When walking along a beach in Mallorca in late April, I noticed some unusual patterns being cast on the seabed by small floating patches of sand. What causes the patches to form and why do their shadows on the seabed have bright fringes around the edges and around the gaps in the middle?
The effect is very specific and perhaps more likely to be seen on the west coast of the British Isles than in Mallorca. The sand will be fine, dry and windblown 鈥� the sort that forms sand dunes. Blown by the wind, the sand will fly a few tens of centimetres above the beach, and some will be trapped on the surface of any water in its path, including a calm sea.
Dry sand takes a few minutes to wet through, and initially there is a 鈥渃ontact angle鈥� between the air/water interface and the surface of the sand grain. The result is that the grain will be kept on the surface rather as if it were on a trampoline, with the water surface curving down around each grain. The surface tension associated with this also pulls in adjacent floating grains to form rafts of grains. Around these 鈥渟and islands鈥� the meniscus of the water effectively forms a convex cylindrical lens.
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鈥淪and grains will be kept on the surface rather as if they were on a trampoline, with the water surface curving around them鈥�
If the depth of the pool below is close to the focal length of this lens then the sun鈥檚 image will be projected as a line around the shadow below, as seen by your questioner. Eventually the sand grains become thoroughly wet and sink.
David Stevenson, Newbury, Berkshire, UK
The effect in question is a result of surface tension. When the sand settles on the surface of the water it presses it down, so that the level of the water under the sand island is slightly below that of the surrounding water, making the surface curve between the two levels. This has the same effect as a convex lens, focusing sunlight on the bottom to create the bright fringe around the island鈥檚 shadow.
Another example of the same effect is seen around the surface-dwelling insects commonly called , which cast disproportionately large and blotchy shadows with a 鈥渟ilver lining鈥� around them. This is because the light that one would normally expect to land directly below the edge of the skater is refracted away from the skater by the curved water surface, effectively enlarging the shadow.
A. Anderson, Ramsgate, Kent, UK