Ok, this would be fun.

[flying dog]

Maybe I should try to build one of these.....

 

http://www.smh.com.au/technology/sci-tech/teens-solar-death-ray-can-melt-concrete-20110204-1afxk.html

 

 

[old man emu]

I don't believe it!!! :csm:

 

Surely it's all smoke and mirrors

 

OME

 

 

[dazza 38]

Myth Busters, tried something similar, it didnt work.

 

 

[Guest davidh10]

I don't believe it!!! :csm:Surely it's all smoke and mirrors

 

OME

Well err... yes and no!

 

Nothing new about this. The actual energy captured is the incident sun's radiation over the area of the reflector, but then concentrated to approximately a point. There are losses in reflection and also not all energy reflected will be absorbed by the target to produce heat.

 

This principle, among others, can be used for solar electricity generation. You can calculate the energy captured from tables that record the solar insolation in your part of the world. Here's an example table, but they are available for Australia too. Even daily measurements.

 

It is worth noting that you can light paper with just an ordinary (quite small) magnifying glass, which is why you should never leave one where it can have the sun fall on it, such as, through a window. Building fires have resulted from just such situations. Typically the ones that are a risk, are the ones on a stand, as there is a possibility that something flammable may be at the focal length of the magnifier.

 

 

[old man emu]

The Mythbusters were trying to see if the story of Archimedes setting alight a fleet of warships by directing the Sun's rays off highly polished bronze shields would work. It didn't.

 

What the young fellow has shown in this example is that if solar heat from multiple reflectors can be focused to one point, then very high temperatures can be produced on the target. This is indeed the principle on which some commercial thermal electricity generation plants operate.

 

It you watch the video carefully, you will see that the maximum heating only occurs within a short range of the mirror apparatus. That range is at the focal length of the reflectors (plus or minus a tad to allow for imperfect measurement). This is one of teh reasons why the Mythbusters rejected the story of Archimedes. The focal length of his array was not long enough to allow the reflected heat to reach the target ship in the harbour. There were other difficullties associated with maintaining the beam on a moving target, and the effects of Solar transit.

 

Having said all that, the young fellow's apparatus is a good Green way to generate extreme temperatures at a fixed point.

 

The question now arises: which gives better results, a reflector made up of a miriad of mirrors, or a single hyperbolic reflector.

 

OME

 

 

[Guest davidh10]

...The question now arises: which gives better results, a reflector made up of a miriad of mirrors, or a single hyperbolic reflector.OME

The generic answer is that a hyperbolic reflector with continuously mirrored surface, will give a more accurate and smaller diameter focal point, depending upon the accuracy of the reflector shape.

The use of a large number of small flat mirrors is a cheaper way of approximating a hyperbolic reflector. Obviously energy that falls between the mirrors is not reflected and thus lost to the system, so that introduces a lower efficiency for a given size of individual mirrors. The more significant trade-off is a less accurate focus, as well as a larger physical focal area, and therefore a lower energy density at the focus. As the array gets larger in comparison to the size of the mirrors, this effect decreases. After all a mirrored hyperbolic reflector is made up of an infinite number of infinitely small plane mirrors.