Waves on water surface

Waves on water surface are caused mainly by gravity (longer waves) and surface tension (shorter waves). If the original flat shape of the water surface is disturbed, both gravity and surface tension try to bring it back to the flat shape, which invokes surface oscillations and consequently wave emergence. Waves on the surface of water are governed by a linear wave equation if they are not too steep. Therefore the superposition principle is valid for them, as one can see in the pictures: waves coming from different drops falling to the water do not influence each other but they penetrate one another and continue on their trip undisturbed. The photographs were taken near puddles and a pond shortly after rain when large drops were falling down from the surrounding trees.