Editing Radar (section)

===Reflection===
{{main|Reflection (physics)}}
[[File:weather radar.jpg|thumb|Brightness can indicate reflectivity as in this 1960 [[weather radar]] image (of [[Hurricane Abby (1960)|Hurricane Abby]]). The radar's frequency, pulse form, polarization, signal processing, and antenna determine what it can observe.]]

If [[Electromagnetic radiation|electromagnetic waves]] travelling through one material meet another material, having a different [[dielectric constant]] or [[diamagnetism|diamagnetic constant]] from the first,
the waves will reflect or scatter from the boundary between the materials. This means that a solid object in [[Earth's atmosphere|air]] or in a [[vacuum]], or a significant change in atomic density between the object and what is surrounding it, will usually scatter radar (radio) waves from its surface. This is particularly true for [[electrical conduction|electrically conductive]] materials such as metal and carbon fibre, making radar well-suited to the detection of aircraft and ships. [[Radar absorbing material]], containing [[Electrical resistance|resistive]] and sometimes [[magnetism|magnetic]] substances, is used on military vehicles to [[stealth technology|reduce radar reflection]]. This is the radio equivalent of painting something a dark colour so that it cannot be seen by the eye at night.

Radar waves scatter in a variety of ways depending on the size (wavelength) of the radio wave and the shape of the target. If the wavelength is much shorter than the target's size, the wave will bounce off in a way similar to the way light is reflected by a [[mirror]]. If the wavelength is much longer than the size of the target, the target may not be visible because of poor reflection. Low-frequency radar technology is dependent on resonances for detection, but not identification, of targets. This is described by [[Rayleigh scattering]], an effect that creates Earth's blue sky and red sunsets. When the two length scales are comparable, there may be [[resonance]]s. Early radars used very long wavelengths that were larger than the targets and thus received a vague signal, whereas many modern systems use shorter wavelengths (a few centimetres or less) that can image objects as small as a loaf of bread.

Short radio waves reflect from curves and corners in a way similar to glint from a rounded piece of glass. The most reflective targets for short wavelengths have 90° angles between the [[reflection (physics)|reflective surfaces]]. A [[corner reflector]] consists of three flat surfaces meeting like the inside corner of a cube. The structure will reflect waves entering its opening directly back to the source. They are commonly used as radar reflectors to make otherwise difficult-to-detect objects easier to detect. Corner reflectors on boats, for example, make them more detectable to avoid collision or during a rescue. For similar reasons, objects intended to avoid detection will not have inside corners or surfaces and edges perpendicular to likely detection directions, which leads to "odd" looking [[stealth aircraft]]. These precautions do not totally eliminate reflection because of [[diffraction]], especially at longer wavelengths. Half wavelength long wires or strips of conducting material, such as [[Chaff (countermeasure)|chaff]], are very reflective but do not direct the scattered energy back toward the source. The extent to which an object reflects or scatters radio waves is called its [[radar cross-section]].