Petal Caustics

Plumarias distort the elastic water surface to generate bright caustic curves beneath. Imaged by Manoj Saranathan of Rochester, Minnesota. ©Manoj Saranathan, shown with permission.

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The flowers are supported by surface tension forces just as some small insects 'walk' on water. At points of contact the surface forms a meniscus. It stretches into strong curves convex towards the air. The tendency for the surface to return to a minimum energy and area - a planar shape in this case - provides the force to support the flower.

The curved meniscus refracts the incoming sunlight. There are regions - turning points - where many refracted rays bundle together with similar deflections and these result in bright lines on the bowl surface called caustics.

A meniscus is not always necessary. Waves on water also deflect sunlight resulting in the dancing caustic patterns at the base of a swimming pool or on a shallow sea bed.

Rainbows are another caustic phenomenon.