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I was wanting to perhaps grow some plants using grow lights. For the sake of efficiency, however, I would like to use some mirrors to reflect this light, ie, keep less of it from uselessly dissipating.

However, I must ask: how much of the actual light energy is a mirror reflecting when it reflects? Is this reflection biased towards certain parts of the spectrum? I thought plants used more than the visible spectrum for energy...

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It depends on the actual materials. If you have a big need and a big budget, you can make some mirrors that have very high reflectivity. 99.9% is achievable in some cases for certain energy ranges.

As energies go up, (UV and higher) it gets harder to reflect cleanly. For sunlight, most of the energy is in the optical, so you can ignore the reflectivity of X-rays and radio waves of your material.

This wikipedia article shows the spectrum for a few materials. Optical Coating

Engineering toolbox suggests 80 - 88% for silver on glass. Reflecting Factor

Most plants use chlorophyll with active energies in blue and red. They do not use non-optical wavelengths for photosynthesis. They may use other wavelengths for other purposes such as display or thermal control.

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