UNIT 16

 

I. Find out the words in the dictionary. Write them down and learn.

 

a filament, incandescence, a source, to emit, to acquire, to decay, ground state, excited state, a light range, to derive, to persist, excitation, steady, removal, to diminish, vapor, coherent, incoherent

 

II. Read the text. Use a dictionary, if necessary.

 

Text: “Luminescence”.

 

Nearly all light-emitters are extremely hot; examples include the sun and other stars, the filament of an electric lamp, and a candle flame. In physical terms they are all incandescent light sources. But there is another smaller group of “cold” sources that emit light by the phenomenon of luminescence. Fluorescent lamps and the luminescent numerals on a clock, which glow in the dark are examples. And a third type of light source – also not incandescent – is the laser.

Both incandescence and luminescence have the same fundamental origin, which results from energy changes within the atoms of the radiating object. Electrons within the atoms acquire energy, are “excited” to higher energy levels, and in decaying again to their ground states emit electromagnetic radiation in the visible light range. In an incandescent object, the exciting energy is externally applied heat; in a luminescent object, it derives from within the object itself or from an external source other than heat. There are two kinds of luminescence. The type that ceases when external excitation stops is known as fluorescence; the type, that persists even in the absence of external excitation, is called phosphorescence.

Certain bacteria and some minerals and organic chemical compounds that readily glow when irradiated by ultraviolet light are examples of fluorescence. Some washing powders contain fluorescent compounds called optical brighteners that make fabrics appear brighter and cleaner.

The cold, steady light given off by certain fungi is an example of phosphorescence: it persists after the removal of the initial stimulus. An inorganic example is the glow of one type of luminous paint, for which the energy source is daylight, which is re-emitted as green light, the glow fading as the “stored” energy diminishes.

Incoherent and coherent light. In a fluorescent lamp, atoms of mercury vapor are bombarded by electrons and emit ultraviolet light. This radiation in turn excites a phosphor coating inside the tube that emits visible light. But not all the mercury atoms are hit by electrons at the same instant. Thus, some are in an excited state while others are in the ground state. The overall light output is therefore a randomly discontinuous series of extremely brief pulses. This kind of light, which is also produced by all normal incandescent sources, is said to be incoherent.

Waves of light from two such sources do not stay in phase (in step) with each other for any length of time. Where they meet, they cannot, therefore, produce the regular pattern of intensification and extinction that is typical of what is called the interference of light waves, even though the wavelengths may be more or less identical and the light thus monochromatic.

An interference pattern results when two sources emit light of the same wavelength and in phase; such light is called coherent. It is produced, by a process similar to fluorescence, in a laser – a name that denotes Light Amplification by Stimulated Emission of Radiation. Stimulated emission can be best understood by considering light as a stream of electromagnetic radiation particles (photons).

 

III. Find a verb in each line and translate it. Translate also the underlined words.

 

a) Relatively, opaque, encounter, uncertain, image;

b) Straight, conventionally, matte, surface, obtain;

c) Identify, randomly, angle, irregular, mirror;

d) Invisible, interference, constitute, equally, refraction;

e) Reflection, perfectly, wavelength, unfamiliar, disappear;

f) Absorb, spectrum, incomplete, extremely, incidence.

 

IV. Find the sentences that can’t be found in the text.

 

1. There is another smaller group of  “cold” sources that emit light by the phenomenon of luminescence.
2. Every source of light is known to have a luminous intensity, which is measurable.
3. In an incandescent object, the exciting energy is externally applied heat; in a luminescent object, it derives from within the object itself or from an external source other than heat. 
4. This radiation in turn excites a phosphor coating inside the tube that emits visible light.
5. The overall light output is therefore a randomly discontinuous series of extremely brief pulses.
6. Incandescence is the emission of light caused by high temperatures.
7. The luminescent substance absorbs the energy and the re-radiation may be immediate, so that excitation and luminescence coincide in time.
8. An interference pattern results when two sources emit light of the same wavelength and in phase; such light is called coherent.

 

V. Find English equivalents in the text.

 

1. пламя свечи ________________________________________________
2. нить накала электрической лампы _____________________________
3. явление люминесценции _____________________________________
4. излучать свет ______________________________________________
5. как температурное свечение, так и люминесценция ______________
6. накаливание _______________________________________________
7. правильная/четкая модель ___________________________________
8. в пределах, внутри _________________________________________
9. область распространения видимого света ______________________
10. основное состояние/нижний уровень __________________________
11. прекращать (ся) ____________________________________________
12. продолжать существовать/оставаться _________________________
13. постоянный/устойчивый ____________________________________
14. некогерентный ____________________________________________
15. одновременно _____________________________________________
16. интерференция световых волн _______________________________
17. угасание, потухание ________________________________________
18. устранение ________________________________________________
19. следовать, проистекать ______________________________________
20. более или менее ____________________________________________
21. поток частиц электромагнитного излучения ____________________

 

VI. Find Russian equivalents to the following expressions in the text.

 

1. light-emitters ________________________________________________
2. are extremely hot _____________________________________________
3. incandescent light sources ______________________________________
4. the same fundamental origin ____________________________________
5. results from __________________________________________________
6. the radiating object ____________________________________________
7. electrons within the atoms ______________________________________
8. higher energy level ____________________________________________
9. in decaying to their ground states _________________________________
10. emit electromagnetic radiation in the visible light range _______________
11. an external source other than heat ________________________________
12. excitation ___________________________________________________
13. organic chemical compounds ____________________________________
14. when irradiated by ultraviolet light ________________________________
15. atoms of mercury vapor_________________________________________
16. are bombarded by electrons _____________________________________
17. a phosphor coating ___________________________________________
18. at the same instance ___________________________________________
19. do not stay in phase ____________________________________________

 

VII. Analyse the structure of the following sentences.

 

1. Electrons within the atoms acquire energy, are “excited” to higher energy levels, and in decaying again to their ground states emit electromagnetic radiation in the visible light range.
2. Certain bacteria and some minerals and organic chemical compounds that readily glow when irradiated by ultraviolet light are examples of fluorescence.
3. The type that ceases when external excitation stops is known as fluorescence.
4. An inorganic example is the glow of one type of luminous paint, for which the energy source is daylight, which is re-emitted as green light, the glow fading as the “stored” energy diminishes.

 

VIII. Fill in the missing words.

 

1. Nearly all light-emitters are extremely hot; examples _______ the sun and other stars, the filament of an electric lamp, and a candle light.
2. In physical terms they are all _______ light sources.
3. Fluorescent lamps and the luminescent numerals on a clock, which _______ in the dark are examples.
4. Both incandescence and luminescence have the same fundamental _______, which results from energy _______ within the atoms of the radiating object.
5. In a luminescent object the exciting energy derives from within the object itself or from _______ source other than heat.
6. The type of luminescence that ______ when external excitation, is called _____.
7. The overall light _______ is a randomly ______ series of extremely brief pulses.
8. This kind of light, which is also produced by all normal _______ sources, is said to be ________.
9. Waves of light from two such sources do not stay in _______ with each other for any length of time.
10. Stimulated _______ can be best understood by considering light as a _______ of electromagnetic radiation particles.   

 

IX. Fill in the prepositions, if necessary.

 

1. Fluorescent lamps and luminescent numerals ___ a clock, which glow ___ the dark are examples.
2. Both incandescence and luminescence have the same fundamental origin, which results ___ energy changes ___ the atoms ___ the radiating object.
3. In a luminescent object the exciting energy derives ___ ___ the object itself or ___ an external source other than heat.
4. Certain bacteria and some minerals and organic chemical compounds that readily glow when irradiated ___ ultraviolet light are examples of fluorescence.
5. ___ a fluorescent lamp, atoms ___ mercury vapor are bombarded ___ electrons and emit ultraviolet light.
6. But not all the mercury atoms are hit ___ electrons ___ the same instant. 
7. Waves ___ light ___ two such sources do not stay ___ phase ___ each other ___ any length ___ time.
8. Stimulated emission can be best understood ___ considering light as a stream ___ electromagnetic radiation particles. 

 

X. Define whether the sentences are true or false.

 

1. Nearly all light-emitters are extremely cold.
2. There is another smaller group of “cold” sources that emit light by the phenomenon of luminescence.
3. And a third type of light source – also incandescent – is the laser.
4. In an incandescent object, the exciting energy derives from within the object itself or from an external source other than heat.
5. There are three kinds of luminescence.
6. The type that ceases when external excitation stops is known as fluorescence; the type that persists even in the absence of external excitation, is called phosphorescence.
7. In a fluorescent lamp, atoms of mercury vapor are bombarded by electrons and emit infrared light.
8. All the mercury atoms are hit by electrons at the same instant.
9. An interference pattern results when two sources emit light of the same wavelength and in phase; such light is called coherent.

 

XI. Answer the question.

 

1. Are all light-emitters extremely hot or cold?
2. What are the examples of hot light-emitters?
3. How are they called in physics?
4. There is also a group of “cold” sources. Is it larger or smaller?
5. In what way do “cold” sources emit light?
6. What are the examples of “cold” sources of light?
7. There is a third type of light source. What is it?
8. Do incandescence and luminescence have the same fundamental origin? What does it result from?
9. How do electrons behave in an atom?
10. What is the source of the exciting energy in an incandescent object? And where does the exciting energy derive from in a luminescent object?
11. Are there two or three kinds of luminescent? What are they?
12. What are the examples of fluorescence?
13. What compounds do some washing powders contain?
14. What are the organic and inorganic examples of phosphorescence?

 

XII. Put questions to the following sentences.

 

1. Both incandescence and luminescence have the same fundamental origin. (General)
2. In an incandescent object the exciting energy is externally applied heat. (Alternative)
3. There are two kinds of luminescence. (Disjunctive)
4. This radiation in turn excites a phosphor coating inside the tube. (Special)
5. An interference pattern results when two sources emit light of the same wavelength and in phase. (Special)

 

XIII. Dictate the following sentences in English to your follow-students. Check them together.

 

1. Both incandescence and luminescence have the same fundamental origin, which results from energy changes within the atoms of the radiating object.
2. In an incandescent object, the exciting energy is externally applied heat.
3. In a luminescent object, it derives from within the object itself or from an external source other than heat.
4. In a fluorescent lamp, atoms of mercury vapor are bombarded by electrons and emit ultraviolet light.
5. The overall light output is therefore a randomly discontinuous series of extremely brief pulses.
6. An interference pattern results when two sources emit light of the same wavelength and in phase; such light is called coherent.
7. It is produced, by process similar to fluorescence, in a laser – a name that denotes Light Amplification by Stimulated Emission of Radiation.

 

XIV. Dictation-translation.

 

1. Почти все тела, которые испускают свет, чрезвычайно горячие; примерами являются солнце и другие звёзды, нить накала электрической лампы и пламя свечи.
2. Существует ещё одна, меньшая группа «холодных» источников, которые испускают свет посредством люминесценции.
3. Как температурное свечение, так и люминесценция имеют один и тот же основной источник, который возникает в результате энергетических изменений внутри атомов излучающего предмета.
4. Вид люминесценции, который продолжает существовать даже в отсутствии внешнего возбуждения, называется фосфоресценцией.
5. Во флуоресцентной лампе атомы паров ртути бомбардируются электронами и излучают ультрафиолетовый свет.
6. Вынужденное излучение можно лучше понять, рассматривая свет как поток частиц электромагнитного излучения (фотонов).