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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 [..]
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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 fr [..]
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double insulation


An electrical appliance which is double insulated does not have an earth wire fitted. The appliance is designed in such a way that the electrical parts can never come into contact with the outer casin [..]
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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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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 ra [..]
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objective lens


In a refracting telescope the objective lens is the large lens which collects the dim light from objects in the night sky. The purpose of the objective is to collect as much light as possible. It does [..]
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deflection plates


These are located in the cathode ray tube of an oscilloscope and are used to deflect the electron beam to the required parts of the screen. One set of parallel plates control horizontal movement of th [..]
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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 [..]
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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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atmosphere


All the gas (mostly air) that surrounds the Earth, extending from the Earth's surface to outer space.
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