Two years ago, four Hungarian scientists published a paper called Optics of sunlit water drops on leaves: conditions under which sunburn is possible in the journal New Phytologist. Given the near-universal belief that water drops can scorch plant leaves on a sunny day (e.g. the RHS book How To Garden: Under a hot midday sun, water droplets on leaves will act as miniature magnifying glasses and may scorch them), you may be surprised or you may not that no one had previously checked to see if this actually happens.First of all, the short answer is no. At various times on a hot, cloudless day in July (in Hungary, where such things still happen), water drops were carefully placed on the surface of horizontal ginkgo and Norway maple leaves and left in the sun until they had evaporated, which generally took an hour or two. Careful examination of the leaves revealed no trace of any damage.Which leaves the interesting question: since water drops undoubtedly can act as small lenses capable of focusing the suns rays, why do they fail to cause any damage? To find out, the researchers carefully calculated the paths of light rays falling on water drops. The first thing to say is that water makes a less effective lens than glass, owing to its lower refractive index. The second is that the shapes of water drops on a leaf vary a lot, depending on how wettable the leaf surface is. On a wettable leaf, such as maple, water spreads out to form a thin, shallow drop. Although such a drop is capable of acting as a lens, it focuses light well below the leaf surface (or it would if the leaf were transparent in practice it doesnt focus it anywhere).
