During photosynthesis, plants emit light humans can’t see, but NASA can see that light from space - and they've even made maps of it!

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References

1Seeing Photosynthesis from Space: NASA Scientists Use Satellites to Measure Plant Health
http://www.nasa.gov/content/goddard/seeing-photosynthesis-from-space-nasa-scientists-use-satellites-to-measure-plant-health/#.U2sW5vldV8E