A new kind of optics has grown up during the last 25 years. Geometrical optics has been studied for centuries—the law of reflection was known
to the ancient Greeks—and wave optics, heralded by Huygens' Treatise on Light, has been studied for more than 300 years. However, in the
mid-1970s it began to be understood that when natural processes focus light, as when sunlight is reflected from the sea at sunset, the light caustics
that are produced have a systematic behaviour previously unrecognized.