In physics and mathematics, **time domain electromagnetics** refers to one of two general groups of techniques (in mathematics, often called *ansätze*) that describe electromagnetic wave motion. In contrast with frequency domain electromagnetics, which are based on the Fourier or Laplace transform, time domain keeps time as an explicit independent variable in descriptive equations or wave motion.

In physics, **electromagnetic radiation** (**EMR**) consists of waves of the electromagnetic (EM) field, which propagate through space and carry electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays. All of these waves form part of the electromagnetic spectrum.

In physics, a **force** is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity, i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newton (N). Force is represented by the symbol **F**.

**Light** or **visible light** is electromagnetic radiation within the portion of the electromagnetic spectrum that is perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 terahertz, between the infrared and the ultraviolet.

**Luminiferous aether** or **ether** was the postulated medium for the propagation of light. It was invoked to explain the ability of the apparently wave-based light to propagate through empty space, something that waves should not be able to do. The assumption of a spatial plenum of luminiferous aether, rather than a spatial vacuum, provided the theoretical medium that was required by wave theories of light.

In Newtonian mechanics, **linear momentum**, **translational momentum**, or simply **momentum** is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If *m* is an object's mass and **v** is its velocity, then the object's momentum **p** is :

In physics, **motion** is the phenomenon in which an object changes its position with respect to time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed and frame of reference to an observer and measuring the change in position of the body relative to that frame with change in time. The branch of physics describing the motion of objects without reference to its cause is called kinematics, while the branch studying forces and their effect on motion is called dynamics.

**Optics** is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.

In optics, the **refractive index** of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.

In physics, mathematics, and related fields, a **wave** is a propagating dynamic disturbance of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (resting) value at some frequency. When the entire waveform moves in one direction, it is said to be a *traveling wave*; by contrast, a pair of superimposed periodic waves traveling in opposite directions makes a *standing wave*. In a standing wave, the amplitude of vibration has nulls at some positions where the wave amplitude appears smaller or even zero. Waves are often described by a *wave equation* or a one-way wave equation for single wave propagation in a defined direction.

The **amplitude** of a periodic variable is a measure of its change in a single period. The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of amplitude, which are all functions of the magnitude of the differences between the variable's extreme values. In older texts, the phase of a periodic function is sometimes called the amplitude.

**Magnetohydrodynamics** is the study of the magnetic properties and behaviour of electrically conducting fluids. Examples of such magnetofluids include plasmas, liquid metals, salt water, and electrolytes. The word "magnetohydrodynamics" is derived from *magneto-* meaning magnetic field, *hydro-* meaning water, and *dynamics* meaning movement. The field of MHD was initiated by Hannes Alfvén, for which he received the Nobel Prize in Physics in 1970.

**Mathematical physics** refers to the development of mathematical methods for application to problems in physics. The *Journal of Mathematical Physics* defines the field as "the application of mathematics to problems in physics and the development of mathematical methods suitable for such applications and for the formulation of physical theories". An alternative definition would also include those mathematics that are inspired by physics.

In signal processing, a **signal** is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The *IEEE Transactions on Signal Processing* includes audio, video, speech, image, sonar, and radar as examples of signal. A signal may also be defined as *any* observable change in a quantity over space or time, even if it does not carry information.

**Finite-difference time-domain** (**FDTD**) or **Yee's method** is a numerical analysis technique used for modeling computational electrodynamics. Since it is a time-domain method, FDTD solutions can cover a wide frequency range with a single simulation run, and treat nonlinear material properties in a natural way.

**Geophysical survey** is the systematic collection of geophysical data for spatial studies. Detection and analysis of the geophysical signals forms the core of Geophysical signal processing. The magnetic and gravitational fields emanating from the Earth's interior hold essential information concerning seismic activities and the internal structure. Hence, detection and analysis of the electric and Magnetic fields is very crucial. As the Electromagnetic and gravitational waves are multi-dimensional signals, all the 1-D transformation techniques can be extended for the analysis of these signals as well. Hence this article also discusses multi-dimensional signal processing techniques.

**Wave radar** is a type of radar for measuring wind waves. Several instruments based on a variety of different concepts and techniques are available, and these are all often called. This article, gives a brief description of the most common ground-based radar remote sensing techniques.

This **glossary of engineering terms** is a list of definitions about the major concepts of **engineering**. Please see the bottom of the page for glossaries of specific fields of engineering.

This **glossary of physics** is a list of definitions of terms and concepts relevant to physics, its sub-disciplines, and related fields, including mechanics, materials science, nuclear physics, particle physics, and thermodynamics. For more inclusive glossaries concerning related fields of science and technology, see Glossary of chemistry terms, Glossary of astronomy, Glossary of areas of mathematics, and Glossary of engineering.

In physics and mathematics, the **spacetime triangle diagram (STTD) technique**, also known as the **Smirnov method of incomplete separation of variables**, is the direct space-time domain method for electromagnetic and scalar wave motion.

**Victor Vasil'evich Borisov** was a Russian physicist and mathematician who contributed to the theory of wave motion, in particular to time domain electromagnetics and localized waves.

- S. M. Rao, E. K. Miller, Time Domain Electromagnetics, Academic Press: San Diego etc., 1999.

- The Virtual Institute for Nonlinear Optics (VINO), a research collaboration devoted to the investigation of X-waves and conical waves in general
- Nolinear X-waves Archived 2008-04-15 at the Wayback Machine page at the nlo.phys.uniroma1.it Archived 2008-04-14 at the Wayback Machine website.

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