​Stealth is the corner stone of 5th gen air fighters.  Stealth tech also called Low Observable (lo) tech is a collection of techniques that makes objects less visible to radar, infrared, sonar and other detection methods. Stealth d doesn't mean it is invisible stealth is used to delay the detection as far as possible. Stealth is applicable to aircraft's  ships, submarines, missiles, satellites & even personals too. Here we only talk about stealth aircraft's.
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Before continuing we are like to give a small description about how a radar works makes further reading easier. The basic idea is the radar antenna to send out a burst of radio energy, which is then reflected back by any object it happens to encounter. The radar antenna measures the time it takes for the reflection to arrive, and with that information can tell how far away the object is.The metal body of an airplane is very good at reflecting radar signals, and this makes it easy to find and track airplanes with radar equipment.
 
One of the main focuses while developing a 5th gen fighter is reducing the Radar Cross Section (RCS) thus improving the stealth capabilities of the fighter. RCS is a measure of how detectable an object is with radar. A larger RCS indicates that an object is more easily detected