Monday, 29 November 2010

Parabolic Reflector

Parabola vs. sphere
If cross-sections of a spherical surface and a parabolic surface were made by slicing each surface in half, these would be the shapes you would see:

I've been wanting to make a parabolic reflector for quite some time now.
As I have heard more and more about them, from Marcus Coates using them to record specific birds in trees for his work 'morning call' and Chris Watson amongst various other sound artists mentioning them on several occasions, I thought it would be worth checking out what all the fuss is about.
I managed to find these instrucions of how to make a parabolic reflector on Instuctables. It also gives a dxf. file which is excelent as I could use this for the Laser cutter machine they have here at Bezalel.
There is also a rather good link to youtube on this page, showing a parabolic reflector being used to cook some bread and fry something (not sure what exactly).

And here is a very useful Java tool for anyone who might be wandering about how to calculate the radius of a parabola, according to it's length and therefore find the focal point - Equation of Parabola.

I was originally going to make a concrete parabola, something like what they used on the coast of britain to detect incoming aeroplanes during the war, using a large inflatable exercise ball.
However, after some research I realised that I had made one of the fundamental misconceptions - that a parabolic dish is part of a sphere. It is not. It is part of a paraboloid.

If you slice a cone with a plane that is parallel to a line on the cone through its vertex (such as UV on this figure), the intersection is a parabola. Here is a proof of this fact.

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