Page 52 - Curriculum Visions Dynamic Book

of this kind are ceramics that contain added metals. They are referred to as doped ceramics.
Lead-activated calcium tungstate is a blue phosphor, manganese-activated zircon is a green phosphor, lead-
or manganese-activated calcium silicate is yellow, and europium-activated yttrium vanadate is red.
In a cathode ray tube a powder of phosphors is deposited on the inside of the glass screen. When a beam of electrons reaches the phosphors, they glow in their distinctive colours (red, blue, and green). Different energy beams make each of the different phosphors glow. In this way one beam makes red phosphors glow, while another makes blue phosphors glow.
Fluorescent tubes are coated on the inside with calcium halophosphate. The tubes contain a mixture
of mercury vapour and an inert gas. When electricity
is sent through the tube, the mercury vapour gives out ultraviolet light. We don’t see these wavelengths, but they
energise the phosphor coating, which then glows white.
Lasers
The first laser was invented in 1960. It used a single crystal of synthetic ruby doped with chromium to amplify light from a flash bulb. The crystal was contained between two mirrors, one completely silvered, the other partly silvered.
In action the light was reflected back and forth between the mirrors, developing a narrow beam.
Ceramics used as conductors
Most ceramics are very good insulators. That is why
they are used in such roles as insulators between
electric wires and the pylons that support them. Electricity substations also used porcelain ceramics for the same purpose. However, there are some ceramics that conduct electricity.
In ceramics the electrons that would flow, for example, in a metal and thereby conduct electricity are firmly held in place. But if special impurities are included in the ceramic, they can act as donors or acceptors of electrons. That allows
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