Backscatter is the reflection of waves, particles, or signals back to the direction they came from. A wave is a disturbance that propagates through Space and Time, usually with transference of Energy. The term is used in several fields of physics, as well as in photography and medical ultrasonography. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. Photography (fә'tɒgrәfi or fә'tɑːgrәfi (from Greek φωτο and γραφία is the process and Art of recording pictures by means of capturing

Contents 1 Backscatter of waves in physical space 1.1 Radar, especially weather radar 1.2 See also 2 Backscatter in waveguides 3 Backscatter in photography 4 Backscatter from spam

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;
• elastic 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. Rayleigh scattering (named after Lord Rayleigh) is the elastic Scattering of Light or other electromagnetic radiation by particles much smaller Mie theory, also called Lorenz-Mie theory or Lorenz-Mie-Debye theory, is a complete analytical solution of Maxwell's equations for the Scattering Alpenglow (from German: Alpenglühen) is an Optical phenomenon. Gegenschein ( very roughy like GAY-guhn-shine German for "counter shine" is a faint brightening of the night sky in the region of the Antisolar A weather radar is a type of Radar used to locate precipitation, calculate its motion estimate its type ( Rain Brillouin scattering, named for Léon Brillouin, occurs when Light in a medium (such as Water or a Crystal) interacts with time dependent Raman scattering or the Raman effect (pronounced — is the inelastic scattering of a Photon. Rutherford backscattering spectrometry (RBS is an analytical technique used in Materials science. In Physics, Bragg's law is the result of experiments into the Diffraction of X-rays or neutrons off Crystal surfaces at certain angles Neutron backscattering is one of several Inelastic neutron scattering techniques The Compton shift formula Klein-Nishina formulaCompton used a combination of three fundamental formulas representing the various aspects of classical and modern physics combining 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

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 these cases, the term "backscattering" just designates the detector location chosen for some practical reasons:

• 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 (from German: Alpenglühen) is an Optical phenomenon. Gegenschein ( very roughy like GAY-guhn-shine German for "counter shine" is a faint brightening of the night sky in the region of the Antisolar In Chemistry, A suspension is a Heterogenous fluid containing Solid particles that are sufficiently large for Sedimentation. 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. A weather radar is a type of Radar used to locate precipitation, calculate its motion estimate its type ( Rain 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. Hail is a form of precipitation which consists of balls or irregular lumps of ice (hailstones Graupel (also called snow pellets) refers to precipitation that forms when supercooled Droplets of Water condense on a Snowflake forming A solid' object is in the States of matter characterized by resistance to Deformation and changes of Volume. Ice is a Solid phase, usually crystalline, of a Non-metalic substance that is liquid or gas at Room temperature, such as Ammonia Another strong return is from melting snow or wet sleet, as they combine size and water reflectivity. "Snowfall" redirects here For other uses see Snow (disambiguation or Snowfall (disambiguation. They often show up as much higher rates of precipitation than actually occurring in what is called a brightband. In Meteorology, precipitation (also known as one class of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric A weather radar is a type of Radar used to locate precipitation, calculate its motion estimate its type ( Rain 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). Rain is Liquid precipitation. On Earth it is the condensation of atmospheric Water vapor into drops heavy enough to fall often making it to A drop or droplet is a small volume of Liquid, bounded completely or almost completely by Free surfaces Surface tension Mist is a phenomenon of small droplets suspended in Air. It can occur as part of natural Weather or Volcanic activity and is common in cold air above Drizzle (also called mizzle) is light precipitation consisting of liquid water drops smaller than that of Rain, and generally smaller than 0 Snow has rather weak backscatter. "Snowfall" redirects here For other uses see Snow (disambiguation or Snowfall (disambiguation.

See also

• Scattering

Backscatter in waveguides

The backscattering method is also employed in fiber optics applications to detect optical faults. 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 An optical fiber (or fibre) is a Glass or Plastic fiber that carries Light along its length Light propagating through a fiber optic cable gradually attenuates due to Rayleigh scattering. An optical fiber cable is a Cable containing one or more Optical fibers The optical fiber elements are typically individually coated with plastic layers and contained Rayleigh scattering (named after Lord Rayleigh) is the elastic Scattering of Light or other electromagnetic radiation by particles much smaller 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. In Physics, attenuation (in some context also called extinction) is the gradual loss in intensity of any kind of Flux through a medium The exponential function is a function in Mathematics. The application of this function to a value x is written as exp( x) Definition and base Logarithmic scales are either defined for ratios of the underlying quantity or one has to agree to measure In mathematics the graph of a function f is the collection of all Ordered pairs ( x, f ( x) If the slope of the graph is steep, then power loss is high. Slope is used to describe the steepness incline gradient or grade of a straight line. If the slope is gentle, then optical fiber has a satisfactory loss characteristic.

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. An optical fiber cable is a Cable containing one or more Optical fibers The optical fiber elements are typically individually coated with plastic layers and contained

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. A flash is a device used in Photography that produces an instantaneous flash of artificial Light 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. Underwater photography is the process of taking Photographs while under water

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 spam

Main article: backscatter (e-mail)

The term "backscatter" is also used to describe a side-effect of email spam, viruses and worms where the spam email triggers a bounced message off of a legitimate server. Backscatter (also known as outscatter, misdirected bounces, blowback or collateral spam) is a side-effect of E-mail spam, viruses E-mail spam, also known as "bulk e-mail" or "junk e-mail" is a subset of spam that involves nearly identical messages sent to numerous recipients by This server sends the reply back to the sender identified in the sender field. However, the sender field is usually forged, and innocent bystanders get backscatter email.

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