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1. {Participle I, II), , be (is, are) , , . .
The fundamental principles of alternating current are presented in this chapter. Included are the basic principles of some alternating current machines.
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2. : only , never before , longer , not until , ( ) .
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Never before has a new class of weapon been attended so much publicity as a guided missile.
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3. nor, neither, so.
Carbon dioxide does not burn, nor does it support combustion.
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4. , as though.
This new branch of science, young as it is, touches on many important practical applications.
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have + + II
, t have ( ), II.
Some airplanes have engines installed in the wings.
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- , ( ), to have , (engines) , II (installed) .
LASERS
A device that has received a great deal of publicity is the laser (Light Amplification by Stimulated Emission of Radiation). This device produces a beam, of light composed of waves that are both monochromatic (all of one wavelength) and coherent (all in the same phase that is, all the peaks coinciding). These properties enable the beam to be used as a source of considerable energy at a sharply defined point, for welding, eye surgery, and similar applications. Because the beam is also, extremely parallel, diverging very much less than ordinary light, it is used in space communications a laser beam that has travelled the quarter of a million miles to the moon is still narrow enough to be useful.
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The principle on which the laser works derives from an earlier device called the maser, which operates at microwave frequencies rather than optical frequencies. This principle is based on simulated emission, that is, the emission of a photon by an atom in an excited state as the result of the impact of a photon from outside of exactly equal energy. In this way the stimulating photon is augmented by the photon from the excited atom.
Thus if an atom in a substance is excited it will emit a photon to bring it back to the ground state. It is stimulated (hit) by a photon containing energy, equal to the difference between the excited and ground states. If a high proportion of the atoms in a substance is pumped to an excited state there is an avalanche effect. A stimulating photon from outside is doubled the first time it hits an excited atom, the two photons resulting then go on to double again by impacts with other excited atoms, and so on. All the photons have exactly equal energy, and are therefore associated with waves of identical wavelength.
A laser consists of a solid or gaseous active medium in which the majority of the atoms can be pumped to an excited state by exposing them to electromagnetic radiation of a different frequency to the stimulating frequency. The active medium consist of (or in the case of a gas is contained in) a transparent cylinder which acts as a resonant cavity the stimulated waves of the same frequency making repeated passages up and down the cylinder. One end of the cylinder has a reflecting surface, and the other has a partially reflecting surface through which the laser beam emerges.
In a ruby laser, for example, the electrons in the chromium atoms of a cylindrical ruby crystal are pumped to an excited level by radiation from a flash tube, thus producing a pulsed beam. Continuous wave lasers can be made using mixtures of inert gases.
Vocabulary notes:
monochromatic -
coherent -
that is, all the peaks coinciding ,
enable the beam to be used
to diverge ( )
frequency -
is augmented (.)
ground state
avalanche effect
two photons resulting then go on to double ,
GRAMMAR EXERCISES
iI. Say if it is true or false.
1. The beam of the laser is used as a source of considerable energy at a sharply defined point.2. The principle on which the laser works is based on stimulating emission.
3. A laser consists of a solid or gaseous active medium.
4. Continuous wave lasers can be made using mixtures of inert gases.
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III. Translate the following sentences with the inversion.
1. Discussed in this chapter are some of the general characteristics inherent to semiconductors. 2. Included in this section is a description of a typical airborne liquid oxygen system. 3. Shown on the photo is the equipment available at many airports to start piston-engined aircraft. 4. Described in this book are all the rockets space-probing craft including the sputniks. 5. Associated with each electron is a wave, which is propagated in the direction of the motion of the electron.
IV. Translate the following sentences with the Construction have+noun+participle"
1. Machines of many types have their operation controlled by a computer. 2. A colliding molecule may have an atom or two knocked out of it. 3. The theory of atomic structure developed by Bohr has the electrons distributed around the nucleus in shells (orbits). 4. The large air-cooled engines have the cylinders arranged radially. 5. An atom which has one or more of its electrons raised to a higher than normal energy level is said to be in an excited state. 6. Current transformers are step-up transformers having their primaries connected in series with one line and their secondaries connected to the ammeter terminals.
V. Translate the text in written form.
