Water Strider Caustics

Imaged by Andrew Kirk of California. The water strider (family Gerridae ) 'walks' on the water surface producing chromatic edged dark patches - not shadows - on the pond bed beneath.    ©Andrew Kirk


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A water surface acts as though it has a tightly stretched elastic skin.   This results from the slight but cumulative attractive forces between water molecules .   Without the ‘skin’ small creatures  would no doubt have evolved in a way that did not involve them walking on water surfaces.   

Small creatures are supported because their legs are not wetted.  They do not penetrate the water surface.   Instead they depress it into a curved meniscus.   The surface tension forces combine to produce a net upthrust that counters the insect’s weight.     With care and guile we can similarly support small metal objects on water. Imagine also walking across a trampoline.

The dipped surface refracts light to form a ‘no-light’ zone around the point where the tip of the insect’s leg would be shadowed.    The no light zone is not a shadow.   It instead marks a region bounded by a bright caustic sheet.

At left is a cross section through a meniscus.    Rays crossing the meniscus closer and closer to its centre are increasingly deflected.     Lower down in the water the deflected rays cross and cluster at the boundary between regions where pairs of rays cross and where there are no rays.   This is a caustic surface and it is intensely bright.      The surface crosses the pond bed to form a bright rim around the no-light zone.

Two processes colour the caustics.   Each colour forms a slightly different caustic sheet.    Light emerging from the water is further split into spectral colours.

Below: Strider Acrobatics - The creature pushes with its back legs producing strong symmetric waves.   Its front legs appear to be raised above the surface because no menisci are present.