Vegetation index

Live green plants absorb solar radiation in the photosynthetically active radiation (PAR) spectral region, which they use as a source of energy in the process of photosynthesis.

Leaf cells have also evolved to scatter solar radiation in the near-infrared spectral region (which carries approximately half of the total incoming solar energy), because the energy level per photon in that domain (wavelengths longer than about 700 nanometers) is not sufficient to be useful to synthesize organic molecules.

A strong absorption at these wavelengths would only result in overheating the plant and possibly damaging the tissues.

Hence, live green plants appear relatively dark in the PAR and relatively bright in the near-infrared.

By contrast, clouds and snow tend to be rather bright in the red (as well as other visible wavelengths) and quite dark in the near-infrared.

The pigment in plant leaves, chlorophyll, strongly absorbs visible light (from 0.4 to 0.7 µm) for use in photosynthesis.

The cell structure of the leaves, on the other hand, strongly reflects near-infrared light (from 0.7 to 1.1 µm).

Source: Wikipedia Normalized Difference Vegetation Index

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