Laser Ranging Retro-reflector Experiment
It consists of series of mirror with the special property of always reflecting an incoming light beam back in the direction it came from. Laser beams aimed through large telescopes on Earth can illuminate these reflectors. The reflected laser beam is observed with the telescope, providing a measurement of the round-trip distance between Earth and the Moon. Laser beams are used because they remain tightly focused for large distances
The Laser Ranging Retro-reflector experiment has produced many important measurements. These include knowledge of the Moon's orbit, the rate at which the Moon is receding from Earth and of variations in the rotation of the Moon
The crewmen reentered the lunar module after about 2 hours 14 minutes of exploration. The astronauts began preparations to reenter the LM The ascent from the lunar surface began at 124:22 GET, 21 hours and 36 minutes after the lunar landing. The command module entered the earth's atmosphere and landed in the Pacific Ocean.