|Karl Ferdinand Braun electromagnetism
A radio antenna broadcasts in all directions. Karl Ferdinand Braun showed that three vertical antennas placed at the points of an equilateral triangle and operated so that the signal from two is delayed by a quarter of a cycle relative to the third have a combined directional broadcast perpendicular to the two that operate in phase and away from the third.
Radio transmitters use phased arrays to direct and strengthen broadcasts toward a target city, and to move the signal up to reflect from the sky at night and down to reflect from the ground during the day. Phased array radar systems scan more quickly than traditional rotating radar dishes and can serve multiple simultaneous uses. Radio-frequency identification systems use phased array antennas to extend their range. Optical phased arrays are used for holography.
Rowers in a crew must pull with exactly the same stroke or risk skewing the sculling, but a photon slipping through more than one slit in a card interferes with itself. Phased arrays and holograms teach us interference is our friend.