Вопрос № 223114 - Английский язык

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A laser is a quantum electronic 1______________. It is a machine for making and concentrating light 2______________ into a very intense beam. The letters LASER stand for Light Amplification by Stimulated Emission of Radiation. The light 3______________ by a laser is much more intense than ordinary light. With ordinary light, all the light waves are of 4_____________ lengths. With lasers, all the light waves are of the same length, and this increases the intensity.

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The laser beam is 1_____________ by exciting the atoms of a suitable material - ruby is one - until most of the atoms have electrons orbiting in a higher energy level than usual. The excitation is then stopped and all the excited electrons fall back together, to their normal orbits, each one emitting a pulse of 2______________ of the same energy. In this way an intense beam of light is generated for a very short time. And 3______________ pulse or wave-train in this beam is in step with every other pulse. In this way a beam of light is obtained which is both monochromatic and coherent and easy to focus. This light is reflected by the mirror at one end of the tube. It can only escape at the other 4______________ of the tube.

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Thermionic emission is the most important and one most commonly used in electron 1_______________. In this 2________________ the metal is heated, resulting in increased thermal or kinetic 3________________ of the unbound electrons. Thus, a greater number of electrons will attain sufficient speed and energy to escape from the surface of the emitter. The number of electrons released per unit area of an emitting surface is related to the absolute 4_________________ of the cathode and a quantity of the work an electron must perform when escaping from the emitting surface.

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When 1__________________ electrons suddenly strike a metallic 2_______________ they give up their kinetic 3_________________ to the electrons and atoms which they strike. Some of the bombarding electrons collide directly with free 4_________________ on the metal surface and may knock them out from the surface. The electrons freed in this way are known as secondary emission electrons, since the primary electrons from some other source must be available to bombard the secondary electron-emitting surface.

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We are finally ready to apply the knowledge we gained in previous sections about electrons, vacuum 1_________________ and transistors to some practical matters. In the following sections we shall consider a variety of circuits employing electron tubes and transistors. Circuits are 2_________________ of tubes or transistors with other components, such as resistors, capacitors and inductors, and form the 3________________ building blocks of electronic systems: radio, automatic computer and so on. To understand the systems, you must be familiar with the circuits that 4_______________ them up.

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Now we shall discuss amplifier 1________________, or more specifically, audio amplifiers. An amplifier is an electron tube or transistor circuit, which 2_________________ up an ac signal applied to its input. It is called a voltage amplifier if the magnitude of the output 3__________________ from the amplifier is considerably greater than that of the input voltage. As a matter of fact the ratio of the output voltage to the 4__________________ voltage is called the amplification or gain of the amplifier.