So, I have an idea that's also sort of questions I can't find the answers to.

I want to focus rays from the sun either using a large parabolic dish covered in mirror shards, or ideally through a large Fresnel lens and then direct those rays down the length of a circular tube with a mirrored interior surface to create a portable solar cutting torch/kiln furnace.
Given a perfect lens, a perfectly circular tube and 100% reflection:

Will the rays pass through one another down the length of the tube and refocus at a distant point?
...and if so, would the same hold true for a bent tube? Could focussed light be redirected to distant points, say even underground?

...and why is there absolutely no information to be found on the reflectiveness of natural obsidian?
-- 2 fries shy of a happy meal, Oct 29 2015

https://www.youtube...watch?v=Sh8EEDuiQ54 [pashute, Nov 01 2015]

Cassegrain focus https://en.wikipedi...assegrain_reflector
Combine this... [neutrinos_shadow, Dec 13 2021]

Heliostat https://en.wikipedia.org/wiki/Heliostat
..with this... [neutrinos_shadow, Dec 13 2021]

Equatorial mount https://en.wikipedi...ki/Equatorial_mount
..on a re-jigged one of these. [neutrinos_shadow, Dec 13 2021]

Odeillo https://en.wikipedi...eillo_solar_furnace
Hot stuff! [neutrinos_shadow, Dec 16 2021]

Could you use a fiber optic instead?
-- travbm, Oct 29 2015


Why not use total internal reflection?
-- RayfordSteele, Oct 29 2015


Same principal I guess eh? but would the light rays re-converge after bending around corners? I try to visualize it, but it hurts.
-- 2 fries shy of a happy meal, Oct 29 2015


//Will the rays pass through one another down the length of the tube and refocus at a distant point?//

You'll need a lens at some point. Think about the point at which the parabolic dish (or Fresnel lens) focusses the rays. They are arriving from all of the dish/lens, and hence at all different angles. They'll therefore bounce down the light-pipe at all different angles.

For instance, a ray that comes from the centre of the dish will travel straight to the focal point, and straight down the middle of the light pipe (if it's straight). A ray that comes from the edge of the dish will enter the pipe at an angle, and will bounce back and forth along the pipe.

So, when the rays exit the pipe, they're going to be heading in all different directions.

And, as I recall from an earlier discussion here, you can't focus light down to a point smaller than the source - in this case the disc of the sun as viewed from Earth.
-- MaxwellBuchanan, Oct 29 2015


hm... I don't want to tighten the focus at all. Because the rays are at an angle to converge when they enter the tube, in my mind I saw them passing through one another at re-convergent intervals down the length of the tube. I get what you're saying though, the straight-line photon will get to the distant focal point slightly faster than the mid-mirror photon, which would arrive faster than the outermost... etc.

Cool.
How about a variation of a Fresnel lens or reflector which, instead of bending concentric rings of light to a focal point, was designed to take the length and curvature of the conduit into account and scatter light accordingly. Could the topology of the lens/reflector needed to do this be figured out by algorithm?
and perhaps even morphed to adapt as needed?
-- 2 fries shy of a happy meal, Oct 29 2015


You can't focus light smaller than the source? I thought it was you can't make it hotter than the source... what if you collimate it first?
-- RayfordSteele, Oct 29 2015


hmmm, that gives me an idea...
-- 2 fries shy of a happy meal, Dec 12 2021


(See linkies)
Using a Cassegrain arrangement to bring the focal point behind the main mirror gets your "hot spot" out of the light path. A heliostat mounted in front of it (on a common frame, but usefully distant) allows you to point the hot spot where you need it, but still get sunlight in the axial direction to the parabola. Some servo-motor & sensors to track the sun (only needs 2) & this is perfectly doable (depending on how large you want to go...). Hmm... since the path of the sun is known, you might get away with no sensors & just precise speed control (assuming you line it all up nicely with South...).
-- neutrinos_shadow, Dec 13 2021


Sweet! You rock.
-- 2 fries shy of a happy meal, Dec 16 2021


Could you do that with thousands of degrees? I'm thinking that there must be a maximum that such a system would work for given how reflective and symmetrical todays' materials are.
-- 2 fries shy of a happy meal, Dec 16 2021


If you rotated it thousands of degrees it wouldn't track the sun so well and would just spin around.
-- pocmloc, Dec 16 2021


Odeillo Solar Furnace (linky) gets to 3,500 degrees. I doubt you will do better, but you might. The idea is, your surfaces (mirrors, prisms, whatever) are "away" from the focus, so the temp gets high at the focus but (since they are more spread out) the surfaces don't. The "worst case" is the secondary mirror, which might need active cooling.
Hmm... [pocmloc]'s comment gave me a thought: the light doesn't care if your mirrors spin around their axes (assuming good optic smoothness...) so you could spin them to get air- cooling.
-- neutrinos_shadow, Dec 16 2021


hmmm...
-- 2 fries shy of a happy meal, Dec 17 2021


We already discussed possible relativistic effects of rotating optics a few years ago
-- pocmloc, Dec 17 2021


Check your e-mail, [2 fries shy of a happy meal] :-)
-- neutrinos_shadow, Feb 06 2022


Dude!
-- 2 fries shy of a happy meal, Feb 07 2022



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