CHARLES H . TOWNES ON JULY 21 , 1969 , astronauts Neil

ON JULY 21, 1969, astronauts Neil Armstrong and Edwin “Buzz” Aldrin set up an array of small reflectors on the moon, facing them toward Earth. At the same time, two teams of astrophysicists, one at the University of California’s Lick Observatory and the other at the University of Texas’s McDonald Observatory, were preparing small instruments on two big telescopes. Ten days later, the Lick team pointed its telescope at the precise location of the reflectors on the moon and sent a small pulse of power into the hardware they had added to it. A few days after that, the McDonald team went through the same steps. In the heart of each telescope, a narrow beam of extraordinarily pure red light emerged from a synthetic ruby crystal, pierced the sky, and entered the near vacuum of space. The two rays were still only a thousand yards wide after traveling 240,000 miles to illuminate the moon-based reflectors. Slightly more than a second after each light beam hit its target, the crews in California and Texas detected its faint reflection. The brief time interval between launch and detection of these light pulses permitted calculation of the distance to the moon to within an inch—a measurement of unprecedented precision. The ruby crystal for each light source was the heart of a laser (an acronym for light amplification by stimulated emission of radiation), which is a device first demonstrated in 1960, just nine years earlier. A laser beam reflected from the moon to measure its distance is only one dramatic illustration of the spectacular quality of laser light. There are many other more mundane, practical uses such as in surveying land and grading roads, as well as myriad everyday uses— THE LIGHT THAT SHIN