In the image below, there is a curve-shaped conduit which transmits solar light. The light is collected at different spots and injected into the conduit at various spots with fiber optics.

As per energy conservation, will the final beam have power equal to the sum of the power of all incoming beams? Any other observations?

Similar phenomenon takes place in "atmospheric ducts" where solar light is trapped and directed along a curved duct.

Can this be done with some cavity/solid-state device on a small scale?

solar light duct


Yes this can be done within the bounds of the second law of thermodynamics. The étendue (or optical grasp) (see the étendue Wikipedia page) measures the entropy of a light beam, so, for a passive system, the étendue of the output beam must equal or be greater than the sum of the input beams' étendues. It is this principle that, for example, prevents one from concentrating the blackbody radiation from a hot body so that it might heat a body hotter than the emitter at the concentration point. It also means that the number of supported waveguige modes of the output must be high enough to be compatible with the required étendue if the coupling is to be 100% efficient. The efficiency of concentration of Solar radiation into a single mode fiber is very low for this reason.

  • $\begingroup$ Thank you, very interesting comment on the optical grasp! I have been trying to read up about devices that can guide many different light beams. I am unclear how can this be done. So far, I found plastic optical fibers and waveguides. Perhaps should post this as a separate question... $\endgroup$ – user142523 Jan 19 '17 at 17:09

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