A question provoked by the now-locked mirror protest thread (I obtained permission to start this one, BTW)…
1000 people are recruited to try to set light to a target by reflecting sunlight onto it, using individual hand-held mirrors. The question is: even if they are all positioned on a special gantry that means they can all see the target and the sun, has any individual any hope of actually reflecting sunlight so as to hit the target?
It was easy enough for me to reflect a small spot of light from my wristwatch glass onto the eye of my geography teacher when I was at school - but that’s because, as the only person doing it, I was able to aim not by observing the angle of my reflector, but by seeing the reflected spot of light and moving it onto the target.
But when there are 999 other people all doing the same, I’ve got no hope of seeing the reflected patch of light and moving it to the target, have I?
I think this means that, without some kind of aiming mechanism, an attempt at creating a distributed, human-powered virtual parabolic reflector is doomed to being at best, rather inefficient. Am I right?
You could do it if you had one person aim at a time. Basically have everyone else hold their mirrors down except for one. Have that one person aim at something, then a second a person move their mirror into position. They would know which spot was theirs because it would be the only one moving. Then repeat over and over with each person.
But enalzi keeping the spot in the right place is going to take constant adjustment by the holder of the mirror. No one can hand-hold a mirror so steady that they can keep a reflected blob of light from it in exactly the right place for any length of time without constant bio-feedback (ie adjusting aim based on seeing where their particular blob of light is).
Once you have a few hundred people aiming, you will have an amorphous blob of bright light on or around the target, and you will be back to the same problem ie you can’t tell which part of the blob is yours, in order to adjust your aim.
Even if you had each mirror on a tripod, and assuming (unrealistically) that you can adjust a mirror in five seconds, the rate at which the sun moves would be higher than the rate at which you could adjust 1000 mirrors.
The only way you could do it would be with 1000 mirrors on stands, each with tracking servos that kept it focussed on the same spot while you set up the other mirrors.
With those kind of numbers, you don’t need to be all that exact, and most people are actually pretty good at estimating angles for aiming.
An aiming mechanism can be as simple as a small hole in the middle of the mirror to sight through. See any picture of a signaling mirror for examples.
A real-life test to see if Archimedes’ “burning glass” legend was possible or practical showed that while it’s not exactly easy to get people who haven’t practiced to aim at the same spot, it is possible.
If your mirrors are at any significant distance from the target, you’ll need to spend some cabbage in order to buy good quality, seriously flat mirrors that are also thick enough to resist bending/distortion when held at various angles relative to gravity. If you just use a bunch of 12"x12" mirrors made of 3/16" glass from the dollar store, you won’t be able to project anything resembling a square of light onto the distant target.
I always wondered how you’re meant to use that - do you sight on a point apparently midway between the sun and your target? Obviously the goal can’t be to place your mirror perpendicular to the direction of the target, so how does sighting through a hole help?
That’s not really a comparable experiment - those mirrors are quite carefully arranged, preconfigured for adjustment and are not handheld. It does demonstrate that a virtual parabolic mirror is possible, but that’s not really a surprise.
CalMeacham explained this once, so if I get it wrong, I hope he’ll correct me. The back of the mirror needs to be at least slightly reflective, too. First, you hold your mirror in front of you so that you can see your target directly through the hole. Then, the mirror will be casting a shadow on the ground in front of you, with a little bright spot in the middle of the shadow where the Sun is shining through the hole. Now, you look at the ground reflected in the back side of your mirror, and adjust your mirror (while keeping the target directly visible) until the reflection of the bright spot in the shadow lines up with the hole from your point of view.
As a very rough rule. Say you want the intensity of the sun to be 1000 times that of normal.
And you have a 1000 mirrors hand held mirrors.
If each mirror is say 4 inches by 4 inches, that means the light reflected from each mirror needs not move any more than that (and preferably a fraction of that ) AT the target. If you are talking any real distance to the target and you have hundreds of other reflections keeping you from telling exactly what your particular reflection is doing…well, lets just say I find it highly improbable that you could pull it off. Optical Ninja’s, maybe. A bunch of rednecks with mirrors? NO.
Wow. Totally missed sleel’s post about signal mirrors, but my link does explain the process. No, you don’t try to guestimate a point midway. You position a virtual spot directly on the target.
Your spot is dependent only upon your own mirror, so all the other points created by everyone else’s mirror won’t affect your aiming.
Don’t forget that the sun itself is moving through the sky, so if you’re taking any significant amount of time, the first group will need to be able to change their aim to compensate for the sun’s change.
IIRC a lens with a 100 inch or so focal length forms an image of the sun that is 1 inch across.
A lens with an F ratio of 4 can barely set fire to an optimum target (something like black painted very dry wood). Note I’ve actually played around with this because of an internet discussion years ago.
A 100 inch focal length lens or mirror with an F 4 ratio would have to have a diameter of 25 inches. That means the intensity of the sun is intensified by 25 squared or 550 times.
You really need significantly more than that for bursting into flames of anything other than toilet paper , so the 1000 mirrors are just gonna barely do, even with a good target and darn good aiming.
Somebody need to get a mirror, reflect an image of the sun onto something XYZ feet apart and see just how “stable” they can keep that image on something XYZ feet away.
I’m confused now. Aren’t you talking about lenses with artificial apertures or something here? A lens with a diameter of a few feet can be used to melt concrete.
The peak temp you can achieve is basically ONLY a function of the F ratio. And its non linear. An F2 is four times as “hot” as an F4 and an F1 is 16 times as “hot” as an F4.
You make the mirror or lens ten times bigger (keeping the same F ratio) and the image becomes tens times bigger as well, equalling things out.
If you get the mirror or lens REALLY small then it breaks down due to other considerations, and then it becomes a “could Smurfs actually build those small fires ?” discussion.
Now, with your 1000 mirrors scenario, there is no focusing per se. But for reasonable distances, your 4 inch square FLAT mirrors will reflect (approximately) a 4 inch square patch of light.
Now, if you use a small mirror and a LONG distance, you get the mirror version of pinhole camera (which I have also played with) and can get a decent image of the sun. That really isnt relevant for this discussion, but they are fun to play with and and I can give you a few pointers, as it is fun to play with.
Their first test was handheld, with an ad-hoc untrained group of about 100 people. They got some effects, though not quite so dramatic. Their second test was the one you’re talking about.
Not speaking to the issue of handheld aiming, but this dude did the experiment of putting hundreds of little mirrors on a giant old-school satellite dish in order to focus the sun and burn things.
The first 15 or so pages are documentation of the trials and tribulations of building the thing, and page 15 or so is where he actually starts to burn stuff with it.
An easier way is to hold your thumb out at arms length and looking through the hole in the mirror, place your thumb over the target. Then tilt the mirror so it shines on your thumb, it will then also be shining on the target.