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Plant leaves absorb visible light for use in photosynthesis, and they reflect near-infrared light. A light-measuring sensor looking at a healthy plant sees very little reflected visible light and lots of near-infrared light. The more healthy green leaves a plant has, the stronger the difference between the amount of visible and near-infrared light it reflects, and the higher the vegetation index. An unhealthy plant will have fewer green leaves, resulting in a lower score on the vegetation index.