1.
Helium
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Helium is a chemical element with symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and its boiling point is the lowest among all the elements. Its abundance is similar to figure in the Sun and in Jupiter. This is due to the high nuclear binding energy of helium-4 with respect to the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of nuclear fusion and radioactive decay. Most helium in the universe is helium-4, and is believed to have formed during the Big Bang. Large amounts of new helium are being created by fusion of hydrogen in stars. Helium is named for the Greek god of the Sun, Helios and it was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer, Janssen observed during the solar eclipse of 1868 while Lockyer observed from Britain. Lockyer was the first to propose that the line was due to a new element, the formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite. In 1903, large reserves of helium were found in gas fields in parts of the United States. Liquid helium is used in cryogenics, particularly in the cooling of superconducting magnets, a well-known but minor use is as a lifting gas in balloons and airships. As with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre, on Earth it is relatively rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the radioactive decay of heavy radioactive elements. Previously, terrestrial helium—a non-renewable resource, because once released into the atmosphere it readily escapes into space—was thought to be in short supply. The first evidence of helium was observed on August 18,1868, the line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India. This line was assumed to be sodium. He concluded that it was caused by an element in the Sun unknown on Earth, Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος
2.
Laser
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A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term laser originated as an acronym for light amplification by stimulated emission of radiation, the first laser was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow. A laser differs from other sources of light in that it emits light coherently, spatial coherence allows a laser to be focused to a tight spot, enabling applications such as laser cutting and lithography. Spatial coherence also allows a laser beam to stay narrow over great distances, Lasers can also have high temporal coherence, which allows them to emit light with a very narrow spectrum, i. e. they can emit a single color of light. Temporal coherence can be used to produce pulses of light as short as a femtosecond, Lasers are distinguished from other light sources by their coherence. Spatial coherence is typically expressed through the output being a narrow beam, Laser beams can be focused to very tiny spots, achieving a very high irradiance, or they can have very low divergence in order to concentrate their power at a great distance. Temporal coherence implies a polarized wave at a single frequency whose phase is correlated over a great distance along the beam. A beam produced by a thermal or other incoherent light source has an amplitude and phase that vary randomly with respect to time and position. Lasers are characterized according to their wavelength in a vacuum, most single wavelength lasers actually produce radiation in several modes having slightly differing frequencies, often not in a single polarization. Although temporal coherence implies monochromaticity, there are lasers that emit a broad spectrum of light or emit different wavelengths of light simultaneously, there are some lasers that are not single spatial mode and consequently have light beams that diverge more than is required by the diffraction limit. However, all devices are classified as lasers based on their method of producing light. Lasers are employed in applications where light of the spatial or temporal coherence could not be produced using simpler technologies. The word laser started as an acronym for light amplification by stimulated emission of radiation, in the early technical literature, especially at Bell Telephone Laboratories, the laser was called an optical maser, this term is now obsolete. A laser that produces light by itself is technically an optical rather than an optical amplifier as suggested by the acronym. It has been noted that the acronym LOSER, for light oscillation by stimulated emission of radiation. With the widespread use of the acronym as a common noun, optical amplifiers have come to be referred to as laser amplifiers. The back-formed verb to lase is frequently used in the field, meaning to produce light, especially in reference to the gain medium of a laser. Further use of the laser and maser in an extended sense, not referring to laser technology or devices, can be seen in usages such as astrophysical maser
3.
Holography
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Holography is the science and practice of making holograms. The hologram itself is not an image and it is usually unintelligible when viewed under diffuse ambient light and it is an encoding of the light field as an interference pattern of seemingly random variations in the opacity, density, or surface profile of the photographic medium. In its pure form, holography requires the use of light for illuminating the subject. A microscopic level of detail throughout the volume of space can be reproduced. Holograms can now also be entirely computer-generated and show objects or scenes that never existed, stage illusions such as Peppers Ghost and other unusual, baffling, or seemingly magical images are also often incorrectly called holograms. The Hungarian-British physicist Dennis Gabor, was awarded the Nobel Prize in Physics in 1971 for his invention and development of the holographic method. His work, done in the late 1940s, was built on pioneering work in the field of X-ray microscopy by other scientists including Mieczysław Wolfke in 1920 and William Lawrence Bragg in 1939. The discovery was a result of research into improving electron microscopes at the British Thomson-Houston Company in Rugby, England. The word holography comes from the Greek words ὅλος and γραφή, early holograms used silver halide photographic emulsions as the recording medium. They were not very efficient as the produced grating absorbed much of the incident light, various methods of converting the variation in transmission to a variation in refractive index were developed which enabled much more efficient holograms to be produced. Several types of holograms can be made, a later refinement, the rainbow transmission hologram, allows more convenient illumination by white light rather than by lasers. Rainbow holograms are used for security and authentication, for example, on credit cards. They are also capable of multicolour-image reproduction, specular holography is a related technique for making three-dimensional images by controlling the motion of specularities on a two-dimensional surface. It works by reflectively or refractively manipulating bundles of light rays, most holograms produced are of static objects but systems for displaying changing scenes on a holographic volumetric display are now being developed. Holograms can also be used to store, retrieve, and process information optically and it was thought that it would be possible to use X-rays to make holograms of very small objects and view them using visible light. Today, holograms with x-rays are generated by using synchrotrons or x-ray free-electron lasers as radiation sources, the reconstruction is then retrieved via computation. Due to the wavelength of x-rays compared to visible light. As free-electron lasers can provide ultrashort and x-ray pulses in the range of femtoseconds which are intense and coherent, in laser holography, the hologram is recorded using a source of laser light, which is very pure in its color and orderly in its composition
4.
Barcode reader
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A barcode reader is an electronic device that can read and output printed barcodes to a computer. Like a flatbed scanner, it consists of a source, a lens. Additionally, nearly all barcode readers contain decoder circuitry analyzing the image data provided by the sensor. Barcode readers can be differentiated by technologies as follows, Pen-type readers consist of a light source, to read a bar code, the person holding the pen must move the tip of it across the bars at a relatively uniform speed. The photodiode measures the intensity of the light reflected back from the source as the tip crosses each bar. The photodiode generates a waveform that is used to measure the widths of the bars, dark bars in the bar code absorb light and white spaces reflect light so that the voltage waveform generated by the photodiode is a representation of the bar and space pattern in the bar code. This waveform is decoded by the scanner in a similar to the way Morse code dots. As with the pen type reader, a photo-diode is used to measure the intensity of the light reflected back from the bar code, CCD readers use an array of hundreds of tiny light sensors lined up in a row in the head of the reader. Each sensor measures the intensity of the light immediately in front of it, two-dimensional imaging scanners are a newer type of bar code reader. They use a camera and image processing techniques to decode the bar code, large field-of-view readers use high resolution industrial cameras to capture multiple bar codes simultaneously. Unlike the simpler single-line laser scanners, they produce a pattern of beams in varying orientations allowing them to read barcodes presented to it at different angles, most of them use a single rotating polygonal mirror and an arrangement of several fixed mirrors to generate their complex scan patterns. Omnidirectional scanners are most familiar through the scanners in supermarkets. Omnidirectional scanners are also better at reading poorly printed, wrinkled, cell phone cameras open up a number of applications for consumers, Movies, DVD/VHS movie catalogs. Music, CD catalogs – play MP3 when scanned, groceries, nutrition information, making shopping lists when the last of an item is used, etc. Personal Property inventory ode scanned into personal finance software when entering, later, scanned receipt images can then be automatically associated with the appropriate entries. Later, the bar codes can be used to weed out paper copies not required to be retained for tax or asset inventory purposes. Receipts scanned on a scanner could be identified and associated with the appropriate entries in finance. Consumer tracking from the retailer perspective, a number of enterprise applications using cell phones are appearing, Access control, inventory reporting, asset tracking
5.
Optics
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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. 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. Most optical phenomena can be accounted for using the classical description of light. Complete electromagnetic descriptions of light are, however, often difficult to apply in practice, practical optics is usually done using simplified models. The most common of these, geometric optics, treats light as a collection of rays that travel in straight lines, physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics. Historically, the model of light was developed first, followed by the wave model of light. Progress in electromagnetic theory in the 19th century led to the discovery that waves were in fact electromagnetic radiation. Some phenomena depend on the fact that light has both wave-like and particle-like properties, explanation of these effects requires quantum mechanics. When considering lights particle-like properties, the light is modelled as a collection of particles called photons, quantum optics deals with the application of quantum mechanics to optical systems. Optical science is relevant to and studied in many related disciplines including astronomy, various engineering fields, photography, practical applications of optics are found in a variety of technologies and everyday objects, including mirrors, lenses, telescopes, microscopes, lasers, and fibre optics. Optics began with the development of lenses by the ancient Egyptians and Mesopotamians, the earliest known lenses, made from polished crystal, often quartz, date from as early as 700 BC for Assyrian lenses such as the Layard/Nimrud lens. The ancient Romans and Greeks filled glass spheres with water to make lenses, the word optics comes from the ancient Greek word ὀπτική, meaning appearance, look. Greek philosophy on optics broke down into two opposing theories on how vision worked, the theory and the emission theory. The intro-mission approach saw vision as coming from objects casting off copies of themselves that were captured by the eye, plato first articulated the emission theory, the idea that visual perception is accomplished by rays emitted by the eyes. He also commented on the parity reversal of mirrors in Timaeus, some hundred years later, Euclid wrote a treatise entitled Optics where he linked vision to geometry, creating geometrical optics. Ptolemy, in his treatise Optics, held a theory of vision, the rays from the eye formed a cone, the vertex being within the eye. The rays were sensitive, and conveyed back to the observer’s intellect about the distance. He summarised much of Euclid and went on to describe a way to measure the angle of refraction, during the Middle Ages, Greek ideas about optics were resurrected and extended by writers in the Muslim world
6.
LaserDisc
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LaserDisc is a home video format and the first commercial optical disc storage medium, initially licensed, sold and marketed as MCA DiscoVision in North America in 1978. It was not a format in Europe and Australia when first released but was popular in the 1990s. Its superior video and audio quality made it a choice among videophiles. The technologies and concepts behind LaserDisc were the foundation for later optical disc formats including Compact Disc, DVD, Optical video recording technology, using a transparent disc, was invented by David Paul Gregg and James Russell in 1958. The Gregg patents were purchased by MCA in 1968, by 1969, Philips had developed a videodisc in reflective mode, which has advantages over the transparent mode. MCA and Philips then decided to combine their efforts and first publicly demonstrated the video disc in 1972. LaserDisc was first available on the market, in Atlanta, Georgia, on December 15,1978, Philips produced the players while MCA produced the discs. The Philips-MCA cooperation was not successful, and discontinued after a few years, several of the scientists responsible for the early research founded Optical Disc Corporation. In 1979, the Museum of Science and Industry in Chicago opened its Newspaper exhibit which used interactive LaserDiscs to allow visitors to search for the front page of any Chicago Tribune newspaper and this was a very early example of public access to electronically stored information in a museum. The first LaserDisc title marketed in North America was the MCA DiscoVision release of Jaws in 1978, the last title released in North America was Paramounts Bringing Out the Dead in 2000. The last Japanese released movie was the Hong Kong film Tokyo Raiders from Golden Harvest, a dozen or so more titles continued to be released in Japan, until the end of 2001. Production of LaserDisc players continued until January 14,2009, when Pioneer stopped making them and it was estimated that in 1998, LaserDisc players were in approximately 2% of U. S. households. By comparison, in 1999, players were in 10% of Japanese households, LaserDisc was released on June 10,1981 in Japan, and a total of 3.6 million LaserDisc players were sold there. A total of 16.8 million LaserDisc players were sold worldwide, by the early 2000s, LaserDisc was completely replaced by DVD in the North American retail marketplace, as neither players nor software were then produced. Players were still exported to North America from Japan until the end of 2001, the format has retained some popularity among American collectors, and to a greater degree in Japan, where the format was better supported and more prevalent during its life. In Europe, LaserDisc always remained an obscure format and it was chosen by the British Broadcasting Corporation for the BBC Domesday Project in the mid-1980s, a school-based project to commemorate 900 years since the original Domesday Book in England. From 1991 up until the early 2000s, the BBC also used LaserDisc technology to play out the channel idents, the standard home video LaserDisc was 30 cm in diameter and made up of two single-sided aluminum discs layered in plastic. Although appearing similar to compact discs or DVDs, LaserDiscs used analog video stored in the domain with analog FM stereo sound
7.
Optical disc
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The encoding material sits atop a thicker substrate which makes up the bulk of the disc and forms a dust defocusing layer. The encoding pattern follows a continuous, spiral path covering the disc surface. Most optical discs exhibit a characteristic iridescence as a result of the diffraction grating formed by its grooves and this side of the disc contains the actual data and is typically coated with a transparent material, usually lacquer. The reverse side of a disc usually has a printed label, sometimes made of paper. Optical discs are usually between 7.6 and 30 cm in diameter, with 12 cm being the most common size, a typical disc is about 1.2 mm thick, while the track pitch ranges from 1.6 µm to 320 nm. An optical disc is designed to support one of three recording types, read-only, recordable, or re-recordable, write-once optical discs commonly have an organic dye recording layer between the substrate and the reflective layer. Rewritable discs typically contain an alloy recording layer composed of a phase change material, most often AgInSbTe, an alloy of silver, indium, antimony, Optical discs are most commonly used for storing music, video, or data and programs for personal computers. The Optical Storage Technology Association promotes standardized optical storage formats, although optical discs are more durable than earlier audio-visual and data storage formats, they are susceptible to environmental and daily-use damage. Libraries and archives enact optical media preservation procedures to ensure continued usability in the optical disc drive or corresponding disc player. For computer data backup and physical data transfer, optical discs such as CDs and DVDs are gradually being replaced with faster, smaller solid-state devices and this trend is expected to continue as USB flash drives continue to increase in capacity and drop in price. Additionally, music purchased or shared over the Internet has significantly reduced the number of audio CDs sold annually. The first recorded use of an optical disc was in the 1884 when Alexander Graham Bell, Chichester Bell. An early optical disc system existed in 1935, named Lichttonorgel, an early analog optical disc used for video recording was invented by David Paul Gregg in 1958 and patented in the US in 1961 and 1969. This form of optical disc was an early form of the DVD. It is of special interest that U. S, patent 4,893,297, filed 1989, issued 1990, generated royalty income for Pioneer Corporations DVA until 2007 —then encompassing the CD, DVD, and Blu-ray systems. In the early 1960s, the Music Corporation of America bought Greggs patents and his company, american inventor James T. Russell has been credited with inventing the first system to record a digital signal on an optical transparent foil which is lit from behind by a high-power halogen lamp. Russells patent application was first filed in 1966 and he was granted a patent in 1970, following litigation, Sony and Philips licensed Russells patents in the 1980s. Both Greggs and Russells disc are floppy media read in transparent mode, in the Netherlands in 1969, Philips Research physicist, Pieter Kramer invented an optical videodisc in reflective mode with a protective layer read by a focused laser beam U. S
8.
Infrared
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It extends from the nominal red edge of the visible spectrum at 700 nanometers, to 1000000 nm. Most of the radiation emitted by objects near room temperature is infrared. Like all EMR, IR carries radiant energy, and behaves both like a wave and like its quantum particle, the photon, slightly more than half of the total energy from the Sun was eventually found to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has an effect on Earths climate. Infrared radiation is emitted or absorbed by molecules when they change their rotational-vibrational movements and it excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines absorption and transmission of photons in the infrared range, Infrared radiation is used in industrial, scientific, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected, Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatuses. Thermal-infrared imaging is used extensively for military and civilian purposes, military applications include target acquisition, surveillance, night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm, Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers to 1 mm. This range of wavelengths corresponds to a range of approximately 430 THz down to 300 GHz. Below infrared is the portion of the electromagnetic spectrum. Sunlight, at a temperature of 5,780 kelvins, is composed of near thermal-spectrum radiation that is slightly more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level, of this energy,527 watts is infrared radiation,445 watts is visible light, and 32 watts is ultraviolet radiation. Nearly all the radiation in sunlight is near infrared, shorter than 4 micrometers. On the surface of Earth, at far lower temperatures than the surface of the Sun, almost all thermal radiation consists of infrared in mid-infrared region, much longer than in sunlight. Of these natural thermal radiation processes only lightning and natural fires are hot enough to produce much visible energy, thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wiens displacement law. Therefore, the band is often subdivided into smaller sections. Due to the nature of the blackbody radiation curves, typical hot objects, such as exhaust pipes, the three regions are used for observation of different temperature ranges, and hence different environments in space
9.
Neon
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Neon is a chemical element with symbol Ne and atomic number 10 and is a noble gas. Neon is a colorless, odorless, inert gas under standard conditions. It was discovered in 1898 as one of the three residual rare inert elements remaining in dry air, after nitrogen, oxygen, argon and carbon dioxide were removed. Neon was the second of three rare gases to be discovered, and was immediately recognized as a new element from its bright red emission spectrum. The name neon is derived from the Greek word, νέον, neuter singular form of νέος, Neon is chemically inert and forms no uncharged chemical compounds. The compounds of neon include ionic molecules, molecules held together by van der Waals forces and clathrates, during cosmic nucleogenesis of the elements, large amounts of neon are built up from the alpha-capture fusion process in stars. Although neon is a common element in the universe and solar system. It composes about 18.2 ppm of air by volume, the reason for neons relative scarcity on Earth and the inner planets is that neon is highly volatile and forms no compounds to fix it to solids. As a result, it escaped from the planetesimals under the warmth of the newly ignited Sun in the early Solar System, even the atmosphere of Jupiter is somewhat depleted of neon, presumably for this reason. It is also lighter than air, causing it to escape even from Earths atmosphere, Neon gives a distinct reddish-orange glow when used in low-voltage neon glow lamps, high-voltage discharge tubes and neon advertising signs. The red emission line from neon also causes the well known red light of helium–neon lasers, Neon is used in some plasma tube and refrigerant applications but has few other commercial uses. It is commercially extracted by the distillation of liquid air. Since air is the source, it is considerably more expensive than helium. Neon, was discovered in 1898 by the British chemists Sir William Ramsay, Neon was discovered when Ramsay chilled a sample of air until it became a liquid, then warmed the liquid and captured the gases as they boiled off. The gases nitrogen, oxygen, and argon had been identified, first to be identified was krypton. The next, after krypton had been removed, was a gas which gave a brilliant red light under spectroscopic discharge and this gas, identified in June, was named neon, the Greek analogue of novum, suggested by Ramsays son. A second gas was reported along with neon, having approximately the same density as argon but with a different spectrum – Ramsay. However, subsequent spectroscopic analysis revealed it to be contaminated with carbon monoxide
10.
Cathode
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A cathode is the electrode from which a conventional current leaves a polarized electrical device. A conventional current describes the direction in which positive electronic charges move, Electrons have a negative charge, so the movement of electrons is opposite to the conventional current flow. Consequently, the mnemonic cathode current departs also means that flow into the devices cathode. Cathode polarity with respect to the anode can be positive or negative and this outward current is carried internally by positive ions moving from the electrolyte to the positive cathode. It is continued externally by electrons moving inwards, this negative charge moving inwards constituting positive current flowing outwards, for example, the Daniell galvanic cells copper electrode is the positive terminal and the cathode. In a recharging battery, or an electrolytic cell performing electrolysis, for example, reversing the current direction in a Daniell galvanic cell would produce an electrolytic cell, where the copper electrode is the positive terminal and the anode. In a diode, the cathode is the terminal at the pointed end of the arrow symbol. Note, electrode naming for diodes is always based on the direction of the forward current, an electrode through which current flows the other way is termed an anode. The use of West to mean the out direction may appear unnecessarily contrived, previously, as related in the first reference cited above, Faraday had used the more straightforward term exode. The reference he used to effect was the Earths magnetic field direction. The flow of electrons is almost always from anode to cathode outside of the cell or device, regardless of the cell or device type, an exception is when a diode reverse-conducts, either by accident or by design. In chemistry, a cathode is the electrode of a cell at which reduction occurs. Another mnemonic is to note the cathode has a c, as does reduction, perhaps most useful would be to remember cathode corresponds to cation and anode corresponds to anion. The cathode can be negative like when the cell is electrolytic, the cathode supplies electrons to the positively charged cations which flow to it from the electrolyte. The cathodic current, in electrochemistry, is the flow of electrons from the interface to a species in solution. The anodic current is the flow of electrons into the anode from a species in solution, in an electrolytic cell, the cathode is where the negative polarity is applied to drive the cell. Common results of reduction at the cathode are hydrogen gas or pure metal from metal ions, when discussing the relative reducing power of two redox agents, the couple for generating the more reducing species is said to be more cathodic with respect to the more easily reduced reagent. When metal ions are reduced from ionic solution, they form a metal surface on the cathode
11.
Spectroscopy
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Spectroscopy /spɛkˈtrɒskəpi/ is the study of the interaction between matter and electromagnetic radiation. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, later the concept was expanded greatly to include any interaction with radiative energy as a function of its wavelength or frequency. Spectroscopic data is represented by an emission spectrum, a plot of the response of interest as a function of wavelength or frequency. Spectroscopy and spectrography are terms used to refer to the measurement of radiation intensity as a function of wavelength and are used to describe experimental spectroscopic methods. Spectral measurement devices are referred to as spectrometers, spectrophotometers, spectrographs or spectral analyzers, daily observations of color can be related to spectroscopy. Neon lighting is an application of atomic spectroscopy. Neon and other noble gases have characteristic emission frequencies, neon lamps use collision of electrons with the gas to excite these emissions. Inks, dyes and paints include chemical compounds selected for their characteristics in order to generate specific colors. A commonly encountered molecular spectrum is that of nitrogen dioxide, gaseous nitrogen dioxide has a characteristic red absorption feature, and this gives air polluted with nitrogen dioxide a reddish-brown color. Rayleigh scattering is a spectroscopic scattering phenomenon that accounts for the color of the sky, Spectroscopy is used in physical and analytical chemistry because atoms and molecules have unique spectra. As a result, these spectra can be used to detect, identify and quantify information about the atoms, Spectroscopy is also used in astronomy and remote sensing on earth. The measured spectra are used to determine the composition and physical properties of astronomical objects. One of the concepts in spectroscopy is a resonance and its corresponding resonant frequency. Resonances were first characterized in mechanical systems such as pendulums, mechanical systems that vibrate or oscillate will experience large amplitude oscillations when they are driven at their resonant frequency. A plot of amplitude vs. excitation frequency will have a peak centered at the resonance frequency and this plot is one type of spectrum, with the peak often referred to as a spectral line, and most spectral lines have a similar appearance. In quantum mechanical systems, the resonance is a coupling of two quantum mechanical stationary states of one system, such as an atom, via an oscillatory source of energy such as a photon. The coupling of the two states is strongest when the energy of the matches the energy difference between the two states. The energy of a photon is related to its frequency by E = h ν where h is Plancks constant, spectra of atoms and molecules often consist of a series of spectral lines, each one representing a resonance between two different quantum states
12.
Optical cavity
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An optical cavity, resonating cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a component of lasers, surrounding the gain medium. They are also used in optical parametric oscillators and some interferometers, light confined in the cavity reflects multiple times producing standing waves for certain resonance frequencies. Different resonator types are distinguished by the lengths of the two mirrors and the distance between them. The geometry must be chosen so that the beam remains stable, resonator types are also designed to meet other criteria such as minimum beam waist or having no focal point inside the cavity. Optical cavities are designed to have a large Q factor, a beam will reflect a large number of times with little attenuation. Therefore, the line width of the beam is very small indeed compared to the frequency of the laser. In general, radiation patterns which are reproduced on every round-trip of the light through the resonator are the most stable, the basic, or fundamental transverse mode of a resonator is a Gaussian beam. The most common types of optical cavities consist of two facing plane or spherical mirrors, the simplest of these is the plane-parallel or Fabry–Pérot cavity, consisting of two opposing flat mirrors. However, this problem is reduced for very short cavities with a small mirror separation distance. Plane-parallel resonators are therefore used in microchip and microcavity lasers. In these cases, rather than using separate mirrors, an optical coating may be directly applied to the laser medium itself. The plane-parallel resonator is also the basis of the Fabry–Pérot interferometer, for a resonator with two mirrors with radii of curvature R1 and R2, there are a number of common cavity configurations. If the two curvatures are equal to half the cavity length, a concentric or spherical resonator results and this type of cavity produces a diffraction-limited beam waist in the centre of the cavity, with large beam diameters at the mirrors, filling the whole mirror aperture. Similar to this is the cavity, with one plane mirror. A common and important design is the confocal resonator, with equal curvature mirrors equal to the cavity length. This design produces the smallest possible beam diameter at the cavity mirrors for a given cavity length, a concave-convex cavity has one convex mirror with a negative radius of curvature. A transparent dielectric sphere, such as a liquid droplet, also forms an optical cavity
