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aberration

Overview page. Subjects: Physics — Medicine and Health.

n. (in optics) a defect in the image formed by an optical device (e.g. a lens). In chromatic aberration the image has coloured fringes as a result of the different extent to...

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absolute space

Overview page. Subjects: Physics.

Space that exists as a background to events and processes and is not affected by objects or other entities in the universe. The idea underpins Newtonian physics, although many physicists...

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absolute time

Overview page. Subjects: Physics.

Time that exists independently of any events or processes in the universe. Like absolute space, absolute time is a basic concept in Newtonian physics.

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abundance

Overview page. Subjects: Chemistry — Physics.

1 The ratio of the total mass of a specified element in the earth's crust to the total mass of the earth's crust, often expressed as a percentage. For example, the abundance of...

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acceptor

Overview page. Subjects: Physics.

A substance that is added as an impurity to a semiconductor because of its ability to accept electrons from the valence bands, causing p-type conduction by the mobile positive holes left.

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acceptor levels

Overview page. Subjects: Physics.

Energy levels of an acceptor atom in a semiconductor, such as aluminium, in silicon. These energy levels are very near the top of the valence band, and therefore cause p-type conduction....

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accumulator

Overview page. Subjects: Chemistry — Physics.

A type of voltaic cell or battery that can be recharged by passing a current through it from an external d.c. supply. The charging current, which is passed in the opposite direction to that...

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acoustoelectronic devices

Overview page. Subjects: Physics.

Devices in which electronic signals are converted into acoustic waves. Acoustoelectronic devices are used in constructing delay lines and also in converting digital data from computers for...

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actinium

Overview page. Subjects: Chemistry — Physics.

Symbol Ac. A radioactive metallic element belonging to group 3 of the periodic table; a.n. 89; mass number of most stable isotope 227 (half-life 21.7 years); m.p. 1050 ± 50°C; b.p. 3200°C...

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actinoid contraction

Overview page. Subjects: Chemistry — Physics.

A smooth decrease in atomic or ionic radius with increasing proton number found in the actinoids.

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