1.
Wavelength
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In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the waves shape repeats, and thus the inverse of the spatial frequency. Wavelength is commonly designated by the Greek letter lambda, the concept can also be applied to periodic waves of non-sinusoidal shape. The term wavelength is also applied to modulated waves. Wavelength depends on the medium that a wave travels through, examples of wave-like phenomena are sound waves, light, water waves and periodic electrical signals in a conductor. A sound wave is a variation in air pressure, while in light and other electromagnetic radiation the strength of the electric, water waves are variations in the height of a body of water. In a crystal lattice vibration, atomic positions vary, wavelength is a measure of the distance between repetitions of a shape feature such as peaks, valleys, or zero-crossings, not a measure of how far any given particle moves. For example, in waves over deep water a particle near the waters surface moves in a circle of the same diameter as the wave height. The range of wavelengths or frequencies for wave phenomena is called a spectrum, the name originated with the visible light spectrum but now can be applied to the entire electromagnetic spectrum as well as to a sound spectrum or vibration spectrum. In linear media, any pattern can be described in terms of the independent propagation of sinusoidal components. The wavelength λ of a sinusoidal waveform traveling at constant speed v is given by λ = v f, in a dispersive medium, the phase speed itself depends upon the frequency of the wave, making the relationship between wavelength and frequency nonlinear. In the case of electromagnetic radiation—such as light—in free space, the speed is the speed of light. Thus the wavelength of a 100 MHz electromagnetic wave is about, the wavelength of visible light ranges from deep red, roughly 700 nm, to violet, roughly 400 nm. For sound waves in air, the speed of sound is 343 m/s, the wavelengths of sound frequencies audible to the human ear are thus between approximately 17 m and 17 mm, respectively. Note that the wavelengths in audible sound are much longer than those in visible light, a standing wave is an undulatory motion that stays in one place. A sinusoidal standing wave includes stationary points of no motion, called nodes, the upper figure shows three standing waves in a box. The walls of the box are considered to require the wave to have nodes at the walls of the box determining which wavelengths are allowed, the stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities. Consequently, wavelength, period, and wave velocity are related just as for a traveling wave, for example, the speed of light can be determined from observation of standing waves in a metal box containing an ideal vacuum. In that case, the k, the magnitude of k, is still in the same relationship with wavelength as shown above
2.
Vacuum
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Vacuum is space void of matter. The word stems from the Latin adjective vacuus for vacant or void, an approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. In engineering and applied physics on the hand, vacuum refers to any space in which the pressure is lower than atmospheric pressure. The Latin term in vacuo is used to describe an object that is surrounded by a vacuum, the quality of a partial vacuum refers to how closely it approaches a perfect vacuum. Other things equal, lower gas pressure means higher-quality vacuum, for example, a typical vacuum cleaner produces enough suction to reduce air pressure by around 20%. Ultra-high vacuum chambers, common in chemistry, physics, and engineering, operate below one trillionth of atmospheric pressure, outer space is an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average. In the electromagnetism in the 19th century, vacuum was thought to be filled with a medium called aether, in modern particle physics, the vacuum state is considered the ground state of matter. Vacuum has been a frequent topic of debate since ancient Greek times. Evangelista Torricelli produced the first laboratory vacuum in 1643, and other techniques were developed as a result of his theories of atmospheric pressure. A torricellian vacuum is created by filling a glass container closed at one end with mercury. Vacuum became an industrial tool in the 20th century with the introduction of incandescent light bulbs and vacuum tubes. The recent development of human spaceflight has raised interest in the impact of vacuum on human health, the word vacuum comes from Latin an empty space, void, noun use of neuter of vacuus, meaning empty, related to vacare, meaning be empty. Vacuum is one of the few words in the English language that contains two consecutive letters u. Historically, there has been dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers debated the existence of a vacuum, or void, in the context of atomism, Aristotle believed that no void could occur naturally, because the denser surrounding material continuum would immediately fill any incipient rarity that might give rise to a void. Almost two thousand years after Plato, René Descartes also proposed a geometrically based alternative theory of atomism, without the problematic nothing–everything dichotomy of void, by the ancient definition however, directional information and magnitude were conceptually distinct. The explanation of a clepsydra or water clock was a topic in the Middle Ages. Although a simple wine skin sufficed to demonstrate a partial vacuum, in principle and he concluded that airs volume can expand to fill available space, and he suggested that the concept of perfect vacuum was incoherent. However, according to Nader El-Bizri, the physicist Ibn al-Haytham and the Mutazili theologians disagreed with Aristotle and Al-Farabi, using geometry, Ibn al-Haytham mathematically demonstrated that place is the imagined three-dimensional void between the inner surfaces of a containing body
3.
Microwave
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Microwaves are a form of electromagnetic radiation with wavelengths ranging from one meter to one millimeter, with frequencies between 300 MHz and 300 GHz. Different sources define different frequency ranges as microwaves, the broad definition includes both UHF and EHF bands. A more common definition in radio engineering is the range between 1 and 100 GHz, in all cases, microwaves include the entire SHF band at minimum. Frequencies in the range are often referred to by their IEEE radar band designations, S, C, X, Ku, K, or Ka band. The prefix micro- in microwave is not meant to suggest a wavelength in the micrometer range and it indicates that microwaves are small, compared to waves used in typical radio broadcasting, in that they have shorter wavelengths. The boundaries between far infrared, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. At the high end of the band they are absorbed by gases in the atmosphere, microwaves are extremely widely used in modern technology. Although at the low end of the band they can pass through building walls enough for useful reception, therefore on the surface of the Earth microwave communication links are limited by the visual horizon to about 30 -40 miles. Microwaves are absorbed by moisture in the atmosphere, and the attenuation increases with frequency, beginning at about 40 GHz, atmospheric gases also begin to absorb microwaves, so above this frequency microwave transmission is limited to a few kilometers. A spectral band structure causes absorption peaks at specific frequencies, in a microwave beam directed at an angle into the sky, a small amount of the power will be randomly scattered as the beam passes through the troposphere. A sensitive receiver beyond the horizon with a high gain antenna focused on that area of the troposphere can pick up the signal. This technique has been used at frequencies between 0.45 and 5 GHz in tropospheric scatter communication systems to communicate beyond the horizon and their short wavelength allows narrow beams of microwaves to be produced by conveniently small high gain antennas from a half meter to 5 meters in diameter. Therefore beams of microwaves are used for point-to-point communication links, an advantage of narrow beams is that they allow frequency reuse by nearby transmitters. Parabolic antennas are the most widely used directive antennas at microwave frequencies, flat microstrip antennas are being increasingly used in consumer devices. Where omnidirectional antennas are required, for example in wireless devices and Wifi routers for wireless LANs, small monopoles, dipole, or patch antennas are used. Due to the high cost and maintenance requirements of waveguide runs, the term microwave also has a more technical meaning in electromagnetics and circuit theory. As a consequence, practical microwave circuits tend to away from the discrete resistors, capacitors. Open-wire and coaxial transmission lines used at lower frequencies are replaced by waveguides and stripline, high-power microwave sources use specialized vacuum tubes to generate microwaves
4.
Radio astronomy
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Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an object was in 1932. Subsequent observations have identified a number of different sources of radio emission and these include stars and galaxies, as well as entirely new classes of objects, such as radio galaxies, quasars, pulsars, and masers. The discovery of the microwave background radiation, regarded as evidence for the Big Bang theory, was made through radio astronomy. Before Jansky observed the Milky Way in the 1930s, physicists speculated that radio waves could be observed from astronomical sources, in the 1860s, James Clerk Maxwells equations had shown that electromagnetic radiation is associated with electricity and magnetism, and could exist at any wavelength. These attempts were unable to detect any emission due to limitations of the instruments. The discovery of the radio reflecting ionosphere in 1902, led physicists to conclude that the layer would bounce any astronomical radio transmission back into space, Karl Jansky made the discovery of the first astronomical radio source serendipitously in the early 1930s. As an engineer with Bell Telephone Laboratories, he was investigating static that interfered with short wave transatlantic voice transmissions, using a large directional antenna, Jansky noticed that his analog pen-and-paper recording system kept recording a repeating signal of unknown origin. Since the signal peaked about every 24 hours, Jansky originally suspected the source of the interference was the Sun crossing the view of his directional antenna. Continued analysis showed that the source was not following the 24-hour daily cycle of the Sun exactly and he concluded that since the Sun were not large emitters of radio noise, the strange radio interference may be generated by interstellar gas and dust in the galaxy. Jansky announced his discovery in 1933 and he wanted to investigate the radio waves from the Milky Way in further detail, but Bell Labs reassigned him to another project, so he did no further work in the field of astronomy. His pioneering efforts in the field of astronomy have been recognized by the naming of the fundamental unit of flux density. Grote Reber was inspired by Janskys work, and built a radio telescope 9m in diameter in his backyard in 1937. He began by repeating Janskys observations, and then conducted the first sky survey in the radio frequencies, on February 27,1942, James Stanley Hey, a British Army research officer, made the first detection of radio waves emitted by the Sun. Later that year George Clark Southworth, at Bell Labs like Jansky, both researchers were bound by wartime security surrounding radar, so Reber, who was not, published his 1944 findings first. Several other people independently discovered solar radiowaves, including E. Schott in Denmark, at Cambridge University, where ionospheric research had taken place during World War II, J. A. This early research soon branched out into the observation of celestial radio sources. Martin Ryle and Antony Hewish at the Cavendish Astrophysics Group developed the technique of Earth-rotation aperture synthesis, the radio astronomy group in Cambridge went on to found the Mullard Radio Astronomy Observatory near Cambridge in the 1950s
5.
Cosmic dust
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Cosmic dust, or Extraterrestrial dust, is dust which exists in outer space, as well as all over planet Earth. Most cosmic dust particles are between a few molecules to 0.1 µm in size, a smaller fraction of all dust in space consists of larger refractory minerals that condensed as matter left the stars. It is called stardust and is included in a section below. The dust density falling to Earth is approximately 10−6/m3 with each grain having a mass between 10−16kg and 10−4 kg, cosmic dust can be further distinguished by its astronomical location, intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust. In the Solar System, interplanetary dust causes the zodiacal light, sources of Solar System dust include comet dust, asteroidal dust, dust from the Kuiper belt, and interstellar dust passing through the Solar System. The terminology has no application for describing materials found on the planet Earth except for dust that has demonstrably fallen to Earth. By one estimate, as much as 40,000 tons of cosmic dust reaches the Earths surface every year, in October 2011, scientists reported that cosmic dust contains complex organic matter that could be created naturally, and rapidly, by stars. In August 2014, scientists announced the collection of interstellar dust particles from the Stardust spacecraft since returning to Earth in 2006. In March 2017, scientists reported that extraterrestrial dust particles have been identified all over planet Earth. According to one of the researchers, “Once I knew what to look for, I found them everywhere. ”Cosmic dust was once solely an annoyance to astronomers, when infrared astronomy began, those dust particles were observed to be significant and vital components of astrophysical processes. Their analysis can reveal information about phenomena like the formation of the Solar System, for example, cosmic dust can drive the mass loss when a star is nearing the end of its life, play a part in the early stages of star formation, and form planets. In the Solar System, dust plays a role in the zodiacal light, Saturns B Ring spokes, the outer diffuse planetary rings at Jupiter, Saturn, Uranus and Neptune. These disparate research areas can be linked by the following theme, the astronomers accumulate observational ‘snapshots’ of dust at different stages of its life and, over time, form a more complete movie of the Universes complicated recycling steps. Parameters such as the initial motion, material properties, intervening plasma. Slightly changing any of these parameters can give significantly different dust dynamical behavior, therefore, one can learn about where that object came from, and what is the intervening medium. Cosmic dust can be detected by indirect methods utilizing the properties of cosmic dust. Cosmic dust can also be detected using a variety of collection methods. Estimates of the influx of extraterrestrial material entering the Earths atmosphere range between 5 and 300 tonnes
6.
Light
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Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to light, which is visible to the human eye and is responsible for the sense of sight. Visible light is defined as having wavelengths in the range of 400–700 nanometres, or 4.00 × 10−7 to 7.00 × 10−7 m. This wavelength means a range of roughly 430–750 terahertz. The main source of light on Earth is the Sun, sunlight provides the energy that green plants use to create sugars mostly in the form of starches, which release energy into the living things that digest them. This process of photosynthesis provides virtually all the used by living things. Historically, another important source of light for humans has been fire, with the development of electric lights and power systems, electric lighting has effectively replaced firelight. Some species of animals generate their own light, a process called bioluminescence, for example, fireflies use light to locate mates, and vampire squids use it to hide themselves from prey. Visible light, as all types of electromagnetic radiation, is experimentally found to always move at this speed in a vacuum. In physics, the term sometimes refers to electromagnetic radiation of any wavelength. In this sense, gamma rays, X-rays, microwaves and radio waves are also light, like all types of light, visible light is emitted and absorbed in tiny packets called photons and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality, the study of light, known as optics, is an important research area in modern physics. Generally, EM radiation, or EMR, is classified by wavelength into radio, microwave, infrared, the behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths, when EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. There exist animals that are sensitive to various types of infrared, infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, which is how these animals detect it, above the range of visible light, ultraviolet light becomes invisible to humans, mostly because it is absorbed by the cornea below 360 nanometers and the internal lens below 400. Furthermore, the rods and cones located in the retina of the eye cannot detect the very short ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses are able to detect ultraviolet, by quantum photon-absorption mechanisms, various sources define visible light as narrowly as 420 to 680 to as broadly as 380 to 800 nm
7.
Electron
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The electron is a subatomic particle, symbol e− or β−, with a negative elementary electric charge. Electrons belong to the first generation of the lepton particle family, the electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the include a intrinsic angular momentum of a half-integer value, expressed in units of the reduced Planck constant. As it is a fermion, no two electrons can occupy the same state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of particles and waves, they can collide with other particles and can be diffracted like light. Since an electron has charge, it has an electric field. Electromagnetic fields produced from other sources will affect the motion of an electron according to the Lorentz force law, electrons radiate or absorb energy in the form of photons when they are accelerated. Laboratory instruments are capable of trapping individual electrons as well as electron plasma by the use of electromagnetic fields, special telescopes can detect electron plasma in outer space. Electrons are involved in applications such as electronics, welding, cathode ray tubes, electron microscopes, radiation therapy, lasers, gaseous ionization detectors. Interactions involving electrons with other particles are of interest in fields such as chemistry. The Coulomb force interaction between the positive protons within atomic nuclei and the negative electrons without, allows the composition of the two known as atoms, ionization or differences in the proportions of negative electrons versus positive nuclei changes the binding energy of an atomic system. The exchange or sharing of the electrons between two or more atoms is the cause of chemical bonding. In 1838, British natural philosopher Richard Laming first hypothesized the concept of a quantity of electric charge to explain the chemical properties of atoms. Irish physicist George Johnstone Stoney named this charge electron in 1891, electrons can also participate in nuclear reactions, such as nucleosynthesis in stars, where they are known as beta particles. Electrons can be created through beta decay of isotopes and in high-energy collisions. The antiparticle of the electron is called the positron, it is identical to the electron except that it carries electrical, when an electron collides with a positron, both particles can be totally annihilated, producing gamma ray photons. The ancient Greeks noticed that amber attracted small objects when rubbed with fur, along with lightning, this phenomenon is one of humanitys earliest recorded experiences with electricity. In his 1600 treatise De Magnete, the English scientist William Gilbert coined the New Latin term electricus, both electric and electricity are derived from the Latin ēlectrum, which came from the Greek word for amber, ἤλεκτρον
8.
Galactic Center
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The Galactic Center is the rotational center of the Milky Way. The estimates for its range from 7.6 to 8.7 kiloparsecs from Earth in the direction of the constellations Sagittarius, Ophiuchus. There is strong evidence consistent with the existence of a black hole at the Galactic Center of the Milky Way. Because of interstellar dust along the line of sight, the Galactic Center cannot be studied at visible, the available information about the Galactic Center comes from observations at gamma ray, hard X-ray, infrared, sub-millimetre and radio wavelengths. In the early 1940s Walter Baade at Mount Wilson Observatory took advantage of wartime conditions in nearby Los Angeles to conduct a search for the center with the 100 inch Hooker Telescope. This gap has been known as Baades Window ever since, by 1954 they had built an 80 feet fixed dish antenna and used it to make a detailed study of an extended, extremely powerful belt of radio emission that was detected in Sagittarius. In 1958 the International Astronomical Union decided to adopt the position of Sagittarius A as the true zero co-ordinate point for the system of latitude and longitude. In the equatorial system the location is, RA 17h 45m 40. 04s. The exact distance between the Solar System and the Galactic Center is not certain, although estimates since 2000 have remained within the range 7. 2–8.8 kpc. The nature of the Milky Ways bar, which extends across the Galactic Center, is also debated, with estimates for its half-length. Certain authors advocate that the Milky Way features two bars, one nestled within the other. The bar is delineated by red-clump stars, however, RR Lyr variables do not trace a prominent Galactic bar. The bar may be surrounded by a called the 5-kpc ring that contains a large fraction of the molecular hydrogen present in the Milky Way. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way, accretion of gas onto the black hole, probably involving a disk around it, would release energy to power the radio source, itself much larger than the black hole. The latter is too small to see with present instruments, a study in 2008 which linked radio telescopes in Hawaii, Arizona and California measured the diameter of Sagittarius A* to be 44 million kilometers. For comparison, the radius of Earths orbit around the Sun is about 150 million kilometers, thus the diameter of the radio source is slightly less than the distance from Mercury to the Sun.3 million solar masses. On 5 January 2015, NASA reported observing an X-ray flare 400 times brighter than usual, the central cubic parsec around Sagittarius A* contains around 10 million stars. Although most of them are old red giant stars, the Galactic Center is also rich in massive stars, more than 100 OB and Wolf–Rayet stars have been identified there so far
9.
Hydrogen
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Hydrogen is a chemical element with chemical symbol H and atomic number 1. With a standard weight of circa 1.008, hydrogen is the lightest element on the periodic table. Its monatomic form is the most abundant chemical substance in the Universe, non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium, has one proton, the universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, since hydrogen readily forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays an important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a charge when it is known as a hydride. The hydrogen cation is written as though composed of a bare proton, Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. Industrial production is mainly from steam reforming natural gas, and less often from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production, mostly for the fertilizer market, Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks. Hydrogen gas is flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol,2 H2 + O2 →2 H2O +572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74%, the explosive reactions may be triggered by spark, heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C, the detection of a burning hydrogen leak may require a flame detector, such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames, the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a mixture of hydrogen to oxygen combined with carbon compounds from the airship skin. H2 reacts with every oxidizing element, the ground state energy level of the electron in a hydrogen atom is −13.6 eV, which is equivalent to an ultraviolet photon of roughly 91 nm wavelength. The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, however, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity. The most complicated treatments allow for the effects of special relativity
10.
Harvard University
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Although never formally affiliated with any denomination, the early College primarily trained Congregationalist and Unitarian clergy. Its curriculum and student body were gradually secularized during the 18th century, james Bryant Conant led the university through the Great Depression and World War II and began to reform the curriculum and liberalize admissions after the war. The undergraduate college became coeducational after its 1977 merger with Radcliffe College, Harvards $34.5 billion financial endowment is the largest of any academic institution. Harvard is a large, highly residential research university, the nominal cost of attendance is high, but the Universitys large endowment allows it to offer generous financial aid packages. Harvards alumni include eight U. S. presidents, several heads of state,62 living billionaires,359 Rhodes Scholars. To date, some 130 Nobel laureates,18 Fields Medalists, Harvard was formed in 1636 by vote of the Great and General Court of the Massachusetts Bay Colony. In 1638, it obtained British North Americas first known printing press, in 1639 it was named Harvard College after deceased clergyman John Harvard an alumnus of the University of Cambridge who had left the school £779 and his scholars library of some 400 volumes. The charter creating the Harvard Corporation was granted in 1650 and it offered a classic curriculum on the English university model—many leaders in the colony had attended the University of Cambridge—but conformed to the tenets of Puritanism. It was never affiliated with any denomination, but many of its earliest graduates went on to become clergymen in Congregational. The leading Boston divine Increase Mather served as president from 1685 to 1701, in 1708, John Leverett became the first president who was not also a clergyman, which marked a turning of the college toward intellectual independence from Puritanism. When the Hollis Professor of Divinity David Tappan died in 1803 and the president of Harvard Joseph Willard died a year later, in 1804, in 1846, the natural history lectures of Louis Agassiz were acclaimed both in New York and on the campus at Harvard College. Agassizs approach was distinctly idealist and posited Americans participation in the Divine Nature, agassizs perspective on science combined observation with intuition and the assumption that a person can grasp the divine plan in all phenomena. When it came to explaining life-forms, Agassiz resorted to matters of shape based on an archetype for his evidence. Charles W. Eliot, president 1869–1909, eliminated the position of Christianity from the curriculum while opening it to student self-direction. While Eliot was the most crucial figure in the secularization of American higher education, he was motivated not by a desire to secularize education, during the 20th century, Harvards international reputation grew as a burgeoning endowment and prominent professors expanded the universitys scope. Rapid enrollment growth continued as new schools were begun and the undergraduate College expanded. Radcliffe College, established in 1879 as sister school of Harvard College, Harvard became a founding member of the Association of American Universities in 1900. In the early 20th century, the student body was predominately old-stock, high-status Protestants, especially Episcopalians, Congregationalists, by the 1970s it was much more diversified
