The product of a wave's frequency times its wavelength is equal to its speed of travel. Rearranging that relationship, it's easy to find that the wavelength of the microwave is the speed of light divided by the frequency, or 0.300 meters. An airplane flies past the radar unit at a distance of 1 kilometer from the unit. That time is equal to the distance the microwaves have to travel divided by their speed. Metals reflect microwaves extremely well, but even if an airplane were made entirely of nonconducting materials, it would still reflect some microwaves.
Microwaves slow down as they travel through materials.
As a microwave slows down upon entry into a nonconducting material, part of the wave reflects from the interface. A tiny metal airplane flies past the radar unit. This remote controlled airplane is only 1 centimeters long. The 1.000 GHz radar unit has great difficulty detecting any reflection from this airplane. However a much higher frequency radar unit operating at 100.0 GHz observes the plane easily.