Caustic comments & reflections

Reflection Caustics & Skypools. Image by Les Cowley June '09.

The dazzling sharp white patterns on the fishing boat hull are 'caustics' generated by sunlight reflected from the undulating sea surface.

Why are there well defined lines rather than fuzzy patches of light? Sunlight is reflected with deviations that pass through minimum or maximum angles. Many rays cluster at these turning points and thus give a bright line. Rainbows with their angles of minimum deviation are also caustics. The diagram below for 2D waves shows how caustics result from regions of sea surface where the curvature undergoes a change from convex to concave. The direction of the reflected rays then passes through a minimum or maximum angle.

Waves and undulations refract sunlight that enters the water. These form rippling and ever shifting caustic patterns on shallow sea or lake beds and in swimming pools.

At lower right in the image there are faint 'skypools'. Concave patches reflect the sky to towards the eye from their centres but reflect the harbour surroundings from their steeper edges. The result is oval patches of blue surrounded by the darker shore and boat colours.