Physics Q & A - The stealth fighter
Cheung Kai-chung (Translation by Janny Leung)    

Radar detection of an aeroplane.
Fig. 1  Radar detection of an aeroplane.

F117 stealth fighter
Fig. 2  The stealth fighter F117 is structured with many faceted flat surfaces.

How can a stealth fighter be invisible?

A stealth fighter is not really invisible; it is just that we cannot detect its presence with radar. Why does it have such a characteristic? Let us explain it with the operation of radar. The so-called "radar detection" refers to a situation in which electromagnetic waves, being emitted by an observer, is reflected to its original direction by the metal surface of an aircraft and then received by the observer (Fig. 1). The observer can rely on the time difference from emission to reception to locate the enemy fighter.

Traditional aircrafts usually have a curved surface in order to minimize air resistance in flight. Therefore, electromagnetic waves impinged in any direction will be partly reflected to their original direction, leading to a large signal on the radar detector. Some modern stealth fighters are structured with a large number of faceted flat surfaces (Fig. 2), and thus electromagnetic waves will not be reflected effectively to their original direction. Apart from that, the materials used for the outer surface is specially selected so that it absorbs the energy of radar signals effectively. These materials are typically carbon, carbon fibre composites, or magnetic ferrite-based substance. As a result, to radar, a stealth fighter is as low-observable as a little bird!