A drinking glass or a water-filled vase can act like a lens to focus light rays into curving projected lines or spots of light. This field of optics is called “caustics.” The word connotes the sense of burning, reminiscent of the way that a light from a lens can burn.
In this arrangement, set up in the morning sunlight, note the difference in the shapes of the caustic projections. Essentially, the objects are imperfect lenses. Also, check out the rich assortment of internal highlights in the glass, and the cast shadows of both forms. The cast shadow of the surface of water itself is visible as the dark shape within the cast shadow at lower left.
I’ve mentioned the digital imaging pioneer Henrik Jensen before. He has produced some striking images of caustic effects, like this cognac glass…
…and this one of a transparent ball. Artists, scientists, and mathematicians in the field of digital imaging have been breaking new ground in understanding this realm of optics, and their work inspires all of us traditional painters.
The hard thing to capture with any digital simulation is the quality found in Nature of infinite surface variation and complex interactivity of light, which causes unexpected nuances like that array of internal highlights.
Caustics are also at play when sunlight is refracted by a gently undulating water surface. The waves act like lenses. They focus a network of dancing lines on objects below the surface—like this Dunkleosteus from Dinotopia: The World Beneath.
Note the chromatic effects on the borders of the caustics above. Keep in mind that underwater caustic effects don’t occur much deeper than 20 or 30 feet. It would look wrong to include them in a deep-sea picture.
Dr. Jensen’s website, with explanations, link.
A 3D digital scene before and after caustics, link.