Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word refers to visible light, the visible spectrum, 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, between the infrared and the ultraviolet. This wavelength means a frequency range of 430–750 terahertz; the main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars in the form of starches, which release energy into the living things that digest them; this process of photosynthesis provides all the energy used by living things. Another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has replaced firelight; some species of animals generate their own light, a process called bioluminescence.
For example, fireflies use light to locate mates, vampire squids use it to hide themselves from prey. The primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, polarization, while its speed in a vacuum, 299,792,458 metres per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation, is experimentally found to always move at this speed in a vacuum. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays and radio waves are light. Like all types of EM radiation, visible light propagates as waves. However, the energy imparted by the waves is absorbed at single locations the way particles are absorbed; the absorbed energy of the EM waves is called a photon, represents the quanta of light. When a wave of light is transformed and absorbed as a photon, the energy of the wave collapses to a single location, this location is where the photon "arrives."
This is. This dual wave-like and particle-like nature of light is known as the wave–particle duality; the study of light, known as optics, is an important research area in modern physics. EM radiation, or EMR, is classified by wavelength into radio waves, infrared, the visible spectrum that we perceive as light, ultraviolet, X-rays, gamma rays; the behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, 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. EMR in the visible light region consists of quanta that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humans because its photons no longer have enough individual energy to cause a lasting molecular change in the visual molecule retinal in the human retina, which change triggers the sensation of vision.
There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. 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, how these animals detect it. Above the range of visible light, ultraviolet light becomes invisible to humans because it is absorbed by the cornea below 360 nm and the internal lens below 400 nm. Furthermore, the rods and cones located in the retina of the human eye cannot detect the 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, in much the same chemical way that humans detect visible light. Various sources define visible light as narrowly as 420–680 nm to as broadly as 380–800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm.
Plant growth is affected by the color spectrum of light, a process known as photomorphogenesis. The speed of light in a vacuum is defined to be 299,792,458 m/s; the fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at this same speed in vacuum. Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit. However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s. Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849.
X-rays make up X-radiation, a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and longer than those of gamma rays. In many languages, X-radiation is referred to with terms meaning Röntgen radiation, after the German scientist Wilhelm Röntgen who discovered these on November 8, 1895, credited as its discoverer, who named it X-radiation to signify an unknown type of radiation. Spelling of X-ray in the English language includes the variants x-ray, X ray. Before their discovery in 1895 X-rays were just a type of unidentified radiation emanating from experimental discharge tubes, they were noticed by scientists investigating cathode rays produced by such tubes, which are energetic electron beams that were first observed in 1869. Many of the early Crookes tubes undoubtedly radiated X-rays, because early researchers noticed effects that were attributable to them, as detailed below.
Crookes tubes created free electrons by ionization of the residual air in the tube by a high DC voltage of anywhere between a few kilovolts and 100 kV. This voltage accelerated the electrons coming from the cathode to a high enough velocity that they created X-rays when they struck the anode or the glass wall of the tube; the earliest experimenter thought to have produced. In 1785 he presented a paper to the Royal Society of London describing the effects of passing electrical currents through a evacuated glass tube, producing a glow created by X-rays; this work was further explored by his assistant Michael Faraday. When Stanford University physics professor Fernando Sanford created his "electric photography" he unknowingly generated and detected X-rays. From 1886 to 1888 he had studied in the Hermann Helmholtz laboratory in Berlin, where he became familiar with the cathode rays generated in vacuum tubes when a voltage was applied across separate electrodes, as studied by Heinrich Hertz and Philipp Lenard.
His letter of January 6, 1893 to The Physical Review was duly published and an article entitled Without Lens or Light, Photographs Taken With Plate and Object in Darkness appeared in the San Francisco Examiner. Starting in 1888, Philipp Lenard, a student of Heinrich Hertz, conducted experiments to see whether cathode rays could pass out of the Crookes tube into the air, he built a Crookes tube with a "window" in the end made of thin aluminum, facing the cathode so the cathode rays would strike it. He found that something came through, that would cause fluorescence, he measured the penetrating power of these rays through various materials. It has been suggested that at least some of these "Lenard rays" were X-rays. In 1889 Ukrainian-born Ivan Pulyui, a lecturer in experimental physics at the Prague Polytechnic who since 1877 had been constructing various designs of gas-filled tubes to investigate their properties, published a paper on how sealed photographic plates became dark when exposed to the emanations from the tubes.
Hermann von Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Röntgen made his announcement, it was formed on the basis of the electromagnetic theory of light. However, he did not work with actual X-rays. In 1894 Nikola Tesla noticed damaged film in his lab that seemed to be associated with Crookes tube experiments and began investigating this radiant energy of "invisible" kinds. After Röntgen identified the X-ray, Tesla began making X-ray images of his own using high voltages and tubes of his own design, as well as Crookes tubes. On November 8, 1895, German physics professor Wilhelm Röntgen stumbled on X-rays while experimenting with Lenard tubes and Crookes tubes and began studying them, he wrote an initial report "On a new kind of ray: A preliminary communication" and on December 28, 1895 submitted it to Würzburg's Physical-Medical Society journal. This was the first paper written on X-rays. Röntgen referred to the radiation as "X"; the name stuck.
They are still referred to as such in many languages, including German, Danish, Swedish, Estonian, Japanese, Georgian and Norwegian. Röntgen received the first Nobel Prize in Physics for his discovery. There are conflicting accounts of his discovery because Röntgen had his lab notes burned after his death, but this is a reconstruction by his biographers: Röntgen was investigating cathode rays from a Crookes tube which he had wrapped in black cardboard so that the visible light from the tube would not interfere, using a fluorescent screen painted with barium platinocyanide, he noticed a faint green glow from the screen, about 1 meter away. Röntgen realized some invisible rays coming from the tube were passing through the cardboard to make the screen glow, he found they could pass through books and papers on his desk. Röntgen threw himself into investigating these unknown rays systematically. Two months after his initial discovery, he published his paper. Röntgen discovered their medical use when he made a picture of his wife's hand on a photographic plate formed due to X-rays.
The photograph of his wife's hand was the first photograph of a human body part using X-rays. When she saw the picture, she said "I have seen my death."The discovery of X-rays stimul
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: electromagnetic radiation, such as radio waves, infrared, visible light, ultraviolet, x-rays, gamma radiation particle radiation, such as alpha radiation, beta radiation, neutron radiation acoustic radiation, such as ultrasound and seismic waves gravitational radiation, radiation that takes the form of gravitational waves, or ripples in the curvature of spacetime. Radiation is categorized as either ionizing or non-ionizing depending on the energy of the radiated particles. Ionizing radiation carries more than 10 eV, enough to ionize atoms and molecules, break chemical bonds; this is an important distinction due to the large difference in harmfulness to living organisms. A common source of ionizing radiation is radioactive materials that emit α, β, or γ radiation, consisting of helium nuclei, electrons or positrons, photons, respectively.
Other sources include X-rays from medical radiography examinations and muons, positrons and other particles that constitute the secondary cosmic rays that are produced after primary cosmic rays interact with Earth's atmosphere. Gamma rays, X-rays and the higher energy range of ultraviolet light constitute the ionizing part of the electromagnetic spectrum; the word "ionize" refers to the breaking of one or more electrons away from an atom, an action that requires the high energies that these electromagnetic waves supply. Further down the spectrum, the non-ionizing lower energies of the lower ultraviolet spectrum cannot ionize atoms, but can disrupt the inter-atomic bonds which form molecules, thereby breaking down molecules rather than atoms; the waves of longer wavelength than UV in visible light and microwave frequencies cannot break bonds but can cause vibrations in the bonds which are sensed as heat. Radio wavelengths and below are not regarded as harmful to biological systems; these are not sharp delineations of the energies.
The word radiation arises from the phenomenon of waves radiating from a source. This aspect leads to a system of measurements and physical units that are applicable to all types of radiation; because such radiation expands as it passes through space, as its energy is conserved, the intensity of all types of radiation from a point source follows an inverse-square law in relation to the distance from its source. Like any ideal law, the inverse-square law approximates a measured radiation intensity to the extent that the source approximates a geometric point. Radiation with sufficiently high energy can ionize atoms. Ionization occurs when an electron is stripped from an electron shell of the atom, which leaves the atom with a net positive charge; because living cells and, more the DNA in those cells can be damaged by this ionization, exposure to ionizing radiation is considered to increase the risk of cancer. Thus "ionizing radiation" is somewhat artificially separated from particle radiation and electromagnetic radiation due to its great potential for biological damage.
While an individual cell is made of trillions of atoms, only a small fraction of those will be ionized at low to moderate radiation powers. The probability of ionizing radiation causing cancer is dependent upon the absorbed dose of the radiation, is a function of the damaging tendency of the type of radiation and the sensitivity of the irradiated organism or tissue. If the source of the ionizing radiation is a radioactive material or a nuclear process such as fission or fusion, there is particle radiation to consider. Particle radiation is subatomic particle accelerated to relativistic speeds by nuclear reactions; because of their momenta they are quite capable of knocking out electrons and ionizing materials, but since most have an electrical charge, they don't have the penetrating power of ionizing radiation. The exception is neutron particles. There are several different kinds of these particles, but the majority are alpha particles, beta particles and protons. Speaking and particles with energies above about 10 electron volts are ionizing.
Particle radiation from radioactive material or cosmic rays invariably carries enough energy to be ionizing. Most ionizing radiation originates from radioactive materials and space, as such is present in the environment, since most rocks and soil have small concentrations of radioactive materials. Since this radiation is invisible and not directly detectable by human senses, instruments such as Geiger counters are required to detect its presence. In some cases, it may lead to secondary emission of visible light upon its interaction with matter, as in the case of Cherenkov radiation and radio-luminescence. Ionizing radiation has many practical uses in medicine and construction, but presents a health hazard if used improperly. Exposure to radiation causes damage to living tissue.
Alan Paige Lightman is an American physicist and social entrepreneur. He has served on the faculties of Harvard and Massachusetts Institute of Technology and is professor of the practice of the humanities at MIT, he is known as the author of the international bestseller Einstein's Dreams. Einstein's Dreams has been adapted into dozens of independent theatrical productions and is one of the most used "common books" on college campuses. Lightman's novel. Lightman was the first professor at MIT to receive a joint appointment in the sciences and the humanities, he is the recipient of five honorary degrees. He is the founder of the Harpswell Foundation, a nonprofit organization whose mission is to advance a new generation of women leaders in Southeast Asia. Lightman was born into a white, upper-middle-class, Jewish family in Memphis and grew up there during the racially divided and inflamed 1950s and 1960s, his paternal great grandfather, immigrated from Hungary to the U. S. in settled in Nashville. Uneducated, “Papa Joe Lightman” started a stone quarry and construction business and built some of the prominent public buildings in Nashville.
Papa Joe’s son, M. A. Lightman's paternal grandfather, started buying movie theaters in the South in 1916, during the silent-film era, created a movie theater circuit spanning half a dozen southern states. M. A. was a larger than life figure. At age forty three, he swam across the Mississippi River. For a number of years, he was president of the Motion Picture Theater Owners of America, he devoted himself to civic action and, among many other activities, was president of the Jewish Welfare Fund and head of fund raising for the all-black Collins Chapel Hospital in Memphis. M. A.’s wife, graduated from the University of Kentucky in Lexington and was well read and kept many books in the house. Lightman’s maternal grandfather, David Garretson, dropped out of school in the eighth grade to support his family after his father died at a young age. David began sweeping the floors of the Crescent Box Factory in New Orleans and rose to become owner and president of the factory. David’s wife, Hattie Levy, was graduated from Wellesley College in 1920 and, for years, would mail young Alan a scrap of paper each week with an obscure new vocabulary word for him to look up and report back to her.
Lightman’s father, second son of M. A. was artistically inclined. He worked. In the early 1960s, Richard played a key role in the civil rights movement by being the first movie theater owner in Memphis to integrate his theaters, only the second business of any kind to do so in that pivotal city. Lightman’s mother, was a ballroom dance teacher and a volunteer Braille typist, making books available to the blind. Much of the above family history can be found in Lightman's memoir Screening Room. From an early age, Lightman was interested in the arts. While in high school, he began writing poetry, his combination of talents in both science and creative writing drew attention as he won city and statewide science fairs as well as won the statewide creative writing competition from the National Council of Teachers of English. He graduated from White Station High School in Memphis. Lightman received his AB degree in physics from Princeton University in 1970, magna cum laude, where he was Phi Beta Kappa and won the Kusaka Memorial Prize in Physics for his senior thesis.
In 1976, Lightman married Jean Greenblatt, a painter and the first female president of the Boston Guild of Artists in that organization's 100+ year history. Alan and Jean have two daughters and Kara. Lightman earned his PhD in theoretical physics from the California Institute of Technology in 1974, where he had received a National Science Foundation pre-doctoral fellowship, his thesis advisor was relativist Kip Thorne. From 1974 to 1976, Lightman was a postdoctoral fellow in astrophysics at Cornell University, he became an Assistant Professor of astronomy at Harvard University from 1976 to 1979 and from 1979 to 1989 a research scientist at the Harvard-Smithsonian Center for Astrophysics. In 1989, Lightman was appointed professor of science and writing and senior lecturer in physics at the Massachusetts Institute of Technology, he was the first professor at MIT to receive a joint appointment in the humanities. In 1995, he was appointed John Burchard Professor of Humanities at MIT, a position that he resigned in 2002 to allow himself more time for writing.
In the late 1990s, Lightman chaired a committee at MIT that established a new Communication Requirement requiring each undergraduate to have a writing and speaking course each of his or her four years at MIT. He teaches at Massachusetts Institute of Technology as Professor of the Practice of the Humanities. In his scientific work, Lightman has made fundamental contributions to the theory of astrophysical processes under extreme temperatures and densities. In particular, his research has focused on relativistic gravitation theory, the structure and behavior of accretion disks, stellar dynamics, radiative processes, relativistic plasmas; some of his significant achievements are his discovery, with Douglas Eardley, of a structural instability in orbiting disks of matter, called accretion disks, that form around massive condensed objects such as black holes, with wide application in astronomy.
Radiology is the medical specialty that uses medical imaging to diagnose and treat diseases within the human body. A variety of imaging techniques such as X-ray radiography, computed tomography, nuclear medicine including positron emission tomography, magnetic resonance imaging are used to diagnose or treat diseases. Interventional radiology is the performance of minimally invasive medical procedures with the guidance of imaging technologies such as X-ray radiography, computed tomography, nuclear medicine including positron emission tomography, magnetic resonance imaging; the modern practice of radiology involves several different healthcare professions working as a team. The radiologist is a medical doctor who has completed the appropriate post-graduate training and interprets medical images, communicates these findings to other physicians by means of a report or verbally, uses imaging to perform minimally invasive medical procedures; the nurse is involved in the care of patients before and after imaging or procedures, including administration of medications, monitoring of vital signs and monitoring of sedated patients.
The radiographer known as a "radiologic technologist" in some countries such as the United States, is a specially trained healthcare professional that uses sophisticated technology and positioning techniques to produce medical images for the radiologist and nurse to interpret. Depending on the individual's training and country of practice, the radiographer may specialize in one of the above-mentioned imaging modalities or have expanded roles in image reporting. Radiographs are produced by transmitting X-rays through a patient; the X-rays are projected through the body onto a detector. Röntgen discovered X-rays on November 8, 1895 and received the first Nobel Prize in Physics for their discovery in 1901. In film-screen radiography, an X-ray tube generates a beam of X-rays, aimed at the patient; the X-rays that pass through the patient are filtered through a device called an grid or X-ray filter, to reduce scatter, strike an undeveloped film, held to a screen of light-emitting phosphors in a light-tight cassette.
The film is developed chemically and an image appears on the film. Film-screen radiography is being replaced by phosphor plate radiography but more by digital radiography and the EOS imaging. In the two latest systems, the X-rays strike sensors that converts the signals generated into digital information, transmitted and converted into an image displayed on a computer screen. In digital radiography the sensors shape a plate, but in the EOS system, a slot-scanning system, a linear sensor vertically scans the patient. Plain radiography was the only imaging modality available during the first 50 years of radiology. Due to its availability and lower costs compared to other modalities, radiography is the first-line test of choice in radiologic diagnosis. Despite the large amount of data in CT scans, MR scans and other digital-based imaging, there are many disease entities in which the classic diagnosis is obtained by plain radiographs. Examples include various types of arthritis and pneumonia, bone tumors, congenital skeletal anomalies, etc.
Mammography and DXA are two applications of low energy projectional radiography, used for the evaluation for breast cancer and osteoporosis, respectively. Fluoroscopy and angiography are special applications of X-ray imaging, in which a fluorescent screen and image intensifier tube is connected to a closed-circuit television system; this augmented with a radiocontrast agent. Radiocontrast agents are administered by swallowing or injecting into the body of the patient to delineate anatomy and functioning of the blood vessels, the genitourinary system, or the gastrointestinal tract. Two radiocontrast agents are presently in common use. Barium sulfate is given rectally for evaluation of the GI tract. Iodine, in multiple proprietary forms, is given by oral, vaginal, intra-arterial or intravenous routes; these radiocontrast agents absorb or scatter X-rays, in conjunction with the real-time imaging, allow demonstration of dynamic processes, such as peristalsis in the digestive tract or blood flow in arteries and veins.
Iodine contrast may be concentrated in abnormal areas more or less than in normal tissues and make abnormalities more conspicuous. Additionally, in specific circumstances, air can be used as a contrast agent for the gastrointestinal system and carbon dioxide can be used as a contrast agent in the venous system. CT imaging uses X-rays in conjunction with computing algorithms to image the body. In CT, an X-ray tube opposite an X-ray detector in a ring-shaped apparatus rotate around a patient, producing a computer-generated cross-sectional image. CT is acquired in the axial plane, with coronal and sagittal images produced by computer reconstruction. Radiocontrast agents are used with CT for enhanced delineation of anatomy. Although radiographs provide higher spatial resolution, CT can detect more subtle variations in attenuation of X-rays. CT exposes the patient to more ionizing radiation than a radiograph. Spiral multidetector CT uses 16, 64, 254 o
A mirror is an object that reflects light in such a way that, for incident light in some range of wavelengths, the reflected light preserves many or most of the detailed physical characteristics of the original light, called specular reflection. This is different from other light-reflecting objects that do not preserve much of the original wave signal other than color and diffuse reflected light, such as flat-white paint; the most familiar type of mirror is the plane mirror. Curved mirrors are used, to produce magnified or diminished images or focus light or distort the reflected image. Mirrors are used for personal grooming or admiring oneself, for viewing the area behind and on the sides on motor vehicles while driving, for decoration, architecture. Mirrors are used in scientific apparatus such as telescopes and lasers and industrial machinery. Most mirrors are designed for visible light. There are many types of glass mirrors, each representing a different manufacturing process and reflection type.
An aluminium glass mirror is made of a float glass manufactured using vacuum coating, i.e. aluminium powder is evaporated onto the exposed surface of the glass in a vacuum chamber and coated with two or more layers of waterproof protective paint. A low aluminium glass mirror is manufactured by coating silver and two layers of protective paint on the back surface of glass. A low aluminium glass mirror is clear, light transmissive and reflects accurate natural colors; this type of glass is used for framing presentations and exhibitions in which a precise color representation of the artwork is essential or when the background color of the frame is predominantly white. A safety glass mirror is made by adhering a special protective film to the back surface of a silver glass mirror, which prevents injuries in case the mirror is broken; this kind of mirror is used for furniture, glass walls, commercial shelves, or public areas. A silkscreen printed glass mirror is produced using inorganic color ink that prints patterns through a special screen onto glass.
Various colors and glass shapes are available. Such a glass mirror is durable and more moisture resistant than ordinary printed glass and can serve for over 20 years; this type of glass is used for decorative purposes. A silver glass mirror is an ordinary mirror, coated on its back surface with silver, which produces images by reflection; this kind of glass mirror is produced by coating a silver, copper film and two or more layers of waterproof paint on the back surface of float glass, which resists acid and moisture. A silver glass mirror provides clear and actual images, is quite durable, is used for furniture and other decorative purposes. Decorative glass mirrors are handcrafted. A variety of shades and glass thickness are available. A beam of light reflects off a mirror at an angle of reflection equal to its angle of incidence; that is, if the beam of light is shining on a mirror's surface, at a θ ° angle vertically it reflects from the point of incidence at a θ ° angle, vertically in the opposite direction.
This law mathematically follows from the interference of a plane wave on a flat boundary. In a plane mirror, a parallel beam of light changes its direction as a whole, while still remaining parallel. In a concave mirror, parallel beams of light become a convergent beam, whose rays intersect in the focus of the mirror. Known as converging mirror In a convex mirror, parallel beams become divergent, with the rays appearing to diverge from a common point of intersection "behind" the mirror. Spherical concave and convex mirrors do not focus parallel rays to a single point due to spherical aberration. However, the ideal of focusing to a point is a used approximation. Parabolic reflectors resolve this. Parabolic reflectors are not suitable for imaging nearby objects because the light rays are not parallel. Objects viewed in a mirror will appear not vertically inverted. However, a mirror does not "swap" left and right any more than it swaps top and bottom. A mirror reverses the forward/backward axis. To be precise, it reverses the object in the direction perpendicular to the mirror surface.
Because left and right are defined relative to front-back and top-bottom, the "flipping" of front and back results in the perception of a left-right reversal in the image. Looking at an image of oneself with the front-back axis flipped results in the perception of an image with its left-right axis flipped; when reflected in the mirror, your right hand remains directly opposite your real right hand, but it is perceived as the left hand of your image. When a person looks into a mirror, the image is front-back reversed, an effect similar to the holl