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Thread: Would my ray gun work?

  1. #31
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    Quote Originally Posted by a1call View Post
    Here is my proverbial 2 cents:

    • The fact that the sun's ray are not perfectly parallel is irreverent
      • They are nearly parallel and the concept is valid in principal/theory given:
        • The concave (secondary) lens has a (much) shorter focal length than the larger collector/primary convex lens
        • The lenses are highly aligned and the focal points of the 2 lenses are are precisely coincided
        • This will produce a nearly parallel ray which is more intense than the incident sun rays since the rays will will be concentrated in a smaller diameter than the incident ray
        • One caveat is that the 2 lens' alignment and foci coincidence (Occupying same point in space) requires Optical-Laboratory precision and can not be achieved by homemade apparatus fabrication.



    BTW, this reminds me of Archimedes' Heat-Ray. A Mirror equivalent with a hole in the middle of the concave mirror is as valid. Forgo the hole and you will end up with a light trap with (theoretically) continuously converging rays (precision, non-parallelism and scattering notwithstanding).
    https://en.wikipedia.org/wiki/Archimedes#Heat_ray
    The fact that the sun's rays are not parallel is not at all irrelevant, it is the one and only reason why the concept doesn't work.

  2. #32
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    Quote Originally Posted by a1call View Post
    Image search Google for: Laser beam expander
    To see images related to the reverse process.
    It is common in laser optics.
    The literature on the web describes the narrowing process difficult to achive due to worsening of divergence.
    FWIW.
    Sunlight is not laser light. Something being achievable with laser light does not mean it can be achieved with sunlight.


    Quote Originally Posted by a1call View Post
    This thread got me thinking. If I am not mistaking:
    • Laser beam divergence is for the most part due to scattering
    • The ratio of scattering over beam diameter will be less the wider the beam is
    • The larger the primary/collector convex lens (or equivalent concave mirror) is the "easier" it will be to narrow parallel beams to a given diameter
      • It is the same principle that produces sharper images/view in a telescope, the larger the area of the collector is.
    Laser beam divergence is limited by diffraction, not scattering. Divergence of focused sunlight is limited by it being an extended source that is about half a degree wide at the distance of Earth.

  3. #33
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    Yes diffraction not scattering. Thank you for the correction.
    With 1/2 degrees of divergence. Diffraction worsening after narrowing notwithstanding, a sun beam of 10 meters in diameter, which its cross diameter area has been narrowed 10 times will have a diameter of:
    10/Sqrt(10)= 3.16 meters
    In other words 10 circles of 3.16 metres have an area equal to one 10 meters in diameter circle.
    A 0.5 degrees divergence of a 3.16 metres in diameter beam will result in a diameter of: 2*100*tan(0.5)+3.16 = 4.91 metres
    In a distance of 100 metres.
    Such a beam will have an intensity
    ((10/2)^2*pi)/((4.91/2)^2*pi)=4.15
    times the original input beam.
    Not much but any target intensification can theoretically be achived, if you are willing to go large enough.
    Last edited by a1call; 2018-Jul-02 at 10:42 PM.

  4. #34
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    Quote Originally Posted by a1call View Post
    Yes diffraction not scattering. Thank you for the correction.
    With 1/2 degrees of divergence. Diffraction worsening after narrowing notwithstanding, a sun beam of 10 meters in diameter, which its cross diameter area has been narrowed 10 times will have a diameter of:
    10/Sqrt(10)= 3.16 meters
    In other words 10 circles of 3.16 metres have an area equal to one 10 meters in diameter circle.
    A 0.5 degrees divergence of a 3.16 metres in diameter beam will result in a diameter of: 2*100*tan(0.5)+3.16 = 4.91 metres
    In a distance of 100 metres.
    Such a beam will have an intensity
    ((10/2)^2*pi)/((4.91/2)^2*pi)=4.15
    times the original input beam.
    Not much but any target intensification can theoretically be achived, if you are willing to go large enough.
    It is impossible to intensify sunlight with optics that do not cover more of the sky as seen from the target than the sun itself does. This is not what the word "raygun" typically implies. And no, it is not possible to achieve any desired intensification, the surface brightness of the sun is a hard physical limit. The best you could possibly do is direct sunlight at the target from all sides in an arrangement nothing at all like a raygun, at which point it will reach equilibrium at ~6000 K.

  5. #35
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    Here is a related discussion and the quote is what I agree with for the record.

    The sun's surface temperature is not an upper limit. The upper limit on power derived from the sun is the power output of the sun.

    Reference https://www.physicsforums.com/thread...-point.315719/

    https://www.physicsforums.com/thread...-point.315719/

  6. #36
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    Quote Originally Posted by a1call View Post
    Here is a related discussion and the quote is what I agree with for the record.




    https://www.physicsforums.com/thread...-point.315719/
    Yes, the upper limit is the power output of the sun...across a surface area equal to that of the sun.

  7. #37
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    Quote Originally Posted by a1call View Post
    Yes diffraction not scattering. Thank you for the correction.
    With 1/2 degrees of divergence. Diffraction worsening after narrowing notwithstanding, a sun beam of 10 meters in diameter, which its cross diameter area has been narrowed 10 times will have a diameter of:
    10/Sqrt(10)= 3.16 meters
    In other words 10 circles of 3.16 metres have an area equal to one 10 meters in diameter circle.
    A 0.5 degrees divergence of a 3.16 metres in diameter beam will result in a diameter of: 2*100*tan(0.5)+3.16 = 4.91 metres
    In a distance of 100 metres.
    Such a beam will have an intensity
    ((10/2)^2*pi)/((4.91/2)^2*pi)=4.15
    times the original input beam.
    Not much but any target intensification can theoretically be achived, if you are willing to go large enough.
    I cannot visualize your geometrical optical line of thought from your words. A sketch might help. I stand by what I wrote in post 15 and illustrated with sketches later. What we get is a projected image of the Sun, with the diverging cone of light that forms it subtending an angle of about 5 degrees for a 10x scope, as measured from the exit pupil. At a distance where its diameter equals that of the objective, it is no more intense than direct sunlight without the scope. Still farther back it becomes less intense.

  8. #38
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    For a sketch images-search the phrase:

    Laser beam expander

    and take any of the 1st few drawings showing a narrow intense parallel input rays expanding to wider, less intense parallel rays and then work it in reverse. I don't think it's that complicated as a concept. Of course as I said before the related web discussions describe the reverse difficult to achieve due to worsening divergence. This is all repeat of what already was said.

    Here is a discussion and quote on the subject:

    Unfortunately what you are trying to correct is a law of light physics. If you make the beam narrower, your divergence will increaUnfortunatelyUnfortunately what you are trying to correct is a law of light physics. If you make the beam narrower, your divergence will increase significantly.

    what you are trying to correct is a law of light physics. If you make the beam narrower, yo significantly.
    se significantly.
    https://laserpointerforums.com/f49/i...ter-60908.html
    Last edited by a1call; 2018-Jul-03 at 02:12 AM.

  9. #39
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    Here is a link to sketch and the product (expander not narrower):
    http://www.cnioptics.com/beam_expander_for_marking.htm

    ETA this page has conceptual drawings showing the lenses:
    https://www.edmundoptics.com/resourc...eam-expanders/
    Last edited by a1call; 2018-Jul-03 at 02:20 AM.

  10. #40
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    a1call, I am not interested in lasers here. Tom asked about concentrating light from the Sun into a beam to be projected a considerable distance beyond the secondary lens. I posted my conclusion in post 15 and followed up with some sketches. Do you disagree with my conclusion?

  11. #41
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    The existence of beam expanders/reducers is rather irrelevant, because you are not dealing with a beam of collimated light.

  12. #42
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    Quote Originally Posted by Hornblower View Post
    a1call, I am not interested in lasers here. Tom asked about concentrating light from the Sun into a beam to be projected a considerable distance beyond the secondary lens. I posted my conclusion in post 15 and followed up with some sketches. Do you disagree with my conclusion?

    I think your sketches in post 21 are not a good representatives of sun rays since your objective is too close to the lens and does not take into account the near parallel rays of the sun.

    The actual dispersion is closer to ideal perfectly parallel rays than what you have sketched.

  13. #43
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    Quote Originally Posted by a1call View Post
    For a sketch images-search the phrase:

    Laser beam expander

    and take any of the 1st few drawings showing a narrow intense parallel input rays expanding to wider, less intense parallel rays and then work it in reverse. I don't think it's that complicated as a concept. Of course as I said before the related web discussions describe the reverse difficult to achieve due to worsening divergence. This is all repeat of what already was said.
    ...linear optics are reversible. Every beam expander is also a beam reducer. Neither is more difficult than the other, and neither is relevant to making "ray guns" that use sunlight.


    Quote Originally Posted by a1call View Post
    I think your sketches in post 21 are not a good representatives of sun rays since your objective is too close to the lens and does not take into account the near parallel rays of the sun.

    The actual dispersion is closer to ideal perfectly parallel rays than what you have sketched.
    Those sketches show exactly why sunlight can't be focused like laser light, the "near parallel" rays of the sun are not parallel. It doesn't matter where you put the objective, etendue is a conserved quantity in optics, and there is no arrangement of lenses that can reduce it. And dispersion is something else entirely...a difficulty when focusing white light with refractive optics, but not the limiting factor in focusing sunlight.

  14. #44
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    the sun's rays are only near parallel if you stop down to use a small area of the sun's surface. we experience rays from all over the visible surface and those extra rays arrive at angles. Therefore a mirror that directs the suns rays in a focussed way on the target would deliver much more heat than the proposed optics. Searchlights use a point source, or as close as you can get, and parabolic mirror. The first lens does not produce a point source, the dispersion spreads the focus point out along the optical axis, so the second lens will not be able to focus on all the rays, basically only on the central part of the sun's image. So the ray disperses. You get a better near parallel ray just using a flat mirror for long distance.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  15. #45
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    I will post some more sketches showing the source to the left of the objective in addition to what I already posted. I omitted that to save space, but it appears that some of us might have trouble visualizing it.

  16. #46
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    Quote Originally Posted by Hornblower View Post
    I will post some more sketches showing the source to the left of the objective in addition to what I already posted. I omitted that to save space, but it appears that some of us might have trouble visualizing it.
    Here are my new sketches:
    Ray tracing i.jpg
    In each case a 1" aperture front lens focuses the incident light at a point back 5", and the back lens has either a positive or negative focal length of 1". The sources are 3" in front of the front lens so I could get everything on the sketch. Moving the sources out to infinity, lengthen the arrow to keep it at the same angular size, and changing the strength of the front lens to keep its focus at 5" will give the same results.

    Figure 1 has a point source on the optical axis. The ray tracing as I explained in previous posts produces concentrated beams of parallel rays emerging from the back lens, along the optical axis. With good quality lenses this beam will remain just as tight for a long distance. In a thought exercise it could go forever. In the real world diffraction would degrade it after a while. With a sufficiently brilliant point source and a stronger back lens closer to the focus, this could be the ray gun Tom was envisioning.

    Figure 2 has a point source off the optical axis, as indicated by a short arrow, with the arrowhead being the source. The key to tracing the exit beam is the peripheral ray through the center of the back lens. In this thin lens approximation that ray is undeviated, and the other rays from the front lens are parallel to it. I remembered that principle from college physics and used it here. Note that the exit beam is inclined to the optical axis several times the inclination of the incident rays. The arrow represents a point on the upper edge of a disk source centered on the optical axis. Tracing the rays from the lower edge will give a beam with the same inclination in the opposite direction. Filling in from the rest of the disk will create a cone of light expanding away from the instrument. In this case, going back just a few inches will expand the pattern to the diameter of the front lens, making the intensity no greater than that from direct lighting by the source. Beyond that it will continue to expand and fade, and will not be an effective ray gun at any great distance.

    The key point here is that not all sets of non-parallel incident rays are created equal. Rays from a nearby point source can be made parallel by carefully focusing the instrument. Rays from different parts of an extended source will inevitably diverge.

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