Angle of Reflection - For radiation incident upon a surface: the angle between the direction of reflection and the normal to the surface at the point of incidence.

Angle of Incidence - For radiation incident upon & surface: the angle between the direction of incidence and the normal to the surface at the point of incidence.

Angle of Refraction - For radiation passing from one medium to another: the angle between the direction of the refracted radiation and the normal to the surface separating the two media at the point of incidence.

Angstrom Unit - A unit of length used in spectroscopy, X-ray diffraction, etc. The international Angstrom was defined by assigning the value 6438.46% A to the wavelength of the cadmium red line in dry normal air 15 T and standard atmospheric pressure. It was 108 cm to within 2 parts in 107. but is now defined as 10-10 m exactly.

Angular Acceleration - It is the rate of change of the angular velocity of an object. If θ is the angular displacemnt of an object, the angular acceleration is,α = d2θ/dt2 = dω/dtwhere  ω = dθ/dt is its angular velocity.

Angualr Momentum - The product of the. moment of inertia of a rotating body or system of bodies, as measured about the axis of ,rotation, and the angular velocity about that axis. Also called moment of mementum. It is a vector quantity.

Anharmonic Oscillator - An oscillator in which the restoring force varies other than linearly with the displacement of the system from its equlibrium position.

Anharmonic Oscillator Spectrum - A spectrum, e.g. a vibration spectrum or a Raman spectrum, for which a full explanation of the details requires the assumption of anharmoncity in the vibrations, i.e. of the existence of anharmonic oscillators.

Anisotropy - The exhibition by a medium of a variation in physical properties with the direction of measurement.

Annihilation - The disappearance of a particle and its anti-particle resulting from their mutual collision, their energy being converted into annihilation radiation or other particles.

Annihilation Radiation - The electromagnetic radiation resulting from annihilation.

Anode Characteristic - The relation between anode current and anode voltage in a thermionic valve or tube.

Anode Current - The current flowing to the anode in a thermionic tube or valve.

Anode Dark Space - In a glow discharge tube: a thin dark region next to, and on the cathode side of, the anode glow.

Anode Fall - The voltage drop between the anode and the positive column in a discharge tube or arc discharge. Also known as anode drop.

Anode Glow - In a glow discharge tube: a thin luminous layer occurring on the surface of the anode.

Anode Rays - Positive ions emitted from the anode in a glow discharge.

Anomaloscope - An instrument used for the detection and classification of defective colour vision.

Anomalous Dispersion - An anomaly in the regular variation of refractive index with wavelength observed in the region of an absorption band.

Anomalous Series - Spectral series for which the quantum defect for the different members of the series does not vary in a smooth fashion with the total quantum number.

Anorthic System - The crystal system exhibiting the least symmetry.

Antenna - See Aerial.

Anti-coincidence Circuit - An electronic circuit that produces a pulse only when anti-coincidence occurs, i.e. when a count occurs in a specified detector unaccompanied by a count in one or more other specified detectors within a given short interval of time.

Anticyclone - A region (perhaps of some 106 km² in area) in which the atmospheric pressure is high and the isobars take the form of closed curves around the point of highest pressure.

Antiferroelectric - A dielectric of high permittivity in which no net spontaneous polarization occurs.

Antiferromagnetism - The existence of a state analogous to the state analogous to the ferromagnetic state put with neighbouring spins anti-parallel instead of parallel. Anti-ferromagnetic exhibit paramagnetism.

Antimatter - Matter composed of anti particles e.g. anti oxygen consists of eith anti protons and anti neutrons with eight positrons that move around these in orbits.

Anti-particle - A particle having the same mass as a given fundamental particle, but whose other properties, while having the same magnitude, may some of them be of opposite sign, e.g. electric charge in the case of the electron and positron, magnetic moment in the case of the neutron and antineutron.

Anti-Stokes Emission - The emission of fluorescent, phosphorescent, or luminescent radiation of shorter wavelength, than that of the incident radiaton.

Aperiodic - Refers to the condition of a potentially oscillatory system in which oscillation does not occur following an initial disturbance, owing to the degree of damping present.

Apertometer - A device for measuring the numerical aperture of a microscope objective.

Aperture - Of an optical system: the diameter of the exit pupil when the system is adjusted to infinity.

Aplantic Lens - A lens in whose construction use is made of aplantic points, whereby it possesses the property of giving a sharp image for rays making large angles with the axis.

Aplanatic Points - Two points on the axis of an optical system which have the property that rays proceeding from one of them shall all converge to, or appear to diverge from, the other.

Aplanatic Refraction (reflection) - Refraction (reflection) in which neither the spherical aberration nor the coma of the incident pencil is changed.

Aplanatic Surface - A surface at which aplanatic refraction occurs.

Apochromatic Lens - A compound lens that is sensibly free from chromatic errors, from spherical aberration for two wavelengths, and from central coma for one wavelength.

Apogee - Of the orbit of a body about the Earth: the point in that orbit which is farthest from the centre of the Earth.

Aquesous Humour - A fluid secreted in the eye, just behind the cornea.

Arago Point - A bright spot which, owing to diffraction, appears at the centre of the shadow of a circular disk.

Arc - An electrical discharge in a gas, generally operated at a relatively low voltage and high current.

Archimedes Principle - States that the vertical hydrostatic force acting on an immersed body is equal to the weight of the fluid displaced by the body.

Ardometer - A type of total radiation pyrometer in which the radiation is concentrated on a small (2-3 mm dian1eter) blackened platinum disk in an evacuated glass bulb.

Areometer - A device for determining the density of a liquid by measuring the loss of weight of a solid of known mass and volume suspended in the liquid.

Artificial Ear - A device employed to calibrate earphones objectively for measurements on the acuity of hearing.

Asterism - In a Laue X -ray diffraction pattern: the elongation of Laue spots into radial streaks. It is attributed to lattice distortion.

Asteroid - One of a large number of small planets, all of diameter less than 770 km, most of which have near-Circular orbits mostly lying those of Mars and Jupiter, at radial distances between 240 and 700 x 106 km.

Astigmatic Foci - The two focal lines produced by an astigmatic system.

Astigmatic Mounting - For a concave diffraction grating: a mounting designed to minimize astigmatism.

Astigmatism - Of an optical system: the production, from a point source, of two short line images (focal lines) at right angles to each other, and at two different distances from the system

Aston Dark Space - In a glow discharge tube: a thin dark region immediately adjacent to the cathode

Astrographic Telescope - A telescope specially designed for the taking of photographs of star fields for the accurate measurement of relative star positions. Sometimes referred to as an Astrograph.

Astrolabe -
(1) A medieval instrument for measuring the altitude of a heavenly body, incorporating horizonal and vertical graduated circles.
(2) A term sometimes applied to the armillary sphere.
(3) A modern optical- instrument (the prismatic astrolabe) designed to measure the precise instant at which a given star has a given altitude (usually 45° or 60°).

Astrometry - The precise measurement of stellar positions, either absolute or differential, usually from photographic plates.

Astronautics - The science of locomotion and travel outside the Earth's atmosphere, involving the problems of artificial satellites and of interplanetary journeys.

Astronmical Triangle - A spherical triangle on the celestial sphere.