Microwave zone plates.

Microwave (and acoustical) zone plates. A 3 cm. microwave transmitter is placed 102 cm. from a microwave zone plate (positive or negative ) constructed of rings of thin metal. When the transmitter is turned on, it sends microwaves through the zone plate which focuses the microwaves at approximately 110 cm from the plate. The microwave receiver emits a tone when it detects microwaves. The receiver is moved around until it finds the focus: the focus will be very loud in contrast to the nonfocused areas. (Both negative and positive zone plates focus the microwaves).