Refraction Caustics. Image by Les Cowley June '09.

The shifting skeins of bright white and spectral coloured lines are 'caustics'.

These ones were seen looking down from above the surface onto the sandy and rock heaped seabed of a crystal clear Mediterranean. The water depth was 0.4 to 1m.

The sea surface has ever changing concave and convex patches. Entering sunlight has its direction changed, is refracted. Many of the refracted rays cluster and cross to form curved sheets of light - caustic sheets. Where they intercept the sea bed we see a bright line.  The same mechanism makes the reflection caustics seen on boat hulls and harbour walls. More about caustics here.

Water refraction is wavelength dependent and the patches of chromatic colour are mainly from a second refraction as light from the sea bed leaves the wavy water surface towards the eye. Particular wave slopes give large colour separations. The colours are best seen live and then they are momentary and unpredictable flashes of jewel like colour.


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