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Plants only absorb certain wavelengths of light. This leaves a lot of light useless. But... imagine putting plants on a wheel and spin that wheel very fast. Then the light is red-shifted and so the plants can absorb a different color of light.

Likewise one might do the same trick with solar panels.

So imagine a scenario where the sun is emitting red light. But a solar panel could only absorb blue light.

Is this possible to use this trick to make the solar panel work? (Assume it is in a frictionelss environment). And how fast would you have to move the solar panel?

As more practical example(?), maybe one could have a ionised gas whose atoms only absorb a certain frequency. But then that gas is rotated very fast in a magnetic field so that now the atoms can absorb the sunlight because of redshifting). Not sure if the energy could be harnessed from this!

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  • $\begingroup$ Have you tried applying Doppler shift equation here? $\endgroup$
    – BioPhysicist
    Oct 8, 2019 at 2:14
  • $\begingroup$ @Aaron I am not familar with the equation. I'll take a look. From the word Dopler I feel like the answer is that you'd have to move it close to the speed of light! $\endgroup$
    – zooby
    Oct 8, 2019 at 2:17
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    $\begingroup$ Also note that plants have adapted to absorb light at the peak of the sunlight spectrum, I believe. $\endgroup$
    – BioPhysicist
    Oct 8, 2019 at 2:17
  • $\begingroup$ @aaron yes, but that still leaves a fair bit not used. e.g. all that green light that is reflected to make plants look green. $\endgroup$
    – zooby
    Oct 8, 2019 at 2:18
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    $\begingroup$ 'course if you red shift the light, then the light it used to be gathering is now out in the longer wavelengths and not being absorbed in a useful way. $\endgroup$
    – Cort Ammon
    Oct 8, 2019 at 4:56

2 Answers 2

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The shortcoming of the arrangement you suggest is that the plant, or solar panel, would only experience the ideal frequency at a single point on the rotation of the wheel. There would be points on the circular path where no red-shit occurred, and points at which the exact opposite of the target shift was experienced. Any tiny amount of additional energy you might hope to obtain from the sunlight would be dwarfed by the energy you would have to expend in achieving the necessary speed of rotation. Granted, in the absence of friction you would be able to recover that energy. However, a number of other practical difficulties confront your scheme. The rotational speed would be so great that even if you managed to create an apparatus that could cope with the centrifugal effect, your plants and the medium in which they were growing would be spattered on the glass of your greenhouse before the machinery had attained a tiny fraction of its intended rotational speed.

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This strategy would not work with plants as they only absorb specific wavelengths (680nm and 700nm). Even if you managed to red shift the light of shorter wavelengths into these wavelengths the original green light would not be utilized. Hence the energy absorbed would still be exactly the same.

The same goes for the solar panels as they also absorb only a fixed set of waveleghts.

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