These are basically the spherical cows of mirrors, right? Perfect reflection so no need to worry about melting or anything like that?
Frictionless spherical cow mirrors in a vacuum.
I’m trying to understand this.
If I have 100 1m2 mirrors all shining onto a single 1m2 high intensity spot, that high intensity spot is going to be much hotter than if it was only heated by the direct light from the sun that naturally hits it. The high intensity spot can never get hotter than the surface of the sun, but we won’t be anywhere near that, so as an upper bound it is irrelevant.
If I have 10 1m2 mirrors all shining onto the high intensity spot, it will be less hot than the 100 mirror case. Or is my intuition wrong?
If the 100 mirrors are shining onto the 10 mirrors, and then the 10 mirrors shine onto the high intensity spot will that be hotter than the 10 mirror case? Any heat absorbed by the 10 mirrors will definitely make it less hot than the 100 mirror case, but accepting some loss, will it be nearly as hot as the 100 mirror case?
That’s almost the exact opposite of what I’ve been saying. You can turn it into multiple suns, at least effectively. But what you can’t do is make the Sun (or a sun) any more intense.
No. At best, it’ll be the exact same hotness. Except that you’re making the system more complicated, and so making it more likely that you’ll accidentally lose some hotness somewhere.
Assuming the mirrors are all at the same distance from the spot, from the point of view of the spot, there’s 10 extra “suns” in the sky for the 10 mirror case, and 100 “suns” for the 100 mirror case. This is fine. The 100 mirror spot gets hotter.
This is where the example breaks down. Now from the point of view of the spot, it would somehow see 10 suns of 10x amplification each, which would mean the source temperature now exceeds the temperature of the sun. This is impossible. You can’t focus light in the way you are describing.
We’ve all got to be careful about conflating temperature with heat flux. They are related. They are not the same.
This is what I don’t understand. I don’t care about how hot the sun appears, I care about how much heat/light/energy is concentrated on the final spot. Are those different things? The final spot isn’t getting heated to 6000K or whatever.
Why can’t a focus light that way? Isn’t it basically just a reflecting telescope?
If I’m collecting 100m2 of energy from the sun, and focusing it on a 10m2 spot, and then focusing that to a 1m2 spot. Where did the energy from the 100m2 go, if it isn’t reaching the final spot? I know some will be lost due to inefficiencies in the system, but all of it?
I’m sure this is a big part of my problem. I don’t know the difference between these things, so I’m using heat, energy, light, and such all interchangeably, when they all probably have very specific, and different, meanings in this context.
In the simplest setup, in the 100 square meters around your target.
But then it isn’t focused on the smaller spot, it is just reflected from one 100m2 spot onto another 100m2 spot that happens to contain some other mirrors in it.
So 100 mirrors, each with motors, sensors, computers, or highly trained 4th graders, whatever, that move each mirror to track the sun and point at the 10 other mirrors. Each mirror doesn’t even have to be flat, they could each be slightly curved to be more efficient, but I think that is just details, not the core of the problem.
Yes, you can focus the 100 mirrors down to the 10 mirrors no problem. But there’s nothing you can do with the 10 mirrors (not by shaping them, not by moving them, nothing) that will continue to focus that down.
Why not, that’s what I’m trying to understand?
How can those 10 mirrors tell the difference between light that is coming directly from the sun, from some high powered spot lights, or is reflected off the other mirrors?
Light coming directly from the Sun is coming from one direction. Light coming from other mirrors is coming from many directions.
Back to etendue.
The brightest you can make a solid angle as viewed by your target is the surface brightness of the original light source.
So if you take the light from the Sun, no matter what you do with you optical chain, where your target is sitting, the final part of the chain they see can never be brighter - in terms of energy per solid angle. If you take your 100 square meters of array, and bounce it via a secondary array, that second array, as seen by your target still has a maximum apparent brightness no more than that of the Sun.
The geometry of the system just ends up pointing the additional light in directions that miss the target.
This might sound like something that should be surmounted by clever optical design, but it isn’t. If you could do so, you could create an optical chain that took light from a cooler source and concentrate it into a hotter target. This means you can build a perpetual motion machine. Somewhere along the way the second law of thermodynamics got violated.
Thank you for the continuing explanations.
How does this correspond to a reflecting telescope? In a simple case a main mirror collects some light, focuses it onto a secondary mirror, which then sends it to an eyepiece. I’m having trouble understanding the difference between using a big mirror to collect lots of light in a telescope, bouncing it around once or twice, and then focusing it. What is the difference between a simple reflecting telescope and the arrangement I described earlier?
Sorry for all of these questions. Sometimes the answer is “you need an upper level college physics course on optics,” but I hope there is some simpler explanation that gets at why a reflecting telescope works but a reflecting ref vaporizer doesn’t.
There is room for confusion because “focus” is being used in two different senses here. When one talks about focusing the image on a telescope one is not talking about concentrating the light - one is talking about making the image true to the original. When one talks about focusing the spot from a magnifying class one is talking about concentrating the light together onto a single point.
In a reflecting telescope, the main mirror starts with a certain amount of light, and the light path never increases that intensity, from then on. Indeed if you have a magnifying eyepiece, the intensity goes down because the light is being spread (ie the image is made larger but less intense).
if it helps, simplify the scenario in this way –
Imagine that you have two people with mirrors who are on the East (sunny) side of the stadium but one is well to the North and one is well to the South of that side of the stadium (we will call them Mr Northeast and Mr Southeast). They reflect the sun to Mrs West, a person with a mirror on the West side of the stadium. There is no way that Mrs West can hold a single mirror so that the spots of light she reflects from Mr Northeast and Mr Southeast can end up in the same place. If she holds her mirror angled so that the light she is receiving from Mr Northeast hits the referee, her mirror must necessarily reflect the light from Mr Southeast somewhere else - her mirror can’t do two jobs at once.
Of course, if we introduce Mr West, then he could reflect Mr Southeast’s spot onto the referee while Mrs West reflected Mr Northeast’s spot onto the referee - but now all we’ve done is replicate what would happen if Mr Southeast and Mr Northeast just aimed their mirrors to reflect onto the referee direct.
Thank you, yes, that is the kind of domain specific terminology that can be very confusing, and which a quick clarification of the specific meanings can help.
I’m not looking to increase the intensity when reflecting it, just change its direction.
I think I’m starting to be able to imagine this. Even if West’s mirror is curved, that curve just collects all of the light from one direction, and sends it to a particular spot. Light from another direction goes to a different spot.
So in a reflecting telescope, all of the light hitting the main mirror is coming from one direction, and is then focused on the secondary mirror, which sends it to the eyepiece? And, yeah, of course when I think about it the eyepiece reduces the intensity, because zoom lens, etc.
So if I build a big reflecting telescope, and make the ref stand where the eyepiece goes, does that work?
Now I’m losing it again, because what if Northeast and Southeast are just two facets in a big reflecting telescope? Is it because a reflecting telescope is designed so everything all focuses in one spot, and the whole scope can move, but the focus point stays in place relative to the mirrors?
Changing the direction is no problem and can occur without loss of intensity (disregarding inefficiencies). However, while I haven’t read the whole thread over to check, my understanding was that this whole tangent started with a suggestion that (say) 50,000 people on the east side of the stadium could reflect sunlight onto 50 people on the west side of the stadium who could then concentrate that sunlight onto the referee.
That can’t happen for reasons given above.
Hell yeah. Assuming it’s a big enough reflector, he’s toast. You are taking all the sunlight falling on your big telescope and concentrating it in one spot. You can do this (as you say) because you are taking parallel rays from the sun, so your reflector can take all that light and reflect it to one place.
As I said, there is no problem with Mr Northeast and Mr Southeast sending their light direct to the ref, thereby concentrating the light on him. And there is no problem with Mr Northeast and Mr Southeast sending their light to Mr and Mrs West, who then send both send their light to the ref. But the latter is pointless since it just adds an extra step to what you have achieved already just using Mr Northeast and Mr Southeast.
I suppose a way of thinking about it is that by using a big primary array of mirrors you are creating a focused image of a tiny part of the surface of the sun. Since the surface of the sun is extremely hot, the reflected image is hot. The bigger your primary array of mirrors, the more intensely you are imaging that tiny part of the surface of the sun, right up to the point where the ultimate perfect array of mirrors would create an image that was as hot as the surface of the sun it was imaging.
The problem is that if you try the same trick again by using a secondary array of mirrors to reflect the image from the primary array of mirrors all you are doing is creating an image of the primary array. And since the image of the primary array is not very hot, the reflected image is the same or less.
Mythbusters tested a Greek story. Highly polished shields were used to direct light at a ship and set it on fire.
Mythbusters couldn’t burn anything.
I was surprised. Most kids have used a mirror to heat up a bug or leaf.