Chaotic reflections within a glittery eye.

The structure is simple: four shiny spheres stacked like cannonballs to form a pyramid with a triangular base. The view between the spheres, however, is spectacularly complex.

The three spheres of one of the pyramid's faces frame an intricate pattern of multiple reflections. Described as a fractal, that pattern displays the complexity of the paths taken by light traveling through the structure.

For example, a light ray entering the inner chamber reflects from sphere to sphere in such a way that, for certain incoming paths, it's impossible to predict from which face the ray will emerge. David Sweet, Edward Ott, and James A. Yorke of the University of Maryland in College Park report this finding in the May 27 NATURE.

Their laboratory model could prove useful for characterizing a physical phenomenon known as chaotic scattering, the researchers say.

To map exit points, Sweet and his coworkers place the four-ball pyramid on a translucent white sheet illuminated from below and put colored poster board outside two of the three exposed faces. A camera in the dark records light emerging from the uncovered opening on the remaining exposed face.

The resulting image registers a pattern of blue, red, white, and black that depends on the light's path. A blue patch within the image, for example, would represent the paths of light rays that reflected off the blue board at some point.

Conversely, when researchers aim a laser into the pyramid at a blue patch in the image, the beam produces a bright spot on the blue board, indicating where the light emerges. The image also contains boundary regions where the four colors inextricably intertwine. A laser beam directed at these areas appears through all four openings because it hits all the colors. Computer simulations reveal that an infinitely narrow laser beam's path would be unpredictable.