Snippets (text quotes and extracts from authoritative sources)

A Snippet is a short quote or extract (typically a phrase, a sentence, or at most a few sentences) from an authoritative source document such as a specification, technical manual, or design manual. Throughout this site, content is often related to supporting Snippets and each Snippet page links back to the content pages that reference it! The Snippet and Note concepts are very closely related and they support each other.

The Snippet concept is also at the heart of the Parsing Analysis recipe for UML® and SysML®

Kind Snippet quote/extract Source UML keywords SysML keywords Keywords
INFO Schematic of a Keplerian refracting telescope. The arrow at (4) is a (notional) representation of the original image; the arrow at (5) is the inverted image at the focal plane; the arrow at (6) is the virtual image that forms in the viewer's visual sphere Wikipedia Keplerian refractor, telescope, refracting telescope, optical telescope
INFO rays that enter the system parallel to the optical axis are focused such that they pass through the rear focal point Wikipedia optics, principal plane, optic axis, focal point, focal length
INFO A principal focus or focal point is a special focus: For a lens, or a spherical or parabolic mirror, it is a point onto which collimated light parallel to the axis is focused. Wikipedia optics, principal focus, focal point, mirror, lens, spherical mirror, parabolic mirror
INFO If the medium surrounding the optical system has a refractive index of 1 (e.g., air or vacuum), then the distance from the principal planes to their corresponding focal points is just the focal length of the system. Wikipedia optics, principal plane, optic axis, focal point, focal length
INFO The front and rear (or back) focal planes are defined as the planes, perpendicular to the optic axis, which pass through the front and rear focal points. Wikipedia optics, focal plane, rear focal plane, optic axis, focal point
INFO An object infinitely far from the optical system forms an image at the rear focal plane. Wikipedia optics, focal plane, rear focal plane
INFO Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by aberrations of the imaging optics. Wikipedia optics, image, light ray, focus point, focus, image point, blur circle
INFO In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Wikipedia optics, image, light ray, focus point, focus, image point
INFO A real image occurs where rays converge, whereas a virtual image occurs where rays only appear to diverge. Wikipedia optics, image, light ray, focus point, virtual image, real image
INFO In ray diagrams ... real rays of light are always represented by full, solid lines; perceived or extrapolated rays of light are represented by dashed lines. Wikipedia optics, image, light ray, focus point, virtual image, real image
INFO A real image ... is an image which is located in the plane of convergence for the light rays that originate from a given object. Wikipedia optics, image, light ray, focus point, virtual image
INFO A converging lens (one that is thicker in the middle than at the edges) or a concave mirror is also capable of producing a virtual image if the object is within the focal length. Such an image will be magnified. Wikipedia optics, image, light ray, focus point, virtual image
INFO A diverging lens (one that is thicker at the edges than the middle) or a convex mirror forms a virtual image. Such an image is reduced in size when compared to the original object. Wikipedia optics, image, light ray, focus point, virtual image
INFO Because the rays never really converge, a virtual image cannot be projected onto a screen. In contrast, a real image can be projected on the screen as it is formed by rays that converge on a real location. Wikipedia optics, image, light ray, focus point, real image, virtual image
INFO In diagrams of optical systems, virtual rays are conventionally represented by dotted lines. Wikipedia optics, image, light ray, focus point, real image, virtual image
INFO a virtual image is found by tracing real rays that emerge from an optical device (lens, mirror, or some combination) backward to perceived or apparent origins of ray divergences. Wikipedia optics, image, light ray, focus point, real image, virtual image
INFO A real image is the collection of focus points actually made by converging rays, while a virtual image is the collection of focus points made by extensions of diverging rays. Wikipedia optics, image, light ray, focus point, real image, virtual image
INFO In optics, an image is defined as the collection of focus points of light rays coming from an object. Wikipedia optics, image, light ray, focus point
INFO The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece. Wikipedia objective
INFO The objective lens or mirror collects light and brings it to focus creating an image. Wikipedia objective
INFO Objectives can be a single lens or mirror, or combinations of several optical elements. Wikipedia optical astronomy, optics, objective, lens, mirror, optical telescope
INFO In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Wikipedia optical astronomy, optics, objective, light, light ray
INFO For example, in a telescope, the aperture stop is typically the edges of the objective lens or mirror (or of the mount that holds it). One then speaks of a telescope as having, for example, a 100-centimeter aperture. Wikipedia optical telescope, telescope, aperture, optics, astronomy, astrophotography
INFO In some contexts, especially in photography and astronomy, aperture refers to the diameter of the aperture stop rather than the physical stop or the opening itself. Wikipedia optical telescope, telescope, aperture, optics, astronomy, astrophotography
INFO A telescope's ability to resolve small detail is directly related to the diameter (or aperture) of its objective (the primary lens or mirror that collects and focuses the light), and its light gathering power is related to the area of the objective. Wikipedia optical telescope, telescope, light, image, lens, mirror, inverting telescope
INFO There are telescope designs that do not present an inverted image such as the Galilean refractor and the Gregorian reflector. These are referred to as erecting telescopes. Wikipedia optical telescope, telescope, light, image, lens, mirror, erecting telescope
INFO Most telescope designs produce an inverted image at the focal plane; these are referred to as inverting telescopes. Wikipedia optical telescope, telescope, light, image, lens, mirror, inverting telescope
INFO This image may be recorded or viewed through an eyepiece, which acts like a magnifying glass. The eye then sees an inverted [DISPUTED] magnified virtual image of the object. Wikipedia optical telescope, telescope, light, image, lens, mirror
INFO The basic scheme is that the primary light-gathering element, the objective (the convex lens or concave mirror used to gather the incoming light), focuses that light from the distant object to a focal plane where it forms a real image. Wikipedia optical telescope, telescope, light, image, lens, mirror
INFO An optical telescope is a telescope that gathers and focuses light, mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct view, or to make a photograph, or to collect data through electronic image sensors. Wikipedia optical telescope, telescope, light, image
INFO Under ideal laboratory conditions, people can see infrared up to at least 1050 nm; children and young adults may perceive ultraviolet wavelengths down to about 310–313 nm. Wikipedia visible light, infrared, ultraviolet
INFO Various sources define visible light as narrowly as 420–680 nm to as broadly as 380–800 nm. Wikipedia visible light
INFO It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of ​1⁄299792458 second. Wikipedia light
INFO The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is defined as 299792458 metres per second (approximately 300000 km/s, or 186000 mi/s) Wikipedia light
INFO The value of c can ... be found by using the relation c = fλ. Wikipedia light, wavelength, frequency, radio frequency
INFO Near Infrared: 2.0 to 2.4: Wavelength (micrometres): K band: Most major optical telescopes and most dedicated infrared telescopes Wikipedia infrared, infrared astronomy, near infrared
INFO Many optical telescopes, such as those at Keck Observatory, operate effectively in the near infrared as well as at visible wavelengths. Wikipedia far infrared, near infrared, infrared astronomy
INFO For this reason, the near infrared region of the spectrum is commonly incorporated as part of the "optical" spectrum, along with the near ultraviolet. Wikipedia far infrared, near infrared, infrared astronomy
INFO Infrared radiation with wavelengths just longer than visible light, known as near-infrared, behaves in a very similar way to visible light, and can be detected using similar solid state devices ... Wikipedia far infrared, near infrared, infrared astronomy
INFO Far infrared (FIR) is a region in the infrared spectrum of electromagnetic radiation. Far infrared is often defined as any radiation with a wavelength of 15 micrometers (μm) to 1 mm Wikipedia infrared, infrared astronomy, far infrared
INFO There are no precisely defined boundaries between the bands of the electromagnetic spectrum; rather they fade into each other like the bands in a rainbow (which is the sub-spectrum of visible light). Wikipedia electromagnetic radiation, electromagnetic spectrum, visible light, light
INFO Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency of approximately 30 PHz) to 400 nm (750 THz), shorter than that of visible light but longer than X-rays. Wikipedia UV radiation, electromagnetic radiation, ultraviolet
INFO Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around 20 kHz to around 300 GHz. Wikipedia radio astronomy, radio frequency, RF
INFO The wavelength of infrared light ranges from 0.75 to 300 micrometers. Infrared falls in between visible radiation, which ranges from 380 to 750 nanometers, and submillimeter waves. Wikipedia infrared, infrared astronomy, mirror, lens
INFO Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared (IR) radiation. Wikipedia infrared, infrared astronomy, mirror, lens
INFO Infrared and optical astronomy are often practiced using the same telescopes, as the same mirrors or lenses are usually effective over a wavelength range that includes both visible and infrared light. Wikipedia infrared, infrared astronomy, mirror, lens
INFO Optical radiation is part of the electromagnetic spectrum. It is subdivided into ultraviolet radiation (UV), the spectrum of light visible for man (VIS) and infrared radiation (IR). It ranges between wavelengths of 100 nm to 1 mm [DISPUTED] Wikipedia optical radiation, optical astronomy, optics, [DISPUTED]
INFO A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. Wikipedia telescope, optical telescope, astronomy
INFO In the 20th century, many new types of telescopes were invented, including radio telescopes in the 1930s and infrared telescopes in the 1960s. Wikipedia telescope, optical telescope, astronomy
INFO The reflecting telescope, which uses mirrors to collect and focus light, was invented within a few decades of the first refracting telescope. Wikipedia telescope, optical telescope, astronomy
INFO An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. Wikipedia eyepiece, ocular lens, lens
INFO All refracting telescopes use the same principles. The combination of an objective lens and some type of eyepiece is used to gather more light than the human eye is able to collect on its own, focus it, and present the viewer with a brighter, clearer, ... Wikipedia
INFO They were used for both terrestrial applications and astronomy. Wikipedia telescope, optical telescope, astronomy
INFO The first known practical telescopes were refracting telescopes invented in the Netherlands at the beginning of the 17th century, by using glass lenses. Wikipedia telescope, optical telescope, astronomy
INFO The word telescope now refers to a wide range of instruments capable of detecting different regions of the electromagnetic spectrum, and in some cases other types of detectors. Wikipedia telescope, optical telescope, astronomy
INFO A telescope is an optical instrument using lenses, curved mirrors [DISPUTED], or a combination of both to observe distant objects ... Wikipedia telescope, optical telescope, astronomy
INFO The atomic number or proton number (symbol Z) of a chemical element is the number of protons found in the nucleus of every atom of that element. Wikipedia
INFO Protons and neutrons are both nucleons, which may be bound together by the nuclear force to form atomic nuclei. Wikipedia
INFO A neutron contains two down quarks with charge −​1⁄3 e and one up quark with charge +​2⁄3 e. Wikipedia neutron
INFO The finite size of the neutron and its magnetic moment both indicate that the neutron is a composite, rather than elementary, particle. Wikipedia neutron
INFO The neutron is classified as a hadron, because it is a composite particle made of quarks. The neutron is also classified as a baryon, because it is composed of three valence quarks. Wikipedia neutron
INFO The neutron has a magnetic moment, however, so the neutron is influenced by magnetic fields. The neutron's magnetic moment has a negative value, because its orientation is opposite to the neutron's spin. Wikipedia neutron
INFO The neutron has no measurable electric charge. With its positive electric charge, the proton is directly influenced by electric fields, whereas the neutron is unaffected by electric fields. Wikipedia neutron
INFO The neutron has a mean square radius of about 0.8×10−15 m, or 0.8 fm, and it is a spin-½ fermion. Wikipedia neutron
INFO The free neutron has a mass of 939,565,413.3 eV/c2, or 1.674927471×10−27 kg, or 1.00866491588 u. Wikipedia neutron
INFO Beta decay, in which neutrons decay to protons, or vice versa, is governed by the weak force, and it requires the emission or absorption of electrons and neutrinos, or their antiparticles. Wikipedia neutron
INFO Neutrons or protons bound in a nucleus can be stable or unstable, however, depending on the nuclide. Wikipedia neutron
INFO The free proton is stable. Wikipedia neutron
INFO This radioactive decay, known as beta decay, is possible because the mass of the neutron is slightly greater than the proton. Wikipedia neutron
INFO A free neutron is unstable, decaying to a proton, electron and antineutrino with a mean lifetime of just under 15 minutes (881.5±1.5 s). Wikipedia neutron
INFO Since protons and neutrons behave similarly within the nucleus, and each has a mass of approximately one atomic mass unit, they are both referred to as nucleons. Wikipedia neutron
INFO Protons and neutrons constitute the nuclei of atoms. Wikipedia neutron
INFO The neutron is a subatomic particle, symbol n or n0, with no electric charge and a mass slightly greater than that of a proton. Wikipedia neutron
INFO The remainder of a proton's mass is due to quantum chromodynamics binding energy, which includes the kinetic energy of the quarks and the energy of the gluon fields that bind the quarks together. Wikipedia
INFO The rest masses of quarks contribute only about 1% of a proton's mass. Wikipedia
INFO Protons are composite particles composed of three valence quarks: two up quarks of charge + 2/3e and one down quark of charge –1/3e. Wikipedia
INFO Although protons were originally considered fundamental or elementary particles, in the modern Standard Model of particle physics, protons are classified as hadrons, like neutrons, the other nucleon. Wikipedia
INFO The number of protons in the nucleus is the defining property of an element, and is referred to as the atomic number (represented by the symbol Z). Since each element has a unique number of protons, each element has its own unique atomic number. Wikipedia
INFO One or more protons are present in the nucleus of every atom; they are a necessary part of the nucleus. Wikipedia
INFO Protons and neutrons, each with masses of approximately one atomic mass unit, are collectively referred to as "nucleons" (particles present in atomic nuclei). Wikipedia
INFO A modern perspective has a proton composed of the valence quarks (up, up, down), the gluons, and transitory pairs of sea quarks. Wikipedia
INFO The two up quarks and one down quark of a proton are held together by the strong force, mediated by gluons. Wikipedia
INFO A proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and a mass slightly less than that of a neutron. Wikipedia
INFO The quarks that determine the quantum numbers of hadrons are called valence quarks; apart from these, any hadron may contain an indefinite number of virtual "sea" quarks, antiquarks, and gluons, which do not influence its quantum numbers. Wikipedia
INFO Hadrons contain, along with the valence quarks that contribute to their quantum numbers, virtual quark–antiquark pairs known as sea quarks. Wikipedia
INFO Elementary fermions are grouped into three generations, each comprising two leptons and two quarks. Wikipedia
INFO Elementary fermions are grouped into three generations, each comprising two leptons and two quarks. The first generation includes up and down quarks, the second strange and charm quarks, and the third bottom and top quarks. Wikipedia
INFO Antiparticles of quarks are called antiquarks, and are denoted by a bar over the symbol for the corresponding quark, such as ū for an up antiquark. Wikipedia
INFO The Standard Model is the theoretical framework describing all the currently known elementary particles. This model contains six flavors of quarks (q), named up (u), down (d), strange (s), charm (c), bottom (b), and top (t). Wikipedia
INFO For every quark flavor there is a corresponding type of antiparticle, known as an antiquark, that differs from the quark only in that some of its properties (such as the electric charge) have equal magnitude but opposite sign. Wikipedia
INFO Because of this, up and down quarks are generally stable and the most common in the universe, whereas strange, charm, bottom, and top quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators). Wikipedia
INFO The heavier quarks rapidly change into up and down quarks through a process of particle decay: the transformation from a higher mass state to a lower mass state. Wikipedia
INFO Up and down quarks have the lowest masses of all quarks. Wikipedia
INFO There are six types, known as flavors, of quarks: up, down, strange, charm, bottom, and top. Wikipedia
INFO A neutrino (denoted by the Greek letter ν) is a fermion (an elementary particle with spin of 1/2) that interacts only via the weak subatomic force and gravity. Wikipedia
INFO Quarks ... are the only known particles whose electric charges are not integer multiples of the elementary charge. Wikipedia
INFO Quarks ... are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions, also known as fundamental forces (electromagnetism, gravitation, strong interaction, and weak interaction). Wikipedia
INFO Quarks have various intrinsic properties, including electric charge, mass, color charge, and spin. Wikipedia
INFO Due to a phenomenon known as color confinement, quarks are never found in isolation; they can be found only within hadrons, which include baryons (such as protons and neutrons) and mesons, or in quark–gluon plasmas. Wikipedia
INFO Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Wikipedia