un-critical in the extreme.
Shame to spoil a nice optical mirror by gilding it!

There were once surplus searchlight reflectors on the market; I used
one, about 28" (?) dia., as a solar furnace; melted slate with it.
Used welder's goggle glass for viewing. Better to have a plane
reflector in such a case, because the spatter from the hot object
falls on the surface.

How to aim a searchlight reflector at the sun in a really-crude
setup: Do an approximate aim. Take a stick (assuming you're outdoors)
long enough to keep your hand away from the focus. Heat it and ignite
it; blow out the flame. Use the smoke (hope there's little wind!) to
image the 3-D pattern, and you'll learn some practical optics
quickly. Naturally, aim for the narrowest "waist". Relative motion of
the sun will necessitate re-aiming every few minutes.

Nice to know fairly big mirrors are in stock.

As to sound, a wet umbrella is a fairly-decent reflector; sound might
bounce off the pavement. Hold the umbrella's shaft vertically, make
some weird noises, and explore the acoustics. (Btw, I prefer golf
umbrellas, because I like to have a dry waist if the wind is about
3 mph. Small umbrellas keep only my head dry... (Raincoat? What's
that? :) )

Nicholas Bodley |@| Waltham, Mass.
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