Worked example - how wide is a laser beam on the Moon? Take a well-collimated beam with divergence 1 milliradian aimed at the Moon, 384,400 km away. Step 1: spot diameter ~ divergence x distance = 10^-3 x 3.844 x 10^8 m = 3.84 x 10^5 m = 384 km. Step 2: compare - the Apollo retroreflector arrays are under a metre wide, so only a minuscule fraction returns; yet observatories detect the returning photons and time them. Step 3: round-trip time ~2.56 s measured to picoseconds gives the Earth-Moon distance to millimetres - precision from directionality.