Backscatter is the reflection of waves, particles, or signals back to the direction they came from. The term is used in several fields of physics, as well as in photography, telecommunication, and e-mail. A wave is a disturbance that propagates through space or spacetime, transferring energy and momentum and sometimes angular momentum. ... This article needs additional references or sources for verification. ... Photography [fәtɑgrәfi:],[foʊtɑgrәfi:] is the process of recording pictures by means of capturing light on a light-sensitive medium, such as a film or electronic sensor. ... Copy of the original phone of Alexander Graham Bell at the Musée des Arts et Métiers in Paris Telecommunication is the transmission of signals over a distance for the purpose of communication. ... Wikipedia does not yet have an article with this exact name. ...

Backscatter of waves in physical space

Backscattering occurs in quite different physical situations. The incoming waves or particles can be deflected from their original direction by quite different mechanisms:

• Rayleigh scattering of electromagnetic waves from small particles, diffuse reflection from large particles, or Mie scattering in the intermediate case, causing alpenglow and gegenschein, and showing up in weather radar;
• inelastic collisions between electromagnetic waves and the transmitting medium (Brillouin scattering and Raman scattering, important in fiber optics, see below;
• inelastic collisions between accelerated ions and a sample (Rutherford backscattering)
• Bragg diffraction from crystals, used in inelastic scattering experiments (neutron backscattering, X-ray backscattering spectroscopy);
• Compton scattering, used in Backscatter X-ray imaging.

Sometimes, the scattering is more or less isotropic, i. e. the incoming particles are scattered randomly in various directions, with no particular preference for backward scattering. In theses cases, the term "backscattering" just designates the detector location chosen for some practical reasons: Rayleigh scattering causing the blue hue of the sky and the reddening at sunset Rayleigh scattering (named after Lord Rayleigh) is the scattering of light, or other electromagnetic radiation, by particles much smaller than the wavelength of the light. ... The Mie theory also called Lorenz-Mie theory is a complete mathematical-physical theory of the scattering of electromagnetic radiation by spherical particles, developed by Gustav Mie in 1908. ... Alpenglow at sunset (Kehlstein, Höher Göll, Hohes Brett in the German Alps). ... Gegenschein (German for counterglow) is a faint brightening of the night sky in the region of the ecliptic directly opposite the Sun. ... Weather radar in Norman, Oklahoma with rainshaft (Source: NOAA) Environment Canada King City (CWKR) weather radar station. ... Brillouin scattering occurs when light in a medium (such as water or a crystal) interacts with density variations and changes its path. ... Raman scattering or the Raman effect is the inelastic scattering of a photon. ... Rutherford backscattering (or RBS, for Rutherford Backscattering Spectrometry) is an analytical technique in materials science. ... In physics, Braggs law is the result of experiments into the diffraction of X-rays or neutrons off crystal surfaces at certain angles, derived by physicists Sir W.H. Bragg and his son Sir W.L. Bragg in 1912, and first presented on 1912-11-11 to the Cambridge... Neutron backscattering is one of several inelastic neutron scattering techniques. ... In physics, Compton scattering or the Compton effect, is the decrease in energy (increase in wavelength) of an X-ray or gamma ray photon, when it interacts with matter. ... In contrast to the traditional X-ray machine, which detects hard and soft materials by the variation in transmission through the target, backscatter X-ray is a newer imaging system which detects the radiation which comes back from the target. ...

• In X-ray imaging, backscattering means just the opposite of transmission imaging;
• in optical fibers, light can only propagate forward or backward. Forward Brillouin or Raman scattering would violate momentum conservation, so inelastic scattering in optical fibers cannot be anything else but backscattering;
• in inelastic neutron or X-ray spectroscopy, backscattering geometry is chosen because it optimizes the energy resolution.

In other cases, the scattering intensity is enhanced in backward direction. This can have different reasons:

• In alpenglow, red light prevails because the blue part of the spectrum is depleted by Rayleigh scattering;
• in gegenschein, constructive interference might play a role (this needs verification);
• in multiple scattering from suspensions like milk, the enhancement of backscattering is connected with weak localization.

Alpenglow at sunset (Kehlstein, Höher Göll, Hohes Brett in the German Alps). ... Gegenschein (German for counterglow) is a faint brightening of the night sky in the region of the ecliptic directly opposite the Sun. ... It has been suggested that this article or section be merged into Mixture. ... Weak localization is a physical effect, which occurs in disordered electronic systems at very low temperatures. ...

Radar, especially weather radar

Backscattering is the principle behind radar systems.

In weather radar, backscattering is proportional to the 6^th power of the diameter of the target multiplied by its inherent reflective properties. Water is almost 4 times more reflective than ice but droplets are much smaller than snow flakes or hail stones. So the backscattering is dependent on a mix of these two factors. The strongest backscatter comes from hail and large graupel (solid ice) due to their sizes. Another strong return is from melting snow or wet sleet, as they combine size and water reflectivity. They often show up as much higher rates of precipitation than actually occurring in what is called a brightband. Rain is a moderate backscatter, being stronger with large drops (such as from a thunderstorm) and much weaker with small droplets (such as mist or drizzle). Snow has rather weak backscatter. Weather radar in Norman, Oklahoma with rainshaft (Source: NOAA) Environment Canada King City (CWKR) weather radar station. ... This does not cite any references or sources. ... Graupel can be any of the following types of solid-ice precipitation: hail - large chunks of ice such as from a strong or severe thunderstorm sleet - small pellets of raindrops that have frozen in mid-air, in winter or a thunderstorm snow pellets - when freezing fog forms 2-5mm balls... For other uses, see Solid (disambiguation). ... Snowflakes by Wilson Bentley, 1902 Ice is the name given to any one of the 14 known solid phases of water. ... Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. ... Sleet is a term used in a variety of ways to describe precipitation intermediate between rain and snow but distinct from hail. ... Look up Rate in Wiktionary, the free dictionary. ... Weather radar in Norman, Oklahoma with rainshaft (Source: NOAA) Environment Canada King City (CWKR) weather radar station. ... Rain is a type of precipitation, a product of the condensation of atmospheric water vapor that is deposited on the earths surface. ... A thunderstorm, also called an electrical storm or lightning storm, is a form of weather characterized by the presence of lightning and its attendant thunder produced from a cumulonimbus cloud. ... See also drop (telecommunication). ... Dramatic morning mist Mist is a phenomenon of a liquid in small droplets floating through air. ... Drizzle is fairly steady, light precipitation. ... Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. ...

See also

• Scattering

Scattering is a general physical process whereby some forms of radiation, such as light, sound or moving particles, for example, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which it passes. ...

Backscatter in waveguides

The backscattering method is also employed in fiber optics applications to detect optical faults. Light propagating through a fiber optic cable gradually attenuates due to Rayleigh scattering. Faults are thus detected by monitoring the variation of part of the Rayleigh backscattered light. Since the backscattered light attenuates exponentially as it travels along the optical fiber cable, the attenuation characteristic is represented in a logarithmic scale graph. If the slope of the graph is steep, then power loss is high. If the slope is gentle, then optical fiber has a satisfactory loss characteristic. Fiber Optic strands An optical fiber in American English or fibre in British English is a transparent thin fiber for transmitting light. ... Fiber Optic strands An optical fiber in American English or fibre in British English is a transparent thin fiber for transmitting light. ... Attenuation is the decrease in intensity of electromagnetic radiation due to absorption or scattering of photons. ... The exponential function is one of the most important functions in mathematics. ... A logarithmic scale is a scale of measurement that uses the logarithm of a physical quantity instead of the quantity itself. ... Look up Slope in Wiktionary, the free dictionary. ...

The loss measurement by the backscattering method allows measurement of a fiber optic cable at one end without cutting the optical fiber hence it can be conveniently used for the construction and maintenance of optical fibers. Fiber Optic strands An optical fiber in American English or fibre in British English is a transparent thin fiber for transmitting light. ...

Backscatter in photography

The term backscatter in photography refers to light from a flash or strobe reflecting back from particles in the lens' field of view causing specks of light to appear in the photo. Photographic backscatter can result from snowflakes, rain or mist, or airborne dust. Backscatter is particularly a problem in underwater photography, where particulate matter can be very dense and include plankton which would otherwise be near transparent. Look up flash in Wiktionary, the free dictionary. ... U-shaped Xenon Flash Lamp A xenon flash lamp is a gas discharge lamp designed to produce extremely intense, incoherent, full-spectrum white light for very short durations. ... Pink Anemonefish hiding in tentacles Underwater photography is the process of taking photographs while underwater. ...

Backscatter can be reduced by offsetting the direction of the photo strobe as far from the angle of the lens as possible. This is normally done by placing the light source high and to one side by placing the strobe on an extendable strobe arm. By having the light come from the side, the reflected light is primarily in the direction of the strobe instead of the camera lens. This is similar to comparing a full moon to a half moon. The full moon is when the moon is lit from almost behind the earth, creating reflection off the whole surface facing the earth. A half moon is when the moon is being lit from one side, making the reflection half the size and the light intensity much less. In photography, the side lighting makes the backscatter less pronounced.

Backscatter can often also be removed digitally after the photo is taken with photo editing software using digital filters or cloning of areas of the photo near the backscatter spots.

Backscatter from denial-of-service attacks

In computer network security, backscatter is a side-effect of a spoofed denial of service (DoS) attack. In this kind of attack, the attacker spoofs (or forges) the source address in IP packets sent to the victim. In general, the victim machine can not distinguish between the spoofed packets and legitimate packets, so the victim responds to the spoofed packets as it normally would. These response packets are known as backscatter. A denial-of-service attack (also, DoS attack) is an attack on a computer system or network that causes a loss of service to users, typically the loss of network connectivity and services by consuming the bandwidth of the victim network or overloading the computational resources of the victim system. ... The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched internetwork. ... In information technology, a packet is a formatted block of information carried by a computer network. ...

If the attacker is spoofing source addresses randomly, the backscatter response packets from the victim will be sent back to random destinations.

The term "backscatter analysis" refers to observing backscatter packets arriving at a statistically significant portion of the IP address space to determine characteristics of DoS attacks and victims. This article or section does not cite any references or sources. ...

An educational animation describing such backscatter can be found on the animations page maintained by CAIDA, the Cooperative Association for Internet Data Analysis. The Cooperative Association for Internet Data Analysis (CAIDA) is a collaborative undertaking among organizations in the commercial, government, and research sectors aimed at promoting greater cooperation in the engineering and maintenance of a robust, scalable global Internet infrastructure. ... The Cooperative Association for Internet Data Analysis (CAIDA) is a collaborative undertaking among organizations in the commercial, government, and research sectors aimed at promoting greater cooperation in the engineering and maintenance of a robust, scalable global Internet infrastructure. ...

Backscatter of email spam

The term "backscatter" is also used to describe a side-effect of email spam, viruses and worms. In this context, an alternate, more distinguishing term "outscatter" is also used, since the traffic isn't directed to the original destination, but to a third party instead. Since a 2002 Klez variant[1], a large proportion of malignant email is sent with a forged sender address, but some mail servers do not take this into account. They generate bounce messages for spam or viruses - which of course go to an innocent party. E-mail spam, also known as bulk or junk e-mail is a subset of spam that involves sending nearly identical messages to numerous recipients by e-mail. ... Klez is a computer worm that propagates via E-mail. ... A bounce message, or Delivery Status Notification (DSN) message or, simply, a bounce is an automated electronic mail message from the receivers mail system, the message tells the sender that the message could not be delivered. ...

Since these messages were not solicited by the recipients, are substantially similar to each other, and are delivered in bulk quantities, they themselves can qualify as unsolicited bulk email or spam. As such, systems that generate e-mail backscatter can end up being listed on various DNSBLs and be in violation of ISPs Terms-of-Service for being abusive. A typical spam advertisement Spam by e-mail is one type of spamming that involves sending identical or nearly identical messages to thousands (or millions) of recipients. ... E-mail spam, also known as bulk or junk e-mail is a subset of spam that involves sending nearly identical messages to numerous recipients by e-mail. ... A DNS Blacklist, or DNSBL (definition below), is a means by which an Internet site may publish a list of IP addresses that some people may want to avoid and in a format which can be easily queried by computer programs on the Internet. ... “ISP” redirects here. ...

Due to controversial aspects of its design, the qmail mailserver is more likely than most to produce such bounces. qmail is a mail transfer agent that runs on Unix. ... qmail is a mail transfer agent that runs on Unix. ...

• Postfix - backscatter page
• SpamLinks - Backscatter
• RFC 3834: Recommendations for Automatic Responses to Electronic Mail.
• Moronic Mail Autoresponders (A FAQ From Hell)
• Why are auto responders bad? (a SpamCop FAQ)
• A DNSBL of Backscatter sources.
• Dontbouncespam.org Why you shouldn't bounce spam