How further apart will I have to hold a magnifying lens for it to be burnt by another magnifying lens focussing the rays of the sun ?

Edit : I have googled for this and there are no answers available. I have tried to do this experiment myself and wasn't able to reach a conclusion.


closed as unclear what you're asking by CuriousOne, John Rennie, ACuriousMind, heather, sammy gerbil Aug 10 '16 at 12:43

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ I don't know how much more clarity can be put into this question, really. $\endgroup$ – happybuddha Aug 14 '16 at 22:26
  • $\begingroup$ I don't really understand what are you trying to do. People reading your question don't know what is in your mind, they only read what you wrote. As you formulate a question, give always many details: 1) what you want 2) how you tried it 3) what didn't work. | "I don't know" is not a detail. $\endgroup$ – peterh Aug 14 '16 at 22:53
  • $\begingroup$ @happybuddha, I appreciate your edit, but I do not understand your experiment (i.e., what you are trying to find the solution too). I would say, try to provide a diagram of what you mean and explain a little further what you mean. It is not so much the work at this point, but just what in the world you are talking about, because I have no idea. $\endgroup$ – heather Aug 14 '16 at 23:01

Three conditions must be fulfilled:

  1. The potentially burned lens is in the focal plane of the other lens;

  2. The other lens is big enough to gather enough solar energy; and

  3. The potentially burned lens is absorbing enough that enough of the the light focussed on is dissipated as heat within it, such that the lens temperature rises either enough to denature plastic / melt glass or set up a differential temperature profile and large enough attendant internal stresses to shivver the lens, as appropriate to the situation on hand.

Are you destructively testing lenses, or are you wanting to ensure a solar system doesn't fry itself?

I'm guessing the aim of your question is the latter. Most lens systems do not have real focusses inside them so this kind of thing is not often a problem, but a notable exception is the Keplerian Telescope and like systems, which must form an intermediate focus inside the lens system so as to put the exit pupil at the viewer's eye / physical output of the device: this kind of arrangement might be used in a relay lens system where you want to confine light into a tight, long tube. Certain endoscopes for example uses this concept.

Bottom line: you must raytrace throughout your system, and, if there are any regions where the beam is squeezed into a tight region whilst propagating through a physical lens element, then you must calculate the rate of dissipation of light into that element, and do a thermal conduction / convection analysis to check that the temperatures reached are not high enough to damage the lens in question.

  • 1
    $\begingroup$ s/solar system/optical system/ ? $\endgroup$ – The Photon Aug 10 '16 at 5:17
  • $\begingroup$ @ThePhoton I actually did mean a solar system: I'm guessing this may be a solar energy application. $\endgroup$ – WetSavannaAnimal Aug 10 '16 at 5:20

Not the answer you're looking for? Browse other questions tagged or ask your own question.