question: Why does sound and 4G waves are not reflected by mirrors ?
comment: Sound absolutely does get reflected by mirrors, and any other flat surface. That's how you get an echo.
passages: Section::::Reception.
   Because the principles are the same, this concept is perhaps easiest to understand when considering an optical fiber. Imperfections in the glass create mirrors that reflect the light back along the fiber.
   BULLET::::- Monolithic suspensions: The mirrors are suspended as pendulums. While steel wires are used for secondary mirrors, GEO’s main mirrors are hanging from so-called ‘monolithic’ suspensions. This means that the wires are made from the same material as the mirror: fused silica. The reason is that fused silica has less mechanical losses, and losses lead to noise.
   In the theory of exterior noise mitigation, reflective surface size mildly detracts from the concept of a noise barrier by reflecting some of the sound into the opposite direction. Sound reflection can affect the acoustic space.
  
  Section::::Other types of reflection.:Seismic reflection.
   Section::::Issues and improvements.
  
  A challenge would derive from multipath effects, which create errors in mapping the delay of a reflection to the distance from the target. Sometimes the transmitted signal is reflected from walls and any home appliances. A major issue that is currently being fixed is the measuring time of flight is difficult since RF signals travel very fast. The MIT team is currently trying to fix this issue by using a technique named FMCW, which stands for frequency modulated carrier wave.
   Section::::Challenges and limitations.:Fresnel zone.
  
  Even if there is a direct line of sight between the transmitting and receiving antenna, reflections from objects near the path of the signal can decrease apparent signal power through phase cancellations. Whether and how much signal is lost from a reflection is determined by the location of the object in the Fresnel zone of the antennas.
  
  Section::::Two-way satellite-only communication.
   Section::::History.:Fresnel and the phase shift.
   where is the wave vector for the transmitted wave (we assume isotropic media, but the transmitted wave is not "yet" assumed to be evanescent).
   A mirror provides the most common model for specular light reflection, and typically consists of a glass sheet with a metallic coating where the significant reflection occurs. Reflection is enhanced in metals by suppression of wave propagation beyond their skin depths. Reflection also occurs at the surface of transparent media, such as water or glass.
   Two or more mirrors aligned exactly parallel and facing each other can give an infinite regress of reflections, called an infinity mirror effect. Some devices use this to generate multiple reflections:
  
  BULLET::::- Fabry–Pérot interferometer
  
  BULLET::::- Laser (which contains an optical cavity)
  
  BULLET::::- 3D Kaleidoscope to concentrate light
  
  BULLET::::- momentum-enhanced solar sail
  
  Section::::Applications.:Technology.:Military applications.
   MirrorLink currently works with Symbian phones (only Nokia Belle phones, not S60v5 phones from many manufacturers), Samsung Galaxy series (on Android Lollipop (5.0)), and Sony Xperia Z series Android phones. Sony audio has launched two audio head units in 2012 Q2, which are MirrorLink compliant. Phone maker Motorola and audio head unit maker Alpine are also members of the group supporting MirrorLink. Alpine will offer MirrorLink based aftermarket systems in the US in 2013.
   BULLET::::- Cold mirrors are dielectric mirrors that reflect the entire visible light spectrum, while efficiently transmitting infrared wavelengths. These are the converse of hot mirrors.
  
  BULLET::::- Corner reflectors use three flat mirrors to reflect light back towards its source, they may also be implemented with prisms that reflect using total internal reflection that have no mirror surfaces. They are used for emergency location, and traffic reflectors.
   Non-electromagnetic waves can also exhibit specular reflection, as in acoustic mirrors which reflect sound, and atomic mirrors, which reflect neutral atoms. For the efficient reflection of atoms from a solid-state mirror, very cold atoms and/or grazing incidence are used in order to provide significant quantum reflection; ridged mirrors are used to enhance the specular reflection of atoms. Neutron reflectometry uses specular reflection to study material surfaces and thin film interfaces in an analogous fashion to x-ray reflectivity.
  
  Section::::See also.
  
  BULLET::::- Fresnel equations, which describe the behavior of light when moving between media of differing refractive index
  
  BULLET::::- Geometric optics
   As example for this model it is assume: a rectilinear free space with two walls, one upper and the other lower, from which two vertical bases are positioned at their ends, these are the transmitting and receiving antennas that it’s locate in such a way that their heights don’t surpass the limits of the top wall; Achieved this the structure acts as free space for its functioning similar to that of a dielectric canyon of signals propagation, since the rays transmitted from the transmitting antenna will collide each side of the upper and lower walls infinity of times (for this example up to 3 reflections) until reaching the receiving antenna. During the course of the rays for each reflection they suffer, part of the energy of the signal is dissipated in each reflection, normally after the third reflection of said ray its resulting component which is a retro-reflected ray is insignificant with a negligible energy.
   BULLET::::- Flying relativistic mirrors use a shockwave (wake wave) in a plasma to produce not only specular but also coherent reflection of high-energy radiation with very short wavelengths, and are used in some types of X-ray lasers. The wake wave is produced by a very intense laser-pulse focused into the low-density plasma, which creates a wall of charged particles and free electrons moving at an extremely high velocity. Based on the Theory of Special Relativity, because of the wake wave's speed, it can reflect radiation with wavelengths much shorter than visible or UV light.
  
  Section::::Bibliography.
   Section::::Applications.:Technology.:Seasonal lighting.
  
  Due to its location in a steep-sided valley, the Italian town of Viganella gets no direct sunlight for seven weeks each winter. In 2006 a €100,000 computer-controlled mirror, 8×5 m, was installed to reflect sunlight into the town's piazza. In early 2007 the similarly situated village of Bondo, Switzerland, was considering applying this solution as well. In 2013, mirrors were installed to reflect sunlight into the town square in the Norwegian town of Rjukan. Mirrors can be used to produce enhanced lighting effects in greenhouses or conservatories.
  
  Section::::Applications.:Architecture.
   The authors of the paper found that a dielectric (plasma) shell may either decrease or increase the echo area of the object. If either one of the two reflections is considerably greater, then the weaker reflection will not contribute much to the overall effect. The authors also stated that the EM signal that penetrates the plasma shell and reflects off the object's surface will drop in intensity while traveling through plasma, as was explained in the prior section.
   It has been suggested that the high reflectivity of low-E windows can contribute to a concentration of solar radiation which can potentially cause damage to their surroundings; damage to the sidings of homes and to automobiles has been reported not only in news stories, but may cause legal issues as well.
  
  Low-e windows may also block radio frequency signals. Buildings without distributed antenna systems may then suffer degraded cell phone reception.
  
  Section::::Reflective thermal insulation.
   But, if the reflection is total, the spatial penetration of the fields into medium 2 must be limited somehow, or else the total extent and hence the total energy of those fields would continue to increase, draining power from medium 1. Total reflection of a continuing wavetrain permits some energy to be stored in medium 2, but does not permit a "continuing" transfer of power from medium 1 to medium 2.
   In acoustics, reflection causes echoes and is used in sonar. In geology, it is important in the study of seismic waves. Reflection is observed with surface waves in bodies of water. Reflection is observed with many types of electromagnetic wave, besides visible light. Reflection of VHF and higher frequencies is important for radio transmission and for radar. Even hard X-rays and gamma rays can be reflected at shallow angles with special "grazing" mirrors.
  
  Section::::Reflection of light.
   Antenna polarization for EME stations must consider that reflection from a smooth surface preserves linear polarization but reverses the sense of circular polarizations.
   Thus, using mostly qualitative reasoning, we can conclude that total internal reflection must be accompanied by a wavelike field in the "external" medium, traveling along the interface in synchronism with the incident and reflected waves, but with some sort of limited spatial penetration into the "external" medium; such a field may be called an "evanescent wave".
   Microscopic mirrors are a core element of many of the largest high-definition televisions and video projectors. A common technology of this type is Texas Instruments' DLP. A DLP chip is a postage stamp-sized microchip whose surface is an array of millions of microscopic mirrors. The picture is created as the individual mirrors move to either reflect light toward the projection surface (pixel on), or toward a light absorbing surface (pixel off).
   BULLET::::- Long-wave infrared: 8 to 12, or 7 to 14 µm (this is the atmospheric window covered by HgCdTe and microbolometers).
  
  BULLET::::- Very-long wave infrared (VLWIR) (12 to about 30 µm, covered by doped silicon).
   Section::::Mechanism.
  
presuppositions: Sound does not reflect off mirrors. Sound doesn't get reflected by mirrors.
corrections: Sound reflects off mirrors and any flat surface. Sound does get reflected by mirrors.
raw_presuppositions: Sound does not reflect off mirrors. Sound doesn't get reflected by mirrors.
raw_corrections: Sound reflects off mirrors and any flat surface. Sound does get reflected by mirrors.