I have aimed a reflection of the sun several times. In college, i used to aim the reflection off my watch onto the professor’s forehead. It was easy, because i could see the spot. (And no one else was doing it.)
But I’ve also used a mirror to signal. Every year on vacation, i spend the day on lake Winnipesaukee, and my husband climbs mt. Chocoroa. And i aim a beam of light miles away to the top of the mountain, and he aims back at the island where i am staying. And if it’s a bright, sunny day, we always succeed. We do it by aiming the light at something nearby (i use a wooden piling) that lines up with the point i plan to aim at, and then moving the beam of light in that direction, in my case, up the piling and a little further.
I’m certain that with a little practice i could aim the reflection off a shiny program onto a person on a soccer field. I don’t think it would be very hard. Would every person simultaneously hit the man? No. But would a whole lot of them? If they are army, and they trained, absolutely. I think this is one of Clark’s hard science stories, like geosynchronous satellites.
It’s tougher when everyone is doing it at once. With one person, you have one light spot, and can tell immediately that it’s your light spot, and aim accordingly. With ten thousand people, you have ten thousand light spots, and so you can’t tell which one is yours.
That said, if the mirror is flat, mirrored on both sides, and has a hole in it, there’s a technique that you can use that doesn’t require distinguishing your spot from others. Your mirror is casting a shadow in front of you, with a bright spot in the middle of the shadow from the hole. Look through the hole at your target, and at the same time, look at the reflection of the mirror’s shadow. When the hole lines up with the target, and at the same time the light spot on the ground lies up with the hole, you’re aimed right. But it would definitely need some training and practice for this.
(thank to @CalMeacham , the board’s resident expert on death rays, for telling me about this trick)
Not particularly. The range of such a weapon is limited by the size of the mirror-zone. For a target at a distance comparable to the size of the mirror, you can get up to thousands of degrees. For a distance a couple hundred times the size of the mirror, it’s no worse than the Sun itself.
Oh, and for the record, while Archimedes probably could have done this (using CalMeacham’s aiming technique), and while he did come up with a number of super-effective weapon technologies, the historical evidence appears to be that he didn’t actually make a solar death ray.
I agree that aiming is going to be the hard thing, but that is a problem that needs to be solved at an individual level. I was saying that there is no particular need for a a coordinated rehearsal.
As far as how to train the light, one possibility is to poke a small hole in the center of the mirrorized surface, and set up a set of crosshairs on a stick. Look through the small hole, line up the cross hairs on your target and then adjust the mirror until it lights up the cross hairs.
My dad built a device similar to this (although there may have been some more parts that I am missing) that would allow you to reflect a light sunlight from a 8"x10" mirror that could be seen at a selected target miles away. We once used in it boy scouts to try to send messages via morse code between a peak and a place in the town near by. It didn’t work very well but mostly because our poor signalling skills made it difficult to distinguish dots and dashes, but we could definately see eachother’s lights.
Okay, I’m not going senile. A long ago banned/suspended poster here claiming to represent a panhandler’s union (whatever that is exactly) wanted to know if you could set cops on fire at protests by doing this.
(1) The programs were highly mirrored. The narrator mentioned women using them to check their hair and makeup.
(2) 50,000 army personnel on the sunny side of the stadium. Everything was planned ahead of time, a bugle sounded when they were supposed to aim their programs. Possibly they had some training in aiming signaling mirrors as a survival skill.
(3) The programs were described as “tabloid sized”, so between 1 and 2 square feet each. Mirror area 50k to 100k square feet making something like a Fresnel parabolic reflector.
The person who organized the thing just intended to blind the ref so that he would be replaced by one not bribed by the other team. Vaporization was not the goal.
The heliograph has been used by various militaries from the beginning, until as late as 1975. In addition to his monumental ego and a MoH, McArthur was a trained heliographer.
Thank you for a.) remembering this and b.) citing me (yay!)
I didn’t come up with this clever solution – this was the result of folks in the Navy during WWII trying to figure out the best way to use a mirror to heliograph your location to search planes. Unfortunately, they didn’t publish it until after the war ended. But the idea’s the important thing (IT’s in the Journal of the Optical Society of America vol. 35 p. 805 (1946) and vol. 35 #2 pp. 110-115 (1946). The suggestion that this method of aiming could have been used in the ancient world appeared in Applied Optics back in 1973 I first stumbled across the idea in Hal Clement’s science fiction novel Cycle of Fire from 1957.
I’ve tried the method, using it to direct sunlight with a double-sided mirror towards a piece of retroreflecting material. It works beautifully. you can but double-sided signalling mirrors with a hole for the aiming in the center at Army-Navy surplus stores.
That sounds much better than just a silver program. But having a lot of experience aiming an ordinary mirror that takes two hands to hold, with no cross hairs or other special features, i claim that with a little bit of practice and training, it would have been perfectly feasible to kill the soccer official with the programs.
Back when they tested this on Mythbusters, they ran into the problem that’s already been mentioned in this thread; you can’t just aim the spot from your mirror when there are a hundred other spots dancing around on the same target. I figured there had to be a way to make some kind of aiming mechanism but I couldn’t quite figure what it was. I must have missed Cal’s post that described it.
I really wish the Mythbusters had used this. I think they even tried the death ray three times. There ought to be enough energy to set a ship on fire, and the aiming device isn’t exactly cutting-edge tech. I hate to see them fail because of something that could have been fixed.
I wonder if we could succeed where the Mythbusters failed. Can we get some volunteers? And a boat?
It’s not so much that there’s thousands of dancing spots of light around the target, but one blob of very bright shimmering light that’s roughly the size of the target and then a bunch of dancing spots around the outside. You “find” your spot by aiming off to the side then moving it back into the bright blob, but once your spot is in the blob, you have no way to differentiate it from the rest. At the distances involved it’s not feasible to curve the mirror to focus the beam because the focal point would be fixed and the target would just have to step back a few feet. So the “points” of light being aimed are going to be the size of the mirror. The best aim the Mythbusters were able to get was a hot spot of roughly 5-8 feet in diameter at any appreciable distance. It was definitely hot, and Jamie wore his fire suit for protection, but they had to move the boat very close to shore and sit perfectly still for the tinder-dry wood of their boat to finally catch fire. There was nothing quick about it.
IIRC from my youth (long ago) a 2 inch magnifying glass producing a 1/8" spot took a few seconds to set a piece of (white) paper smouldering. That’s 256-times concentration. A thousand or more, even badly aimed, should produce 3rd degree burns and set the black stripes of the ref’s shirt on fire. The heat of vapourization would suggest the guy would collapse in flames long before he actuually evaporated.
Puzzlegal has the aiming technique right. You tilt the mirror down to see where the reflection is nearby, then tilt it upward when it is properly lined up. Not perfect, but close enough I assume.
Instead of tracking a concentration factor, it’s much easier to track a sky-angle. As I go outside right now, I look up in the sky, and most of it is cool, but there’s a very small patch of the sky, a circle a half-degree in diameter, that’s at around 6000 K. If, instead, I’m an ant underneath your magnifying glass, I look up and see the entire magnifying glass, subtending an angle that depends on its distance, and all of that area of the sky appears to be at 6000 K. If I’m the referee on the field, then if the Army hooligans are aiming well, half of the stadium appears to be at 6000 K. If I’m an invading Roman soldier, and the Syracusian army on the hillside by the beach is aiming well, then the entire hillside appears to be at 6000 K. And if I’m in a well-designed solar furnace, with mirrors completely surrounding me, then everything around me appears to be at 6000 K.