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Interference

When two trains of waves having the same wavelength and amplitude of vibration and traveling in the same direction are superimposed, they do not always produce increased illumination but may neutralize each other. The principles of this phenomenon, known as interference, are best understood from the original experiment. made by Thomas Young.

Behind a screen containing a small pinhole S (Fig. 18 ) a source of light is placed. A second screen, in which there are two small holes A and B, is placed in front of the first one in such a way that the openings A and В are equally illuminated from S. If light is allowed to pass through either A Or В there appears a bright spot on a third screen at D. If, however, light passes through both boles A and В simultaneously, we shall observe a series of bright and dark bands instead of bright spot on the third screen.

To see the meaning of this fact one should consider in detail Fig. 18. Let DM be a perpendicular to AB at a point F midway between A and B. Since D is equidistant from A and B, light leaving A and B simultaneously will reach D at the same time and be in phase. Hence, the waves from A and B will coincide and therefore reinforce each other. The illumination at this point will be a maximum. If some point E above or below D is chosen, the light from A will have to travel the distance AE to reach E, while that from B will have to cover the distance BE which is greater than AE; this will result in the light from B arriving later than that from A. Provided E is chosen for enough from D, that ВС, i.e. the difference between BE and AE, is equal to one-half wavelength of light, then the light from A will be ahead of the light from B by one-half wavelength or one-half period of vibration. It follows that when the waves coming from A produce displacement in one direction, those coming from B cause a displacement in the opposite direction. The two waves will thus neutralize each other and there will be no light at E.

If E is chosen still farther away from D so that ВС is equal to one whole wavelength, the light from both sources, on reaching the screen, will again be in phase and so produce an illumination of the screen at that point. By choosing points still farther away from D, alternate dark and light fringes (bands, stripes) will be found on the screen according to whether ВС is equal to an odd or even number of half-wave lengths of light.

Because of the importance of this experiment in proving that light is a wave motion, many modifications of it have been devised. These were designed to remove any criticism of the experiment and make the reality of the interference phenomenon as certain as possible. The simplest of those modifications is one in which for the screen with the two pinholes Fresnel substituted a biprism having a very obtuse angle. By means of this prism any disturbance of the light that might arise when it passed through the apertures A and В was successfully avoided. Fresnel also replaced the biprism by two mirrors inclined at a small angle to each other. These experiments resulted in precisely the same effect as that observed by Young and so the reality of the interference of light was established beyond any possibility of doubt.

In several modem branches of science and industry interference is the phenomenon a number of most up-to-date methods and instruments are based upon, such as devices for testing the shape and parallelism of optical parts; instruments for measuring lengths, viz. geodimeters, that is electro-optical rangefinders; interference microscope the building of which is one of the latest scientific developments; interference filters enabling a very narrow transmission band to be produced; and, last but not least, the method of holography, that is, the production of a spatial (a three dimensional) image through the use of the interference of laser beams.

Fig. 17 Young's interference experiment

I. Remember the following word combinations, common to many scientific texts; find sentences with them in the text; compose your own sentences with them:

1. it follows that..., 2. the production of... through the use of .... 3. to make as certain as possible, 4. up-to-date methods, 5. to see the meaning of this fact, one should consider..., 6. the experiment results in... , 7. instruments for testing, 8. to remove any criticism of the experiment..., 9. it is best understood from..., 10. the production of instruments and devices, 11. last but not least, 12. to be one of the latest scientific developments, 13. let it be, 14. it is precisely the same effect as was received earlier, 15. provided that, 16. hence, 17. the instruments are based upon the optical phenomenon, 18. by means of..., 19. to establish something beyond any possibility of doubt.

II. Insert prepositions into the gaps when it is necessary.

1. Light passes ... both holes ... the screen. 2. A source of light is placed ... a screen. 3. Let us place a screen ... a candle. 4. The rays ... light reach the point ... the same time. 5. Let AB be a perpendicular ... a screen ... a point C. 6. The phenomenon is best understood ... the experiment made ... the scientist. 7. The distance is equal ... the whole wave length. 8. A period ... vibration is equal ... the known value. 9. ... choosing points ... the screen, we'll observe ... the illumination ... points. 10. Can we substitute a biprism ... the screen? 11. The mirrors ... which the biprism is substituted are inclined ... a small angle ... each other. 12. The experiments, observed ... the student, resulted ... the same effects. 13. What instruments do you know ... the latest developments ... Russian engineers? 14. What principles are the instruments based ... ? 15. What are the devices ... testing the parallelism ... optical parts? 16. The three-dimensional image is produced ... the use ... the interference ... laser beams. 17. There is a short distance ... equidistant objects. 18. Was a small candle placed ... the screen or ... it? 19. Is this scientific fact established ... any possibility of doubt?

III. Answer the following questions:

1. Who was the first to make an original experiment that showed interference? 2. Do trains of waves with the same wavelength and amplitude of vibration and traveling in the same direction produce an increased illumination when superimposed? 3. What is the best way to understand the principles of interference? 4. Is interference used in science and industry as main principle upon which the optical instruments are based? 5.What optical instruments can be named among the latest scientific developments? 6. Is the method of holography based upon the principle of interference? 7. What instruments for measuring lengths and based upon interference do you know? 8. What do interference filters serve to produce?

IV. Find and qualify passive constructions:

1. The principles of optics can be best understood from experiments. 2. These parts of screen are equally illuminated with a luminous body. 3. The source of light is placed in front of the screen. 4. Interference microscope has been built recently in our country. 5. Spatial images can be produced through the use of interference of laser beams. 6. A lot of instruments and devices are based upon the phenomenon of interference. 7. Can the openings be found in the screen? 8. The disturbance of light was avoided." 9: These experiments will be observed and discussed by scientists. 10. Some new tools are being created by the scientists of this laboratory. 11. A number of modifications of the original experiment have been designed to prove the reality of it as certain as possible. 12. The ray of light was being observed by the student with the help of a special device during the experiment made by our group.

V. Find in the text 5 sentences with Passive constructions, translate them.

VI. Put the following sentences into the Passive Voice:

I. One can read about this optical phenomenon in the text-book for second-year students. 2. We place a second screen not far from the first screen. 3. The ray of light will reach the screen. 4. The beam of light covers the distance AB to arrive at the screen. 5. A simple experiment will show the same result as the original experiment; 7. Students carried out a lot of investigations into various optical phenomena. 8. The scientist has substituted a biprism for the screen with two pinholes. 9. Russian engineers have already achieved great progress in optical instrument making. 10. We can consider different facts to come to know the principles of this phenomenon.

VII. Find and qualify functions of Infinitives; translate the sentences:

1. These optical instruments are to be produced through the use of interference. 2. The electro-optical rangefinder enables long distances to be measured. 3. Can the disturbance of light be avoided? 4. The rays of light are to pass through the opening to reach the screen. 5. The modifications of the experiment were designed to prove it. 6. To establish that light is a wave motion many experiments were produced. 7. The scientist made many modifications of the experiment to remove any criticism of it. 8. I observe the illumination of the screen become more intensive. 9. Let us consider the optical phenomenon in detail to see the meaning of it. 10. We observe a bright spot appear on the screen.

VII. Find Infinitives and discuss their functions:

1. There is a set of devices to test the shape of optical parts. 2. We can see a beam of light produce a displacement of its direction. 3. We consider the two waves to neutralize each other. 4. The. results of the two experiments seem to indicate that interference takes place here. 5. The author of the paper claims to have been the first to arrive at such a conclusion. 6. The truth of Thomas Young's discovery is to be found in everyday life. 7. If I were invited I would be very glad to take part in the conference on problems of interference application. 8. There are very few people, if any, to be following this old method. 9. We had to repeat the experiment to check up the results obtained. 10. He may stop his experiments. 11. To support this view, it seemed important to analyze the experiment that is said to go well under these conditions.

VIII. Find in the text 5 sentences with Infinitive, translate them.

IX. Translate sentences, paying attention to the functions of Participle:

1. We can see a screen containing a pinhole. 2. There are a lot of experiments using interference as the main principle. 3. Many modern methods based upon interference are used in up-to-date science and industry. 4. Having observed a series of bright and dark bands the student described their pattern. 5. Light passing through the pinholes on the screen, we can notice a bright spot on the second screen. 6. The experiment will result in the light coming through equally illuminated pinholes. 7. He intends to remake the original experiment staged by Thomas Young. 8. Light having been sent through the holes simultaneously, we can see the specific pattern on the screen. 9. Russian scientists are working out new methods of distance measuring. 10. Tests for parallelism of optical parts can be performed by optical instruments based on interference.

X. Find in the text 5 sentences with Participle, translate them.

XI. Translate sentences, pay attention to the functions of Gerund:

1. After passing through the holes light produces a bright spot on the screen. 2. The displacement of the light rays resulted from the biprism having been set. 3. On reaching the screen the light waves will coincide and reinforce each other. 4. By choosing points for the pinholes the students began to reproduce the original experiment. 5. Inventing modifications of the experiment serves to prove it beyond any possibility of doubt.

XII. Translate sentences, pay attention to the functions of Gerund and Participle:

1. There is no hope of their solving this complex engineering problem at present. 2. After covering the distance AB the rays of light reached the screen under investigation later than those traveling from C. 3. Having passed through the pinholes the rays produce an increasing illumination of the screen. 4. Devices for testing the shape and parallelism of optical parts are being designed by them now. 5. Now we are staging a lot of experiments because of their importance for proving this phenomenon. 6. By substituting another source of light for this luminous body we can get different conditions of the experiment.

DIFFRACTION

I. Give Russian equivalents to the following word combinations and words:

A sharp image, a straight line, a geometric shadow, a sound wave, a water wave, a wave front, to spread, an obstacle, a vibrating particle, a spherical wavelet, a secondary wave, a demarcation line.

II. Translate and memorize the following expressions:

A circular aperture, a magnifying glass, a degree of diffraction, an intensity of illumination, a striking illustration, a luminous point, a brilliant spot, a series of bright and dark bands, a primary wave front, a target surface, an opaque strip, complete darkness, a narrow slit, a bright ring.

III. Remember the following expressions:

Long-wave (infrared) radiation, short-wave (visible) radiation, transmitted light, reflected light, reflecting surface, transmitting medium, to illustrate the phenomenon, to show in the figure, to account for the fact, to consider the experiment, in effect, let us assume that, it may be added that, we may suppose that, it should be noted that, to find its way into industry (research).

IV. Give English equivalents:

Светящаяся точка, непрозрачная полоса, пламя свечи, равномерное

освещение, прямые линии, узкая щель, собирающая линза, маленькое круглое отверстие, отчетливая разделительная линия, полная темнота, центральная точка, колеблющиеся частицы, геометрическая тень.

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