physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

, backscatter (or backscattering) is the reflection of

wave In physics, mathematics, engineering, and related fields, a wave is a propagating dynamic disturbance (change from List of types of equilibrium, equilibrium) of one or more quantities. ''Periodic waves'' oscillate repeatedly about an equilibrium ...

particle In the physical sciences, a particle (or corpuscle in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from s ...

s, or

signal A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology. In ...

s back to the direction from which they came. It is usually a

diffuse reflection Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle as in the case of specular reflection. An ''ideal'' dif ...

due to

scattering In physics, scattering is a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including particles and radiat ...

, as opposed to

specular reflection Specular reflection, or regular reflection, is the mirror-like reflection (physics), reflection of waves, such as light, from a surface. The law of reflection states that a reflected ray (optics), ray of light emerges from the reflecting surf ...

as from a

mirror A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...

, although specular backscattering can occur at normal incidence with a surface. Backscattering has important applications in

astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

photography Photography is the visual arts, art, application, and practice of creating images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is empl ...

, and

medical ultrasonography Medical ultrasound includes Medical diagnosis, diagnostic techniques (mainly medical imaging, imaging) using ultrasound, as well as therapeutic ultrasound, therapeutic applications of ultrasound. In diagnosis, it is used to create an image of ...

. The opposite effect is

forward scatter Forward scattering is the deflection of waves by small angles so that they continue to move in close to the same direction as before the scattering. It can occur with all types of waves, for instance light, ultraviolet radiation, X-rays as well as ...

, e.g. when a

translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable light scattering by particles, scattering of light. On a macroscopic scale ...

material like a cloud diffuses

sunlight Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...

, giving soft light.

Backscatter of waves in physical space

Backscattering can occur in quite different physical situations, where the incoming waves or particles are deflected from their original direction by different mechanisms: *

Diffuse reflection Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle as in the case of specular reflection. An ''ideal'' dif ...

from large particles and

Mie scattering In electromagnetism, the Mie solution to Maxwell's equations (also known as the Lorenz–Mie solution, the Lorenz–Mie–Debye solution or Mie scattering) describes the scattering of an electromagnetic plane wave by a homogeneous sphere. The sol ...

, causing alpenglow and

gegenschein Gegenschein (; ; ) or counterglow is a faintly bright spot in the night sky centered at the antisolar point. The backscatter of sunlight by interplanetary dust causes this optical phenomenon, being a zodiacal light and part of its zodiacal light ...

, and showing up in

weather radar A weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation (meteorology), precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern w ...

; *

Inelastic collision An inelastic collision, in contrast to an elastic collision, is a collision in which kinetic energy is not conserved due to the action of internal friction. In collisions of macroscopic bodies, some kinetic energy is turned into vibrational ener ...

s between electromagnetic waves and the transmitting medium ( Brillouin scattering and

Raman scattering In chemistry and physics, Raman scattering or the Raman effect () is the inelastic scattering of photons by matter, meaning that there is both an exchange of energy and a change in the light's direction. Typically this effect involves vibrationa ...

), important in fiber optics, see below; *

Elastic collision In physics, an elastic collision occurs between two physical objects in which the total kinetic energy of the two bodies remains the same. In an ideal, perfectly elastic collision, there is no net loss of kinetic energy into other forms such a ...

s between accelerated ions and a sample ( Rutherford backscattering) * Bragg diffraction from crystals, used in inelastic scattering experiments ( neutron backscattering,

X-ray backscattering spectroscopy An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

); *

Compton scattering Compton scattering (or the Compton effect) is the quantum theory of high frequency photons scattering following an interaction with a charged particle, usually an electron. Specifically, when the photon hits electrons, it releases loosely bound e ...

, used in

Backscatter X-ray Backscatter X-ray is an advanced X-ray imaging technology. Traditional X-ray machines detect hard and soft materials by the variation in x-ray intensity transmitted through the target. In contrast, backscatter X-ray detects the radiation that r ...

imaging. * ''Stimulated backscatter'', observed in non-linear optics, and described by a class of solutions to the three-wave equation. 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 inelastic neutron or X-ray spectroscopy, backscattering geometry is chosen because it optimizes the energy resolution; *in

, backscattered light is that which is reflected with a phase angle of less than 90°. 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 Rayleigh scattering ( ) is the scattering or deflection of light, or other electromagnetic radiation, by particles with a size much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scat ...

. *In

, constructive interference might play a role. * Coherent backscattering is observed in random media; for visible light most typically in suspensions like milk. Due to

weak localization Weak localization is a physical effect which occurs in disordered electronic systems at very low temperatures. The effect manifests itself as a ''positive'' correction to the resistivity of a metal or semiconductor. The name emphasizes the fact tha ...

, enhanced multiple scattering is observed in back direction. ** The Back Scattering Alignment (BSA) coordinate system is often used in

radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...

applications ** The Forward Scattering Alignment (FSA) coordinate system is primarily used in optical applications Backscattering properties of a target are wavelength dependent and can also be polarization dependent. Sensor systems using multiple wavelengths or polarizations can thus be used to infer additional information about target properties.

Radar, especially weather radar

Backscattering is the principle behind

systems. In

, backscattering is proportional to the 6th power of the diameter of the target multiplied by its inherent reflective properties, provided the wavelength is larger than the particle diameter (

). 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 Hail is a form of solid Precipitation (meteorology), precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailsto ...

and large

graupel Graupel (; ), also called soft hail or hominy snow or granular snow or snow pellets, is precipitation that forms when supercooled water droplets in air are collected and freeze on falling snowflakes, forming balls of crisp, opaque rime. Gra ...

(

solid Solid is a state of matter where molecules are closely packed and can not slide past each other. Solids resist compression, expansion, or external forces that would alter its shape, with the degree to which they are resisted dependent upon the ...

ice Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 ° C, 32 ° F, or 273.15 K. It occurs naturally on Earth, on other planets, in Oort cloud objects, and as interstellar ice. As a naturally oc ...

) due to their sizes, but non-Rayleigh (

) effects can confuse interpretation. Another strong return is from melting

snow Snow consists of individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water througho ...

or wet sleet, as they combine size and water reflectivity. They often show up as much higher

rates Rate or rates may refer to: Finance * Rate (company), an American residential mortgage company formerly known as Guaranteed Rate * Rates (tax), a type of taxation system in the United Kingdom used to fund local government * Exchange rate, rate ...

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

than actually occurring in what is called a '' brightband.''

Rain Rain is a form of precipitation where water drop (liquid), droplets that have condensation, condensed from Water vapor#In Earth's atmosphere, atmospheric water vapor fall under gravity. Rain is a major component of the water cycle and is res ...

is a moderate backscatter, being stronger with large drops (such as from a

thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustics, acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorm ...

) and much weaker with small

droplet A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the end of a tube or other surface boundary, producing a hanging drop called a pendant drop. Dro ...

s (such as

mist Mist is a phenomenon caused by small droplets of water suspended in the cold air, usually by condensation. Physically, it is an example of a Dispersion (chemistry), dispersion. It is most commonly seen where water vapor in warm, moist air meets ...

drizzle Drizzle is a light precipitation which consists of liquid water drops that are smaller than those of rain – generally smaller than in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation r ...

Snow Snow consists of individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water througho ...

has rather weak backscatter. Dual polarization weather radars measure backscatter at horizontal and vertical polarizations to infer shape information from the ratio of the vertical and horizontal signals.

In waveguides

The backscattering method is also employed in

fiber optics An optical fiber, or optical fibre, is a flexible glass or plastic fiber that can transmit light from one end to the other. Such fibers find wide usage in fiber-optic communications, where they permit transmission over longer distances and at ...

applications to detect optical faults. Light propagating through a

fiber-optic cable A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to an electrical cable but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with p ...

gradually attenuates due to

. 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 A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to an electrical cable but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with p ...

, the attenuation characteristic is represented in a

logarithmic scale A logarithmic scale (or log scale) is a method used to display numerical data that spans a broad range of values, especially when there are significant differences among the magnitudes of the numbers involved. Unlike a linear Scale (measurement) ...

graph Graph may refer to: Mathematics *Graph (discrete mathematics), a structure made of vertices and edges **Graph theory, the study of such graphs and their properties *Graph (topology), a topological space resembling a graph in the sense of discret ...

. If the

slope In mathematics, the slope or gradient of a Line (mathematics), line is a number that describes the direction (geometry), direction of the line on a plane (geometry), plane. Often denoted by the letter ''m'', slope is calculated as the ratio of t ...

of the graph is steep, then power loss is high. 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.

In photography

The term backscatter in photography refers to light from a flash,or

strobe A strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produce regular flashes of light. It is one of a number of devices that can be used as a stroboscope. The word originated from the Ancient Greek ('), meaning ...

or video lights reflecting back from particles in the lens's field of view causing specks of light to appear in the photo. This gives rise to what are sometimes referred to as ''orb artifacts''. Photographic backscatter can result from snowflakes, rain or mist, or airborne dust. Due to the size limitations of the modern compact and ultra-compact cameras, especially digital cameras, the distance between the lens and the built-in flash has decreased, thereby decreasing the angle of light reflection to the lens and increasing the likelihood of light reflection off normally sub-visible particles. Hence, the orb artifact is commonplace with small digital or film camera photographs.

Cynthia Baron Cynthia Baron is an American film scholar and professor of Theatre and Film Studies at the Bowling Green State University Bowling Green State University (BGSU) is a Public university, public research university in Bowling Green, Ohio, United ...

.
Adobe Photoshop Forensics: Sleuths, Truths, and Fauxtography
'. Cengage Learning; 2008. . p. 310–.

References

{{Authority control Scattering

Backscatter of waves in physical space

Radar, especially weather radar

In waveguides

In photography

See also

References