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"It generally takes much less water to deflect visible light and reduce the efficiency of a silicon solar panel than it takes to absorb the UV that is producing

by physcab 17y ago

"It generally takes much less water to deflect visible light and reduce the efficiency of a silicon solar panel than it takes to absorb the UV that is producing power through the melanin." I'm confused by this statement. I'll try to clarify. Melanin's absorption spectrum can be found here: http://www.cl.cam.ac.uk/~jgd1000/melanin.html http://www.cl.cam.ac.uk/~jgd1000/melanin.html As you can see, it has high absorption from 300nm-700nm which is in the UV-Vis range. Similarly, silicon's absorption spectrum is here: http://www.wsi.tum.de/Research/StutzmanngroupE25/Research/Photovoltaics/tabid/220/Default.aspx http://www.wsi.tum.de/Research/StutzmanngroupE25/Research/Ph... So it absorbs much more in those wavelengths so that's why you're bound to get higher efficiencies. Alternatively, water's absorption spectrum can be found here: http://en.wikipedia.org/wiki/File:Water_absorption_spectrum.png http://en.wikipedia.org/wiki/File:Water_absorption_spectrum.... Water does not have a high absorption in this region. So clouds will not absorb wavelengths in this region. And sorry to nitpick, but nothing deflects light. Light is absorbed, transmitted, and emitted. If it is emitted in the direction from which it came, its called reflectance.

madh 17y ago
What about refraction?
- physcab 17y ago
  It's similar phenomena. When light impinges on a material it is absorbed. Electrons in the ground state are lifted into a higher state, and after a very short time (nanoseconds) they relax, re-emitting that light. That light goes in every direction possible, but the light that you perceive is that fraction of the light that is emitted back in your direction...or in a refracted direction.
  - kragen 17y ago
    I do not believe that this is how refraction works. It would imply that light of frequencies other than those corresponding to the bandgap energies of the material would proceed unrefracted, or be absorbed. Furthermore, it would render coherent light incoherent, since nanoseconds are very large compared to the frequencies involved in visible light, and this incoherence would result in the light being re-emitted in every direction, not a particular refracted direction. In short, although the phenomenon you are describing is real, it is very different from refraction. It is phosphorescence and fluorescence. Refraction works because the permittivity of the transparent material is different from the permittivity of vacuum. See http://en.wikipedia.org/wiki/Permittivity http://en.wikipedia.org/wiki/Permittivity.
lutorm 17y ago
Light can scatter (Rayleigh scattering, Thompson scattering), which does not involve an absorption/emission event (unless you are talking about resonant scattering for example in the Lyman Alpha line). That's how clouds work. The albedo of clouds is very high, so little is absorbed. Nevertheless, the small water drops scatter the light.
- physcab 17y ago
  Sure. I was thinking more from the material perspective, but yes there are other phenomena to consider.
ilitirit 17y ago
> And sorry to nitpick, but nothing deflects light. Light is absorbed, transmitted, and emitted. If it is emitted in the direction from which it came, its called reflectance. Sorry to nitpick even further, but IIUC the angle (not the direction) of reflection equals the angle (not direction) of incidence.

by physcab 17y ago

madh 17y ago

What about refraction?

physcab 17y ago
It's similar phenomena. When light impinges on a material it is absorbed. Electrons in the ground state are lifted into a higher state, and after a very short time (nanoseconds) they relax, re-emitting that light. That light goes in every direction possible, but the light that you perceive is that fraction of the light that is emitted back in your direction...or in a refracted direction.
- kragen 17y ago
  I do not believe that this is how refraction works. It would imply that light of frequencies other than those corresponding to the bandgap energies of the material would proceed unrefracted, or be absorbed. Furthermore, it would render coherent light incoherent, since nanoseconds are very large compared to the frequencies involved in visible light, and this incoherence would result in the light being re-emitted in every direction, not a particular refracted direction. In short, although the phenomenon you are describing is real, it is very different from refraction. It is phosphorescence and fluorescence. Refraction works because the permittivity of the transparent material is different from the permittivity of vacuum. See http://en.wikipedia.org/wiki/Permittivity http://en.wikipedia.org/wiki/Permittivity.

lutorm 17y ago

Light can scatter (Rayleigh scattering, Thompson scattering), which does not involve an absorption/emission event (unless you are talking about resonant scattering for example in the Lyman Alpha line). That's how clouds work. The albedo of clouds is very high, so little is absorbed. Nevertheless, the small water drops scatter the light.

physcab 17y ago
Sure. I was thinking more from the material perspective, but yes there are other phenomena to consider.

ilitirit 17y ago

> And sorry to nitpick, but nothing deflects light. Light is absorbed, transmitted, and emitted. If it is emitted in the direction from which it came, its called reflectance. Sorry to nitpick even further, but IIUC the angle (not the direction) of reflection equals the angle (not direction) of incidence.