Though we cannot perceive it with our eyes, everything around us (including plants) reflect wavelengths of light in visible and non-visible spectrum. Taking into account how much amount of a certain wavelength is reflected, we can access the current status of plants.
Our system monitors how much sunlight is reflected by the plants. We know that, plants have chlorophyll which absorb sunlight for photosynthesis . If a plant is healthy, it will have large amount of chlorophyll on it’s leaves and will absorb good amount of sunlight and reflect quite less of it and vice-versa. We take into account this basic principle in identifying crop health status of an agricultural land.