I once read a collection of Arthur C Clarke short stories and in one of them the main character visits a football / soccer game in South America where the referee is under threat, so much so that he wears a bullet-proof vest.
At one point in the game one of the teams supporters, say 10’000 people, use hand-held mirrors to concentrate sunlight on the referee who is promptly killed although they only intended to temporarily blind him, it’s implied that he’s vapourised entirely.
Now Clarke is usually reasonable realistic in his stories but I find this unlikely to say the least, what would actually happen?
I don’t think you can spontaneously get 10,000 people to sufficiently aim that number of hand mirrors to do the job. At the very least, it would require rehearsal.
The thing is, you’d assume a referee could move, move away from a hotspot like that.
But… given sufficiently focused sunlight… yeah, you could skill someone with sunlight. Even cremate them. There are a lot of variables involved that affect how quickly it could happen and how thoroughly “cooked” the target is but it’s certainly possible.
I haven’t done the math (and wouldn’t be able to) but I rather doubt that even that many handheld mirrors could add up to a meaningful amount of sunlight-damage to kill someone. Especially since each handheld mirrors is likely to reflect the sunlight in a very dispersed, not concentrated, manner. And for death to be immediate, and vaporize a person? Again, dubious.
Concentrated solar power generator systems use mirrors to heat water into steam. They use very big mirrors, though. Archimedes is supposed to have designed a solar weapon using mirrors capable of setting fire to ships; in a test in 1973, Dr. Ioannis Sakkas set fire to a tar-covered dummy ship at a distance of 160 feet using large, handheld mirrors. So it is probably doable.
As I understand it the maximum temperature possible in a mirror or lens system is the surface temperature of the Sun; 9,930°F, enough to vapourise a person. I think if 10000 people had a handheld mirror as big as themselves, the ref would be in trouble.
And even more importantly they are acutely well focused.
I guess like many boys who grew up in less protected times, on occasion using a simple hand held magnifying lens to burn paper was reasonably harmless fun. But you never did it with wet or even moist paper. If the ref was constructed of tissue paper and motionless, with considerable nea improbable organisation from the stands they might be able induce something sort of singe. But the ref is human and 60% water and also in motion which blurs the focus and allows for air cooling.
Mythbusters did the Archimedes Death Ray bit and yes, if the invasion fleet politely moored 50m off shore and didn’t move nor say splash water on the focal points you could char dry wood to the point of a sustained smoulder. Defenders would have been better served lobbing molotov cocktails or at least burning tar with a catapult.
And if the crowd of 10,000 at a soccer match were holding body sized reflective mirrors and the opposition scored a goal they’d probably smash them in frustration and the ref would be in much greater danger from thrown shards of glass.
If I remeber that story correctly, the mirrors were actually the game program which cover was printed on shiny paper. The fans holding them were members of the Army, so it’s possible they practiced it beforehand.
However, there’s still problems. The people holding the “mirrors” have no feedback as to whether they’re aiming correctly. The ref is likely 30 or more meters away. If a mirror holder is somewhat off, their light will be hitting the grass near the ref, but they can’t tell that their mirror is aimed at the grass and not the ref. There are likely hundreds of others aimed at that spot or near it. If they change their aim, it’s likely to be to another spot of grass, also illuminated by hundreds of others. So I’d guess there’s probably only a small fraction of the mirrors aimed correctly, perhaps 10% or less.
I also doubt that shiny paper will reflect at the same efficiency as a glass mirror.
With a hand mirror it might be possible to use the technique one uses with a signaling mirror: put the mirror close to your eye, make a V with your fingers at arm’s length, place the target in the V, use the fingers to aim the beam.
Not that it would help much. I can’t imagine getting the spots tight enough.
That’s definitely not the weakest part of this story. I figure an ingenious person could sort out the aiming and issues with the flexible program cover (no specifications on how thin the program is or how windy the day is).
The program could be stiff, but I guess it would work better if bend a bit.
The army got fifty thousand copies and free tickets to the sun side of the stadium.
11x17 inches is 187 square inches = 0.120645 m2 * 50000 = 6032.25 m2 but how many calories that would collect, I don’t know.
I don’t know that it would require a rehearsal. Each individual has the sole job of keeping their reflected light on the Ref. If each can do it individually then they can do it en mass without coordination.
Let’s say you want to shine your light on the ref. So you aim your mirror roughly at them. You see the glint of light a few feet to their left, and a few moments later you’ve adjusted and aimed directly at them.
Now let’s try again, only the whole stadium has the same idea. You aim your mirror roughly at the ref. At the same time, ten thousand other people do the same. There are now ten thousand randomly dancing points of light on the field, overlapping each other and wiggling around as their owners try to identify their own beam and properly aim it.
