Considering a Death Ray science project - is this video real?

Attacklass is looking to do a science fair type project. We’ve been looking at this video and considering making a similar death ray, and then adapting it to work as a solar power type project, something like:

Death ray heats water in cylinder / water boils / steam turns turbine blades generating electricity / light bulb lights up.

But - is this video real? Can 5000 mirrors and a satellite dish put out that kind of power?

Solar Furnaces routinely get to temperatures of 538 C, and can go up to 3500C . You can melt metals with that kind of heat.

http://en.wikipedia.org/wiki/Solar_furnace

Rob Cockerham from cockeyed.com built something similar. He called it the Light Sharpener. See his step by step construction and follow on experiments here: http://www.cockeyed.com/incredible/solardish/dish01.shtml

Doable, but waay more work than a youngster’s science project ambition can support.

You can probably make a small-scale model without much fuss.

Question:

Why a bunch of little mirrors, instead of one large one like a telescope?

Because it’s much easier and cheaper to manufacture and mount a bunch of small mirrors than to correctly manufacture a massive mirror.

8" Primary Telescope Mirror
Price: $405.95

CMC fnord!

In Scouts we made a parabolic mirror to boil water with just Al foil, a blackened tin at the focus and some wire for the frame. With actual mirrors and a better frame, I can easily believe that video is for real.

You do have to constantly move it to follow the sun regardless of construction type, though I can easily imagine a clockwork setup for that, or an adaptation of an automatic telescope mount.

When I was in high school, one of my classmates built a spiral fresnel reflector and succeeded in boiling water in a pot that had been painted flat black.

I’m not sure what material he used, but his reflector was something like 6 or 8 feet across. I think he had something like 4 sheets of 1/8" plywood or veneer and laminated them into an 8x8 sheet before cutting it down to a circle and then doing the spiral cut.

I imagine it’s easier to glue little mirrors onegto an existing parabolic surface (eg. scrap satellite dish) then it is to make a new parabolic surface.

Also, the smaller the mirrors the more accurate the reflection. The extremes, if you tried to glue 6" square mirrors onto a small satellite dish there probably wouldn’t be enough convergence to do anything fun (apparently in this case fun = burn things)

“Destroyed in a building fire”, huh? A mere coincidence, I presume… :wink:

OK, so it’s not JUST cost, right?

It WOULD be much more expensive to build a nice, perfect parabolic mirror, but the approach of using many mirrors also allows for a more accurate focus point? So if we could use pixel size mirrors and point them all accurately it would be even more effective?

See the link I posted above from cockeyed.com. Gluing 4 inch square mirrors to an existing parabolic satellite dish is exactly what that guy did and burned a ton of stuff up.

One possible way to burn the crap outa stuff using the sun on a smaller scale.

Line a Wok “dish/pan/pot” with aluminum foil. You need to start with a fresh totally unwrinkled roll of aluminum foil. Carefully unroll some and cut into pie slice shapes that you use to line the Wok.

If you can do it “right”, there should be a pretty intense focal point in there somewhere.

Not quite sure what you’re saying but …

A perfectly formed parabolic mirror would be ideal for focusing the Sun’s rays to the maximum degree optically possible. At the opposite extreme, 3 big flat mirrors set in a concave pattern would do a poor job of focussing the Sun’s rays on a point. For any given total mirror surface area, the smaller each mirror is and the more of them you have, the closer your complete reflector approaches the parabolic ideal.

But no mirror made up of multiple flat plates is as good as a true parabola of the same size.

A perfectly formed parabolic mirror is very expensive. You can create a much larger mirror out of flat mirror plates and achieve the same effect for less cost. For any given overal reflector size, more & smaller mirrors will give a more precise focus & hotter hot spot, but will also be more effort to build and calibrate. Effort costs money even if the amount of glass & individual mirror quality remains the same.

Make sense?
Also,there’s no such thing as pixels in the real world. Those are totally an artifact of computer displays or printing machines. So you can’t have a “pixel-sized mirror.” You can have a continuous mirror so smooth that its imperfections are well below the size that matters to the wavelengths of the light you’re trying to reflect. But that’s a tolerance of nanometers, which is why mirrors that smooth are so darn expensive; like hundreds of millions of dollars for one a few feet across. Which is why Rob Cockerham went with the *relatively few large flat mirrors *approach to make his device which was also a few feet across.

Magnifying make-up mirrors are quite useful for starting fires. Same principle as the video, just smaller.

Well sure, but that’s a HUGE dish! The video shows a fairly small dish. 4" mirrors on that small dish wouldn’t do much of anything (my guess). I’d argue (just because) that the 4" mirrors on the huge dish are roughly equivalent to the smaller mirrors on the small dish. Though to be fair, we should build one of each both ways and see!

I’d guess if you had pixel-sized mirrors you’d start to run into mechanical issue getting the darn things aligned exactly right on the existing parabolic surface. In my mind there’s gotta be a sweet spot in between “mirror the size of a grain of sand” and “mirror the size of the entire parabola”.

Just before the focus spot, put a diverging lens. Then there will be a parallel beam of concentrated sunlight leaving the collector. Add a couple of mirrors to bounce and aim the beam, and there’s your solar death ray!

The tricky part is to figure out how much the diverging lens has to diverge. Not that I’ve spent a lot of time thinking about this or anything… :: innocent look ::

Another thought: I was pretty worried looking at the kid in the video, as his hand came in pretty close to the “Death beam”. How much time in the beam’s focus point before you go from hand to charred stump?