Physical optics
Encyclopedia
In physics
, physical optics, or wave optics, is the branch of optics
which studies interference, diffraction
, polarization, and other phenomena for which the ray approximation of geometric optics is not valid. This usage tends not to include effects such as quantum noise
in optical communication
, which is studied in the sub-branch of coherence theory
.
commonly used in optics, electrical engineering
and applied physics
. In this context, it is an intermediate method between geometric optics, which ignores wave
effects, and full wave electromagnetism
, which is a precise theory
. The word "physical" means that it is more physical than geometric or ray
optics and not that it is an exact physical theory.
This approximation consists of using ray optics to estimate the field on a surface and then integrating
that field over the surface to calculate the transmitted or scattered field. This resembles the Born approximation
, in that the details of the problem are treated as a perturbation
.
In optics, it is a standard way of estimating diffraction effects. In radio
, this approximation is used to estimate some effects that resemble optical effects. It models several interference, diffraction and polarization effects but not the dependence of diffraction on polarization. Since it is a high frequency approximation, it is often more accurate in optics than for radio.
In optics, it typically consists of integrating ray estimated field over a lens, mirror or aperture to calculate the transmitted or scattered field.
In radar
scattering
it usually means taking the current that would be found on a tangent
plane
of similar material as the current at each point on the front, i. e. the geometrically illuminated part, of a scatterer
. Current on the shadowed parts is taken as zero. The approximate scattered field is then obtained by an integral over these approximate currents. This is useful for bodies with large smooth convex shapes and for lossy (low reflection) surfaces.
The ray optics field or current is generally not accurate near edges or shadow boundaries, unless supplemented by diffraction and creeping wave
calculations.
The theory of physical optics has some defects in the evaluation of the scattered fields. For example the diffracted fields, which are evaluated by the method of physical optics, are incorrect. In 2004, Y. Z. Umul has introduced an improved theory that leads to the exact solutions to wave diffraction problems by conducting scatterers, based on three axioms. This unique idea was also independently suggested by Shijo et al, four years after Umul's work.
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, physical optics, or wave optics, is the branch of optics
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
which studies interference, diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
, polarization, and other phenomena for which the ray approximation of geometric optics is not valid. This usage tends not to include effects such as quantum noise
Quantum noise
Quantum noise is uncertainty of some physical quantity due to its quantum origin.In the case of number of particles , the quantum noise is also called shot noise. Most optical communications use amplitude modulation...
in optical communication
Optical communication
Optical communication is any form of telecommunication that uses light as the transmission medium.An optical communication system consists of a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the...
, which is studied in the sub-branch of coherence theory
Coherence theory
In physics, coherence theory is the study of optical effects arising from partially coherent light and radio sources. Partially coherent sources are sources where the coherence time or coherence length are limited by bandwidth, by thermal noise, or by other effect...
.
The physical optics approximation
Physical optics is also the name of an approximationApproximation
An approximation is a representation of something that is not exact, but still close enough to be useful. Although approximation is most often applied to numbers, it is also frequently applied to such things as mathematical functions, shapes, and physical laws.Approximations may be used because...
commonly used in optics, electrical engineering
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
and applied physics
Applied physics
Applied physics is a general term for physics which is intended for a particular technological or practical use.It is usually considered as a bridge or a connection between "pure" physics and engineering....
. In this context, it is an intermediate method between geometric optics, which ignores wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
effects, and full wave electromagnetism
Electromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
, which is a precise theory
Theory
The English word theory was derived from a technical term in Ancient Greek philosophy. The word theoria, , meant "a looking at, viewing, beholding", and referring to contemplation or speculation, as opposed to action...
. The word "physical" means that it is more physical than geometric or ray
Ray (optics)
In optics, a ray is an idealized narrow beam of light. Rays are used to model the propagation of light through an optical system, by dividing the real light field up into discrete rays that can be computationally propagated through the system by the techniques of ray tracing. This allows even very...
optics and not that it is an exact physical theory.
This approximation consists of using ray optics to estimate the field on a surface and then integrating
Integral
Integration is an important concept in mathematics and, together with its inverse, differentiation, is one of the two main operations in calculus...
that field over the surface to calculate the transmitted or scattered field. This resembles the Born approximation
Born approximation
In scattering theory and, in particular in quantum mechanics, the Born approximation consists of taking the incident field in place of the total field as the driving field at each point in the scatterer. Born approximation is named after Max Born, winner of the 1954 Nobel Prize for physics.It is...
, in that the details of the problem are treated as a perturbation
Perturbation theory
Perturbation theory comprises mathematical methods that are used to find an approximate solution to a problem which cannot be solved exactly, by starting from the exact solution of a related problem...
.
In optics, it is a standard way of estimating diffraction effects. In radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
, this approximation is used to estimate some effects that resemble optical effects. It models several interference, diffraction and polarization effects but not the dependence of diffraction on polarization. Since it is a high frequency approximation, it is often more accurate in optics than for radio.
In optics, it typically consists of integrating ray estimated field over a lens, mirror or aperture to calculate the transmitted or scattered field.
In radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
it usually means taking the current that would be found on a tangent
Tangent
In geometry, the tangent line to a plane curve at a given point is the straight line that "just touches" the curve at that point. More precisely, a straight line is said to be a tangent of a curve at a point on the curve if the line passes through the point on the curve and has slope where f...
plane
Plane (mathematics)
In mathematics, a plane is a flat, two-dimensional surface. A plane is the two dimensional analogue of a point , a line and a space...
of similar material as the current at each point on the front, i. e. the geometrically illuminated part, of a scatterer
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
. Current on the shadowed parts is taken as zero. The approximate scattered field is then obtained by an integral over these approximate currents. This is useful for bodies with large smooth convex shapes and for lossy (low reflection) surfaces.
The ray optics field or current is generally not accurate near edges or shadow boundaries, unless supplemented by diffraction and creeping wave
Creeping wave
According to the principle of diffraction, when a wave front passes an obstruction, it spreads out into the shadowed space. A creeping wave, in electromagnetism or acoustics is the wave that is diffracted around the shadowed surface of a smooth body such as a sphere.Creeping waves greatly extend...
calculations.
The theory of physical optics has some defects in the evaluation of the scattered fields. For example the diffracted fields, which are evaluated by the method of physical optics, are incorrect. In 2004, Y. Z. Umul has introduced an improved theory that leads to the exact solutions to wave diffraction problems by conducting scatterers, based on three axioms. This unique idea was also independently suggested by Shijo et al, four years after Umul's work.