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Definitions (421)

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Parallel Beam


A parallel beam is one which does not diverge or converge - its width stays the same as the beam travels out from its source. Parallel beams are produced using curved (parabolic) reflector dishes (e.g. in car headlamps, satellite transmitter dishes, mobile phone mast relay dishes). Parallel light beams can also be produced using converging lenses ( [..]
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Principle of Reversibility


This applies to light rays passing through any system of mirrors or lenses. Basically, it means if a ray of light follows a path when going, say, from left to right, then another ray of light going from right to left will follow the same path in reverse.
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Becquerel


This is a measure of the activity of a radioactive sample. One Bq means one nucleus decay per second. So a sample with an activity of 12 Bq is undergoing an average of 12 nuclear decays per second.
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Comet


A comet is a small body orbiting our Sun, usually at great distance from Earth and with a highly elliptical orbit. They are composed of 'ices' and dust. As a comet approaches the inner solar system, the heat of the Sun vapourises volatile material from the surface and pushes the material directly away from the Sun, producing the 'ion [..]
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Semiconductor


A material whose ability to conduct electricity is roughly half way between that of an insulator and that of a conductor. Semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, transistors, solar cells, and integrated circuits.
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Semiconductor


A material that is half way between a non-conductor and a conductor in its ability to carry an electric current. It has special conduction properties.
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Boiling


The process by which liquids change into gases at temperatures at or above the boiling point. It occurs throughout the whole of the liquid.
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Cable


Two or three insulated wires surrounded by an outer sleeve of rubber or plastic. A cable is not designed to be flexible or to be moved about.
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Cathode


(1) In an electrolytic cell, the cathode attracts and reacts with positively charged ions. (2) In a cathode ray tube (and vacuum tubes in general) the cathode collects and conducts electrons. (3) The term 'cathode' applies to electrodes in other devices too.
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Cathode Ray Tube


This is an evacuated tube containing an electron gun (a source of electrons) and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam. The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets, etc.
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