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# Refractive index

In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how light propagates through that medium. [1]

## Abbe number

In optics and lens design, the Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material's dispersion (variation of refractive index versus wavelength), with high values of V indicating low dispersion.

## Abbe refractometer

An Abbe refractometer is a bench-top device for the high-precision measurement of an index of refraction.

## Absolute value

In mathematics, the absolute value or modulus of a real number is the non-negative value of without regard to its sign.

In physics, absorption of electromagnetic radiation is the way in which the energy of a photon is taken up by matter, typically the electrons of an atom.

## Active laser medium

The active laser medium (also called gain medium or lasing medium) is the source of optical gain within a laser.

## Aerogel

Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas.

## Agriculture

Agriculture is the cultivation of land and breeding of animals and plants to provide food, fiber, medicinal plants and other products to sustain and enhance life.

## Angle of incidence (optics)

In geometric optics, the angle of incidence is the angle between a ray incident on a surface and the line perpendicular to the surface at the point of incidence, called the normal.

## Anisotropy

Anisotropy, is the property of being directionally dependent, which implies different properties in different directions, as opposed to isotropy.

## Annulus (mathematics)

In mathematics, an annulus (the Latin word for "little ring" is anulus/annulus, with plural anuli/annuli) is a ring-shaped object, a region bounded by two concentric circles.

## Anti-reflective coating

An antireflective or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses and other optical elements to reduce reflection.

## Aqueous solution

An aqueous solution is a solution in which the solvent is water.

## Atmosphere of Earth

The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth's gravity.

## Atomic form factor

In physics, the atomic form factor, or atomic scattering factor, is a measure of the scattering amplitude of a wave by an isolated atom.

## Attenuation

In physics, attenuation or, in some contexts, extinction is the gradual loss of flux intensity through a medium.

## Attenuation coefficient

Attenuation coefficient or narrow beam attenuation coefficient of the volume of a material characterizes how easily it can be penetrated by a beam of light, sound, particles, or other energy or matter.

## Barium oxide

Barium oxide, BaO, is a white hygroscopic non-flammable compound.

## Beam splitter

A beam splitter is an optical device that splits a beam of light in two.

## Beer–Lambert law

The Beer–Lambert law, also known as Beer's law, the Lambert–Beer law, or the Beer–Lambert–Bouguer law relates the attenuation of light to the properties of the material through which the light is travelling.

## Birefringence

Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light.

## Brewster's angle

Brewster's angle (also known as the polarization angle) is an angle of incidence at which light with a particular polarization is perfectly transmitted through a transparent dielectric surface, with no reflection.

## Bright-field microscopy

Bright-field microscopy is the simplest of all the optical microscopy illumination techniques.

## Brix

Degrees Brix (symbol °Bx) is the sugar content of an aqueous solution.

## Calculation of glass properties

The calculation of glass properties (glass modeling) is used to predict glass properties of interest or glass behavior under certain conditions (e.g., during production) without experimental investigation, based on past data and experience, with the intention to save time, material, financial, and environmental resources, or to gain scientific insight.

## Canonical coordinates

In mathematics and classical mechanics, canonical coordinates are sets of coordinates on phase space which can be used to describe a physical system at any given point in time.

## Carbon dioxide

Carbon dioxide (chemical formula) is a colorless gas with a density about 60% higher than that of dry air.

## Charles Hutton

Charles Hutton FRS FRSE LLD (14 August 1737 – 27 January 1823) was a British mathematician and surveyor.

## Chatoyancy

In gemology, chatoyancy, or chatoyance or cat's eye effect, is an optical reflectance effect seen in certain gemstones.

## Chemical industry

The chemical industry comprises the companies that produce industrial chemicals.

## Chemical substance

A chemical substance, also known as a pure substance, is a form of matter that consists of molecules of the same composition and structure.

## Chemistry

Chemistry is the scientific discipline involved with compounds composed of atoms, i.e. elements, and molecules, i.e. combinations of atoms: their composition, structure, properties, behavior and the changes they undergo during a reaction with other compounds.

## Chromatic aberration

In optics, chromatic aberration (abbreviated CA; also called chromatic distortion and spherochromatism) is an effect resulting from dispersion in which there is a failure of a lens to focus all colors to the same convergence point.

## Circolo Matematico di Palermo

The Circolo Matematico di Palermo (Mathematical Circle of Palermo) is an Italian mathematical society, founded in Palermo by Sicilian geometer Giovanni B. Guccia in 1884.

The classical electron radius is a combination of fundamental physical quantities that define a length scale for problems involving electrons interacting with electromagnetic radiation.

## Clausius–Mossotti relation

The Clausius–Mossotti relation expresses the dielectric constant (relative permittivity) εr of a material in terms of the atomic polarizibility α of the material's constituent atoms and/or molecules, or a homogeneous mixture thereof.

## Color

Color (American English) or colour (Commonwealth English) is the characteristic of human visual perception described through color categories, with names such as red, orange, yellow, green, blue, or purple.

## Complex number

A complex number is a number that can be expressed in the form, where and are real numbers, and is a solution of the equation.

## Core (optical fiber)

The core of a conventional optical fiber is a cylinder of glass or plastic that runs along the fiber's length.

## Crystal

A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions.

## Crystal optics

Crystal optics is the branch of optics that describes the behaviour of light in anisotropic media, that is, media (such as crystals) in which light behaves differently depending on which direction the light is propagating.

## Cubic zirconia

Cubic zirconia (CZ) is the cubic crystalline form of zirconium dioxide (ZrO2).

## David Brewster

Sir David Brewster KH PRSE FRS FSA(Scot) FSSA MICE (11 December 178110 February 1868) was a British scientist, inventor, author, and academic administrator.

## Density

The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume.

## Diamond

Diamond is a solid form of carbon with a diamond cubic crystal structure.

## Differential interference contrast microscopy

Differential interference contrast (DIC) microscopy, also known as Nomarski interference contrast (NIC) or Nomarski microscopy, is an optical microscopy technique used to enhance the contrast in unstained, transparent samples.

## Diffraction

--> Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit.

## Digital handheld refractometer

A digital handheld refractometer is an instrument for measuring the refractive index of materials.

## Dimensionless quantity

In dimensional analysis, a dimensionless quantity is a quantity to which no physical dimension is assigned.

## Dispersion (optics)

In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency.

## Electric field

An electric field is a vector field surrounding an electric charge that exerts force on other charges, attracting or repelling them.

## Electric susceptibility

In electricity (electromagnetism), the electric susceptibility (\chi_; Latin: susceptibilis "receptive") is a dimensionless proportionality constant that indicates the degree of polarization of a dielectric material in response to an applied electric field.

In physics, electromagnetic radiation (EM radiation or EMR) refers to the waves (or their quanta, photons) of the electromagnetic field, propagating (radiating) through space-time, carrying electromagnetic radiant energy.

## Electromagnetic spectrum

The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.

## Electron

The electron is a subatomic particle, symbol or, whose electric charge is negative one elementary charge.

## Ellipsometry

Ellipsometry is an optical technique for investigating the dielectric properties (complex refractive index or dielectric function) of thin films.

## Envelope (waves)

In physics and engineering, the envelope of an oscillating signal is a smooth curve outlining its extremes.

## Ernst Abbe

Ernst Karl Abbe HonFRMS (23 January 1840 – 14 January 1905) was a German physicist, optical scientist, entrepreneur, and social reformer.

## Ethanol

Ethanol, also called alcohol, ethyl alcohol, grain alcohol, and drinking alcohol, is a chemical compound, a simple alcohol with the chemical formula.

## Extreme ultraviolet

Extreme ultraviolet radiation (EUV or XUV) or high-energy ultraviolet radiation is electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths from 124 nm down to 10 nm, and therefore (by the Planck–Einstein equation) having photons with energies from 10 eV up to 124 eV (corresponding to 124 nm to 10 nm respectively).

## Fermat's principle

In optics, Fermat's principle or the principle of least time, named after French mathematician Pierre de Fermat, is the principle that the path taken between two points by a ray of light is the path that can be traversed in the least time.

## Fizeau experiment

The Fizeau experiment was carried out by Hippolyte Fizeau in 1851 to measure the relative speeds of light in moving water.

## Flint glass

Flint glass is optical glass that has relatively high refractive index and low Abbe number (high dispersion).

## Focal length

The focal length of an optical system is a measure of how strongly the system converges or diverges light.

## Fourier optics

Fourier optics is the study of classical optics using Fourier transforms (FTs), in which the wave is regarded as a superposition of plane waves that are not related to any identifiable sources; instead they are the natural modes of the propagation medium itself.

## Francis Hauksbee

Francis Hauksbee the Elder FRS (1660–1713), also known as Francis Hawksbee, was an 18th-century English scientist best known for his work on electricity and electrostatic repulsion.

## Fraunhofer lines

In physics and optics, the Fraunhofer lines are a set of spectral lines named after the German physicist Joseph von Fraunhofer (1787–1826).

## Frequency

Frequency is the number of occurrences of a repeating event per unit of time.

## Fresnel equations

The Fresnel equations (or Fresnel coefficients) describe the reflection and transmission of light (or electromagnetic radiation in general) when incident on an interface between different optical media.

## Fused quartz

Fused quartz or fused silica is glass consisting of silica in amorphous (non-crystalline) form.

## Gas

Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).

## Gemology

Gemology or gemmology is the science dealing with natural and artificial gemstone materials.

## Gemstone

A gemstone (also called a gem, fine gem, jewel, precious stone, or semi-precious stone) is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments.

## Geometrical optics

Geometrical optics, or ray optics, describes light propagation in terms of rays.

## Germanium

Germanium is a chemical element with symbol Ge and atomic number 32.

The Gladstone–Dale relation (J. H. Gladstone and T. P. Dale, 1864) is a mathematical relation used for optical analysis of liquids, the determination of composition from optical measurements.

## Glass

Glass is a non-crystalline amorphous solid that is often transparent and has widespread practical, technological, and decorative usage in, for example, window panes, tableware, and optoelectronics.

Gradient-index (GRIN) optics is the branch of optics covering optical effects produced by a gradual variation of the refractive index of a material.

## Grape

A grape is a fruit, botanically a berry, of the deciduous woody vines of the flowering plant genus Vitis.

## Group velocity

The group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes—known as the modulation or envelope of the wave—propagates through space.

## Helium

Helium (from lit) is a chemical element with symbol He and atomic number 2.

## Hermann Minkowski

Hermann Minkowski (22 June 1864 – 12 January 1909) was a German mathematician and professor at Königsberg, Zürich and Göttingen.

## High-refractive-index polymer

A high-refractive-index polymer (HRIP) is a polymer that has a refractive index greater than 1.50.

## Hydrogen

Hydrogen is a chemical element with symbol H and atomic number 1.

## Ice

Ice is water frozen into a solid state.

## Ideal gas law

The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas.

## Imaginary number

An imaginary number is a complex number that can be written as a real number multiplied by the imaginary unit,j is usually used in Engineering contexts where i has other meanings (such as electrical current) which is defined by its property.

## Index ellipsoid

In optics, an index ellipsoid is a diagram of an ellipsoid that depicts the orientation and relative magnitude of refractive indices in a crystal.

## Index-matching material

In optics, an index-matching material is a substance, usually a liquid, cement (adhesive), or gel, which has an index of refraction that closely approximates that of another object (such as a lens, material, fiber-optic, etc.). When two substances with the same index are in contact, light passes from one to the other with neither reflection nor refraction.

## Information

Information is any entity or form that provides the answer to a question of some kind or resolves uncertainty.

## Infrared

Infrared radiation (IR) is electromagnetic radiation (EMR) with longer wavelengths than those of visible light, and is therefore generally invisible to the human eye (although IR at wavelengths up to 1050 nm from specially pulsed lasers can be seen by humans under certain conditions). It is sometimes called infrared light.

## Inline process refractometer

Inline process refractometers are a type of refractometer designed for the continuous measurement of a fluid flowing through a pipe or inside a tank.

## Integral

In mathematics, an integral assigns numbers to functions in a way that can describe displacement, area, volume, and other concepts that arise by combining infinitesimal data.

## Intensity (physics)

In physics, intensity is the power transferred per unit area, where the area is measured on the plane perpendicular to the direction of propagation of the energy.

## Interferometry

Interferometry is a family of techniques in which waves, usually electromagnetic waves, are superimposed causing the phenomenon of interference in order to extract information.

## Ionosphere

The ionosphere is the ionized part of Earth's upper atmosphere, from about to altitude, a region that includes the thermosphere and parts of the mesosphere and exosphere.

## Isaac Newton

Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, astronomer, theologian, author and physicist (described in his own day as a "natural philosopher") who is widely recognised as one of the most influential scientists of all time, and a key figure in the scientific revolution.

## Isotropy

Isotropy is uniformity in all orientations; it is derived from the Greek isos (ἴσος, "equal") and tropos (τρόπος, "way").

## John Herschel

Sir John Frederick William Herschel, 1st Baronet (7 March 1792 – 11 May 1871) was an English polymath, mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint, and did botanical work.

## Joseph von Fraunhofer

Joseph Ritter von Fraunhofer (6 March 1787 – 7 June 1826) was a Bavarian physicist and optical lens manufacturer.

## Kerr effect

The Kerr effect, also called the quadratic electro-optic (QEO) effect, is a change in the refractive index of a material in response to an applied electric field.

## Kramers–Kronig relations

The Kramers–Kronig relations are bidirectional mathematical relations, connecting the real and imaginary parts of any complex function that is analytic in the upper half-plane.

## Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation.

## Laser schlieren deflectometry

Laser schlieren deflectometry (LSD) is a method for a high-speed measurement of the gas temperature in microscopic dimensions, in particular for temperature peaks under dynamic conditions at atmospheric pressure.

Lead(II) oxide, also called lead monoxide, is the inorganic compound with the molecular formula PbO.

## Lens (optics)

A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction.

## Light

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum.

## Liquid

A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure.

## List of refractive indices

Many materials have a well-characterized refractive index, but these indexes depend strongly upon the frequency of light.

## Lithium oxide

Lithium oxide (2O) or lithia is an inorganic chemical compound.

## Magnesium oxide

Magnesium oxide (MgO), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide).

## Magnetic field

A magnetic field is a vector field that describes the magnetic influence of electrical currents and magnetized materials.

## Magnetic susceptibility

In electromagnetism, the magnetic susceptibility (Latin: susceptibilis, "receptive"; denoted) is one measure of the magnetic properties of a material.

## Mass attenuation coefficient

The mass attenuation coefficient, mass extinction coefficient, or mass narrow beam attenuation coefficient of the volume of a material characterizes how easily it can be penetrated by a beam of light, sound, particles, or other energy or matter.

## Mathematical descriptions of opacity

When an electromagnetic wave travels through a medium in which it gets attenuated (this is called an "opaque" or "attenuating" medium), it undergoes exponential decay as described by the Beer–Lambert law.

## Mathematical optimization

In mathematics, computer science and operations research, mathematical optimization or mathematical programming, alternatively spelled optimisation, is the selection of a best element (with regard to some criterion) from some set of available alternatives.

## Max Abraham

Max Abraham (26 March 1875 – 16 November 1922) was a German physicist.

## Maxwell's equations

Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric circuits.

## Metamaterial

A metamaterial (from the Greek word μετά meta, meaning "beyond") is a material engineered to have a property that is not found in nature.

## Microscope

A microscope (from the μικρός, mikrós, "small" and σκοπεῖν, skopeîn, "to look" or "see") is an instrument used to see objects that are too small to be seen by the naked eye.

## Mirage

A mirage is a naturally occurring optical phenomenon in which light rays bend to produce a displaced image of distant objects or the sky.

## Moissanite

Moissanite is naturally occurring silicon carbide and its various crystalline polymorphs.

## Molar mass

In chemistry, the molar mass M is a physical property defined as the mass of a given substance (chemical element or chemical compound) divided by the amount of substance.

## Molar refractivity

Molar refractivity, A, is a measure of the total polarizability of a mole of a substance and is dependent on the temperature, the index of refraction, and the pressure.

## Mole (unit)

The mole, symbol mol, is the SI unit of amount of substance.

## Momentum

In Newtonian mechanics, linear momentum, translational momentum, or simply momentum (pl. momenta) is the product of the mass and velocity of an object.

## Nanometre

The nanometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American spelling) is a unit of length in the metric system, equal to one billionth (short scale) of a metre (m).

## Nature Photonics

Nature Photonics is a peer-reviewed scientific journal published by the Nature Publishing Group.

## Nature Publishing Group

Nature Publishing Group is a division of the international scientific publishing company Springer Nature that publishes academic journals, magazines, online databases, and services in science and medicine.

## Negative refraction

Negative refraction is the name for an electromagnetic phenomenon where light rays are refracted at an interface in the reverse sense to that normally expected.

## Nonlinear optics

Nonlinear optics (NLO) is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light.

## Normal (geometry)

In geometry, a normal is an object such as a line or vector that is perpendicular to a given object.

## Numerical aperture

In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light.

## Objective (optics)

In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image.

## Oil immersion

In light microscopy, oil immersion is a technique used to increase the resolving power of a microscope.

## Olive oil

Olive oil is a liquid fat obtained from olives (the fruit of Olea europaea; family Oleaceae), a traditional tree crop of the Mediterranean Basin.

## Optic axis of a crystal

An optic axis of a crystal is a direction in which a ray of transmitted light suffers no birefringence (double refraction).

## Optical fiber

An optical fiber or optical fibre is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair.

## Optical instrument

An optical instrument either processes light waves to enhance an image for viewing, or analyzes light waves (or photons) to determine one of a number of characteristic properties.

## Optical medium

An optical medium is material through which electromagnetic waves propagate.

## Optical path length

In optics, optical path length (OPL) or optical distance is the product of the geometric length of the path light follows through the system, and the index of refraction of the medium through which it propagates(OP.

## Optical properties of water and ice

The refractive index of water at 20 °C is 1.3330.

## Optical resolution

Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.

## Optics

Optics is the branch of physics which involves the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it.

## Oxford University Press

Oxford University Press (OUP) is the largest university press in the world, and the second oldest after Cambridge University Press.

## Parallel (geometry)

In geometry, parallel lines are lines in a plane which do not meet; that is, two lines in a plane that do not intersect or touch each other at any point are said to be parallel.

## Penetration depth

Penetration depth is a measure of how deep light or any electromagnetic radiation can penetrate into a material.

## Permeability (electromagnetism)

In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself.

## Permittivity

In electromagnetism, absolute permittivity, often simply called permittivity, usually denoted by the Greek letter ε (epsilon), is the measure of resistance that is encountered when forming an electric field in a particular medium.

## Pharmaceutical industry

The pharmaceutical industry (or medicine industry) is the commercial industry that discovers, develops, produces, and markets drugs or pharmaceutical drugs for use as different types of medicine and medications.

## Phase (waves)

Phase is the position of a point in time (an instant) on a waveform cycle.

## Phase velocity

The phase velocity of a wave is the rate at which the phase of the wave propagates in space.

## Phase-contrast imaging

Phase-contrast imaging is a method of imaging that has a range of different applications.

## Phase-contrast microscopy

Phase-contrast microscopy is an optical microscopy technique that converts phase shifts in light passing through a transparent specimen to brightness changes in the image.

## Phase-contrast X-ray imaging

Phase-contrast X-ray imaging (PCI) or phase-sensitive X-ray imaging is a general term for different technical methods that use information concerning changes in the phase of an X-ray beam that passes through an object in order to create its images.

## Photodetector

Photosensors or photodetectors are sensors of light or other electromagnetic energy.

## Photoelasticity

Photoelasticity describes changes in the optical properties of a material under mechanical deformation.

## Physics-Uspekhi

Physics-Uspekhi (Advances in Physical Sciences) is a peer-reviewed scientific journal.

## Plane of incidence

In describing reflection and refraction in optics, the plane of incidence (also called the meridional plane) is the plane which contains the surface normal and the propagation vector of the incoming radiation.

## Plane wave

In the physics of wave propagation, a plane wave (also spelled planewave) is a wave whose wavefronts (surfaces of constant phase) are infinite parallel planes.

## Plasma (physics)

Plasma (Henry George Liddell, Robert Scott, A Greek English Lexicon, on Perseus) is one of the four fundamental states of matter, and was first described by chemist Irving Langmuir in the 1920s.

## Plastic

Plastic is material consisting of any of a wide range of synthetic or semi-synthetic organic compounds that are malleable and so can be molded into solid objects.

## Pockels effect

The Pockels effect (after Friedrich Carl Alwin Pockels who studied the effect in 1893), or Pockels electro-optic effect, changes or produces birefringence in an optical medium induced by an electric field.

## Point reflection

In geometry, a point reflection or inversion in a point (or inversion through a point, or central inversion) is a type of isometry of Euclidean space.

## Polarizability

Polarizability is the ability to form instantaneous dipoles.

## Polarization (waves)

Polarization (also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations.

## Polarizer

A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations.

## Poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA), also known as acrylic or acrylic glass as well as by the trade names Crylux, Plexiglas, Acrylite, Lucite, and Perspex among several others (see below), is a transparent thermoplastic often used in sheet form as a lightweight or shatter-resistant alternative to glass.

## Polycarbonate

Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate groups in their chemical structures.

## Pressure

Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed.

## Prism

In optics, a prism is a transparent optical element with flat, polished surfaces that refract light.

## Prism coupler

A prism coupler is a prism designed to couple a substantial fraction of the power contained in a beam of light (e.g., a laser beam) into a thin film to be used as a waveguide without the need for precision polishing of the edge of the film, without the need for sub-micrometer alignment precision of the beam and the edge of the film, and without the need for matching the numerical aperture of the beam to the film.

## Process control

Automatic process control in continuous production processes is a combination of control engineering and chemical engineering disciplines that uses industrial control systems to achieve a production level of consistency, economy and safety which could not be achieved purely by human manual control.

## Quality control

Quality control, or QC for short, is a process by which entities review the quality of all factors involved in production.

Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another, or into various parts of the atmosphere.

Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light.

Radius of curvature (ROC) has specific meaning and sign convention in optical design.

## Rainbow

A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky.

## Real image

In optics, a real image is an image which is located in the plane of convergence for the light rays that originate from a given object.

## Real number

In mathematics, a real number is a value of a continuous quantity that can represent a distance along a line.

## Reflectance

Reflectance of the surface of a material is its effectiveness in reflecting radiant energy.

## Reflection (physics)

Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated.

## Refraction

Refraction is the change in direction of wave propagation due to a change in its transmission medium.

A.

## Refractometer

A refractometer is a laboratory or field device for the measurement of an index of refraction (refractometry).

## Relative permittivity

The relative permittivity of a material is its (absolute) permittivity expressed as a ratio relative to the permittivity of vacuum.

## Resonance

In physics, resonance is a phenomenon in which a vibrating system or external force drives another system to oscillate with greater amplitude at specific frequencies.

## Sapphire

Sapphire is a precious gemstone, a variety of the mineral corundum, an aluminium oxide.

## 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 paths due to localized non-uniformities in the medium through which they pass.

## Self-focusing

Self-focusing is a non-linear optical process induced by the change in refractive index of materials exposed to intense electromagnetic radiation.

## Self-phase modulation

Self-phase modulation (SPM) is a nonlinear optical effect of light-matter interaction.

## Sellmeier equation

The Sellmeier equation is an empirical relationship between refractive index and wavelength for a particular transparent medium.

## Signal velocity

The signal velocity is the speed at which a wave carries information.

## Skywave

In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere.

## Snell's law

Snell's law (also known as Snell–Descartes law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.

## Soda–lime glass

Soda–lime glass, also called soda–lime–silica glass, is the most prevalent type of glass, used for windowpanes and glass containers (bottles and jars) for beverages, food, and some commodity items.

## Sodium

Sodium is a chemical element with symbol Na (from Latin natrium) and atomic number 11.

## Solid

Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma).

## Solution

In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances.

## Sound

In physics, sound is a vibration that typically propagates as an audible wave of pressure, through a transmission medium such as a gas, liquid or solid.

## Spatial frequency

In mathematics, physics, and engineering, spatial frequency is a characteristic of any structure that is periodic across position in space.

## Spectral line

A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies.

## Speed of light

The speed of light in vacuum, commonly denoted, is a universal physical constant important in many areas of physics.

## Standard conditions for temperature and pressure

Standard conditions for temperature and pressure are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data.

## Stimulated emission

Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level.

## Superlens

A superlens, or super lens, is a lens which uses metamaterials to go beyond the diffraction limit.

## Superposition principle

In physics and systems theory, the superposition principle, also known as superposition property, states that, for all linear systems, the net response caused by two or more stimuli is the sum of the responses that would have been caused by each stimulus individually.

## Telescope

A telescope is an optical instrument that aids in the observation of remote objects by collecting electromagnetic radiation (such as visible light).

## Temperature

Temperature is a physical quantity expressing hot and cold.

## Tensor

In mathematics, tensors are geometric objects that describe linear relations between geometric vectors, scalars, and other tensors.

## Theory of relativity

The theory of relativity usually encompasses two interrelated theories by Albert Einstein: special relativity and general relativity.

## Thin lens

In optics, a thin lens is a lens with a thickness (distance along the optical axis between the two surfaces of the lens) that is negligible compared to the radii of curvature of the lens surfaces.

## Thomas Young (scientist)

Thomas Young FRS (13 June 1773 – 10 May 1829) was a British polymath and physician.

## Total internal reflection

Total internal reflection is the phenomenon which occurs when a propagated wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface.

## Transparency and translucency

In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without being scattered.

## United States Department of Energy

The United States Department of Energy (DOE) is a cabinet-level department of the United States Government concerned with the United States' policies regarding energy and safety in handling nuclear material.

## Vacuum

Vacuum is space devoid of matter.

## Water

Water is a transparent, tasteless, odorless, and nearly colorless chemical substance that is the main constituent of Earth's streams, lakes, and oceans, and the fluids of most living organisms.

## Wave

In physics, a wave is a disturbance that transfers energy through matter or space, with little or no associated mass transport.

## Wave interference

In physics, interference is a phenomenon in which two waves superpose to form a resultant wave of greater, lower, or the same amplitude.

## Wavelength

In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.

## Waveplate

A waveplate or retarder is an optical device that alters the polarization state of a light wave travelling through it.

## Winemaking

Winemaking or vinification is the production of wine, starting with the selection of the fruit, its fermentation into alcohol, and the bottling of the finished liquid.