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Hadrons - Elementary particles which have stron interaction properties,as opposed to leptons. Hadrons include mesons and baryons.
Haidinger Brushes - Faint yellow brushes seen when looking at a bright surface through a nicol prism. They are believed to be due to an effect at the fovea of the eye.
Haidinger Fringes - Interference fringes seen with thick, flat plates near normal incidence. The fringes of the Fabry-Perot interferometer are of this type.
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Half-life, Effective - Of a particular radionuclide in a system: the time required for the amount of that radionuclide to be reduced to half its value as a consequence both of radioactive decay and other processes such as biological elimination or burn-up, when the rate of removal is approximately exponential.
Half-life, Radioactive - Or a single radioactive decay process: the time required for the activity to decrease to half its value. It is the time taken for half of the nuclei to decay.
Half-Period Zones: Fresnel Zones - Annular elements in which it is convenient to divide a wave front when determining the amplitude at a given point resulting from Fresnel diffraction. The zones are such that radiation from one zone reaching the point is one half-period out of phase with that from the adjacent zones. The resultant amplitude is half the amplitude that would result if all but the first (axial) zone were blocked out. If a screen (known as a zoneplate) is interposed so as to obstruct alternate zones, the intensity at the point of interest can be made very large.
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Hard and Soft Radiatio - Qualitative terms describing the penetrating power of the radiation, "hard" denoting a relatively high degree of penetration and "soft" a relatively low degree. What is hard in one context may be soft in another.
Hard Magnetic Material - Generally taken to mean a material which has a coercivity of more than about 100 oersteds or 7.96 x 103
A/m. The associated value of the product of field strength and flux density is the quantity of interest for permanent magnets.
Hardness - Of a solid: its resistance to local deformation by such means as indentation or scratching. It is customarily expressed as: (a) Scratch hardness, which is based on the ability of one solid to scratch another. Here the Mohs scale (defined separately) may be used in which ten minerals arc selected as standards, from talc (hardness 1) to diamond (hardness 10);
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(b) Indentation hardness, to determine which permanent deformation is made by an indenter and the hardness expressed in terms of the load and area of
indentation formed. The indenter may be in the shape of a ball (Brinell and Rockwell hardnesses), cone (Rockwell hardness), or pyramid (Vickers hardness): or (c) Dynamic or rebound hardness, in which the hardness is expressed in terms of the indentation made by a bouncing indenter falling under gravity, or of the height to which the indenter rebounds under specified conditions (as in the Herbert pendulum and Scleroscope tests).
Hartmann Dispersion Formula - For a prism; the equation:
where n is the refractive index, h0 the wavelength, and C and h0 are constants.
Hartmann Test. A test for aberration and coma in an optical system in which the images formed by different parts of the field are compared in position and sharpness.
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Hartmann Wavelength Formula - For a prism: the equation where λ is the unknown wavelength of a spectral line on a spectroscopic plate, λ0, C, and do are constants (found by calibration), and d is the distance of the spectral line measured along the plate from a fixed point.
Hartree Method: Self-Consistent Field Method - An iteration method used in the determination of solutions of the Schrödinger equation for atomic problems. See a1so: Slater method.
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Halation - The formation of haloes on photographic images of bright objects due to light reflected back into the emulsion from the rear of the supporting film.
Haze - (1) Dry haze: reduced visibility in the atmosphere arising from very small (less than 1 µm) particles of smoke, dust, or salt. It is yellowish in colour. (2) Wet haze or mist: reduced visibility in the atmosphere arising from small water droplets. It is grey in colour. See also: Fog.
Harmonic Motion - A vibratory motion represented by wher x is the displacement of a given point, A and B are constants, and ωt is the phase at time t. Simple examples are the oscillations of a mass at the end of a coiled spring, the elastic displacement of a beam from its. equilibrium position, and a sinusoidal wave motion.
Harmonic Oscillator - An oscillator in which the restoring force varies linearly with the displacement of the system from its equilibrium position.
Hartley Oscillator - One of the first vacuum-tube ( triode) oscillators to be devised. It consists essentially of a parallel resonant circuit connected between the grid and anode, the cathode being connected to a tapping point along the coil.
Halt-Shadow Device - A device used to divide a field of view into two parts to facilitate the detection by the eye of small changes in a dark field of view by balancing one half against the other, reference, half. Such devices are used extensively in measurements made with polarized light and also in photometers.
Half-Value Layer: Half-Value Thickness - Of a specified substance for a given beam of ionizing radiation: that thickness which, when introduced into the path of the beam, reduces the value of a specified radiation quantity by one-half. It is sometimes expressed in terms of mass per unit area.
Half-Wave Circuit - Acircuit in which current flows during alternate half-cycles only.
Half-Wave Plate - A plate of doubly refracting material which. for a given wavelength, resolves a beam of linearly polarized light which
traverses the plate, into two components with a path difference of half that wavelength, i.e. a phase-difference of .
Half-Width - Of a peak on a distribution curve, e.g. a spectral line: the full width of the peak measured at half the height.
Hall Coefficient - The coefficient R in the expression for the Hall e.m.f.,RBJT, where B is the magnetic flux density, J the current density,
and Tthe thickness of the specimen in the direction of the e.m.f. It is potential gradient for unit flux density and unit current density.
Hall Effect - The development of a transverse electric potential difference (the Hall e.m.f.) across a current-carrying conductor or semiconductor when a magnetic field is applied at right angles to the current. The potential gradient, electric current, and magnetic field are mutually perpendicular.
Halo - (1) In atmospheric optics: a coloured circle of light apparently
.surrounding a light source. It may arise from the reflection (either external or internal) of light from the Sun or Moon at the surfaces of ice crystals, from the concentration of light in certain directions due to refraction at minimum deviation of light passing through the crystals, or from a similar concentration arising from the preferred orientation of the crystals. (2) In X-ray and electron diffraction: one of the broad rings appearing in the X-ray or electron diffraction pattern of a material which cannot be regarded as truly crystalline. Such haloes occur with liquids and gases as well as with solids.
Hamilton Principle - Concerns the potential energy V and the kinetic energy T of a mechanical system. For a conservative system the principle postulates that the integral where t is the time, shall have a stationary value. The integrand (T-V) is known as the Lagrangian function. The Hamilton principle is a more general from of the principle of least action and Fermat's principle.
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