Sunburn caused by water

Solution

But the situation described will only arise, if certain conditions are fulfilled:

1. The light has to be focussed in such a way so that the focal spot is located exactly at the height of the leaf. If a bigger focal area is produced on the leaf, the light will not cause any damage.

2. If a focal point occurs, it must be situated next to the water drop. If the light is bundled under the water drop, the water will protect the leaf. Due to the reason that water has got a relatively high heat capacity, it is able to cool the leaf sufficiently.

These conditions again depend on special determinants.

• As you can see in the picture, the drop has to be located a little bit remote from the leaf to be able to focus the rays of light in a focal point, instead of solely producing a focal area. For this purpose the leaf must have fine hairs to keep the drop at a distance.

• These hairs are very short. Therefore the drops must have a great refractive power to get the focal point directly upon the leaf and not behind the leaf. So the drops must be shaped specifically. Only in case of a strong curvature their refractive power is high enough. Thus only small drops produce focal points on the leaves because they are curved more strongly.



• In order that the focal point appears next to the water drop on the leaf, the sun must not shine vertically upon the leaf. Its rays of light must hit the drop laterally.

• Apart from that, the drop will probably have evaporated before it could damage the leaf.

Thus a water drop can become dangerous for a leaf. But the wide variety of conditions make it quite unlikely.
So if your plants look „thirsty“ in the summer heat, you do not have to hesitate to water them.