The inverse square law describes how light intensity drops as the distance from a light source increases. If the distance from the source is doubled, the light at the leaf surface falls to one quarter; if the distance is tripled, it falls to one ninth. This matters most for point-like artificial lights placed close to plants, where small changes in height can cause large changes in PPFD.
In plant care, the inverse square law helps explain why a grow light that performs well at 6 inches (about 15 cm) can be much less effective at 12 inches (about 30 cm). It is less exact for large fixtures, reflective setups, or sunlight, because those do not behave like a single point source over normal gardening distances. Even so, it is a useful rule for understanding why fixture distance strongly affects light available for photosynthesis and daily light integral, or DLI.
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