Interference. General Considerations.

Everyone has noticed the changing colors of a soap bubble, or a thin film of oil floating on water. These are perhaps the most common manifestations of interference, a phenomenon that is observed when two or more beams of light from a common source arrive, along different paths, at the same region of space. In each of these two cases, the interfering beams are those reflected at the two surfaces of a thin film – a film of soapy water in air, or a film of oil between air and water.

The corpuscular model of light does not afford any sim­ple interpretation of interference phenomena. The wave model, however, suggests a most natural interpretation. Indeed, the phenomena observed when two light beams are superimposed are closely analogous to those associated with the superposition of two surface waves, such as are produced by two pebbles fal­ling into a pool of water.

A careful observer will notice that in certain regions, where the crest of one wave arrives simul­taneously with the trough of the other, the effects of the two waves almost cancel, while in other regions, where a crest meets a crest or a trough meets a trough, the superposition of the two waves produces a more violent disturbance than do the individual waves. Similarly, in the region where two coherent monochromatic light beams overlap, there are places where the light intensity is practically zero and others where it is particularly strong. The situation is more complicated if white light is used. As we have already mentioned, white light is a superposition of light of different colors, each color gives a separate set of maxima and minima, and these combine to give the color effects mentioned above.

In order to interpret interference phenomena it is necessary to specify the wave model of light more precisely than we have done so far. We found that we could account for the phenomena of geometrical optics by assuming that light waves consist of a succession of very short pulses, without making any assump­tions as to the exact shape of these pulses. However, knowledge of the shape of light waves, which geometrical optics ignores, is essential for the interpretation of interference phenomena.

Our analysis of interference phenomena is based upon the superposition principle, which can be stated as follows: the instantaneous optical disturbance at a point where two or more light waves cross is the sum of the optical disturbances that would be produced by¹each of the waves separately. This statement implies that the propagation of a light wave is not affected by the presence of other light waves in the same medium, the observation that two light beams whose paths cross in a region of space continue unperturbed²beyond this region is an experimental test of the superposition principle. In the electromagnetic theory of light the rigorous validity of the super­ position principle for light waves is guaranteed by the linear character of Maxwell’s equations.

3800 п. зн.

Words to be learnt:

to account – считать, рассматривать; счет; to account for объяснять

to afford – быть в состоянии что-то сделать

to cancel – уничтожать

exact – точный

to overlap – частично совпадать

Служебные слова: in order to... – чтобы; so far... – до сих пор; beyond – за пределами, вне.

Exercises

1. Look through the text above and find equivalents of the following Russian phrases:

тонкая пленка нефти, наиболее часто встречающиеся проявления, изменяющиеся цвета, внимательный наблюдатель, наложение света, последовательность импульсов, делать предположение, точная форма, создавать более сильное возмущение, пучки монохромати­ческого света, частично совпадают, чтобы объяснить явление интерференции, определить более точно, последовательность корот­ких импульсов, утверждение (положение) означает, уравнение Максвелла.

2. Complete the sentences below with the appropriate word or word-combination:

1) The natural interpretation of interference phenomena can be given by...

a. a corpuscular theory of light b. wave model

2) In regions where a crest meets a crest or a trough meets a trough, the superposition of the two

waves produces a violent…

a. interference

b. disturbance

3) White light is a superposition of light of…

a. different colors

b. similar colors

4) According to the superposition principle the instantaneous optical disturbance is a sum of

optical disturbances produced by

a. all the waves together

b. each of the waves separately

3. Translate and memorize the definitions below:

Interference is a phenomenon when two or more beams of light from а common source arrive, along different paths, at the same region of space. White light is a superposition of light of different colors. Coherent waves are waves whose phase difference remains constant in time.

Grammar Revision

4. Translate the sentences focusing on the functions of ‘that’:

that	conjunction	Object Clause	что
		Subject Clause	то, что
		Relative Clause	который
	Indicative Pronoun			тот, та, то, те
	used to substitute for a singular or plural (those) noun

1. That light has many different effects is known to everybody. 2) If sources have a phase difference that varies rapidly and irregularly with time, they are said to be incoherent. 3) We found that light waves consist of a succession of very short pulses. 4) The fact is that optical disturbance is directly related with the light intensity. 5) The interfering beams are those reflected at the two surfa­ces of a beam film. 6) Interference phenomena are similar to those produced by Fresnel mirrors. 7) This phenomenon is identical with that mentioned above. 8) The effects of light are closely analogous to those associated with waves.

5. Translate the sentences focusing on the functions of the Gerund:

by, in	measuring	измеряя, путем измерения, при измерении
on (upon), after		измерив, после измерения
without		не измеряя, без измерения
before		до измерения, прежде чем измерить

1. The image formed by the returning light can then be observed directly without being weakened by reflection. 2) The ray in passing from air to water is refracted towards the normal. 3) Before discussing the results of experiments, we must state that it is impossible to observe interference between light waves emitted by independent sources. 4) By measuring the angle between two mirrors we can determine the distance between two sources. 5) A phenomenon that is observed in superimposing - two or more beams of light is known as interference. 6) It is impossible to speak of the optical disturbance without mentioning the light intensity. 7) On passing a narrow beam of sunlight through a glass prism Lomonosov found that the patch of light on a screen was broa­dened out into a band of colors: red, orange, yellow, green, blue, indigo and violet.

6. Answer the questions about Text 1A:

1) What is interference? 2) What are the most common manifestations of interference? 3) What happens when the crest of one wave arrives simultane­ously with the trough of the other? 4) In what case can a violent disturbance be produced? 5) What does the superposition principle state? 6) Does the presence of other light wave effect the propagation of a light wave? 7) What confirms the validity of superposition principle expe­rimentally?

7. Translate the following text into English:

Чтобы наблюдать явления интерференции, необходимо исполь­зовать когерентные источники света, т.е. источники, разность фаз которых постоянна во времени. Такие источники можно создать путем использования одного источника и его изображения или используя два различных изображения одного и того же источника.

8. Write an abstract of Text 1A:

• выделите наиболее значимую информацию из каждого абзаца;

• обобщите полученные сведения;

• используйте, слова – клише: сообщается о..., излагается..., приводится…; -

• избегайте сложных предложений.

9. Use the structural scheme above to speak about interference.

10. Read Text 1В (time limit 3-4 min.) and answer the following questions:

1. Какая единица измерения взята за основу измерения в интерферометрии?

2. Какое изобретение способствовало расширению возмож­ностей проведения измерений

в интерферометрии?

3. Какова цель применения голографической интерферометрии?

TEXT 1B OPTICAL INTERFEROMETERS

Optical interferometers are very important scientific tools. The standard unit of length is based on interferometric measu­rement using the wavelength of krypton 86 and our view of the whole world of Physics is greatly dependent on the unit of len­gth. Einstein's relativity theories were constructed to explain the results of some very important interferometric experiments made bу Michelson and Moorly. The invention of holographic interferometry has in the last years to a great extent widened the possibilities of interferometric measurements.

Hologram interferometry can be used for measurement of di­mension deformation, vibration and changes in refrac­tive index. It is a general agreement that hologram interferometry has the following advantages over ordinary interferometry: I) Displacement of non-optical rough surfaces that scatter light in a diffuse way can be measured with interferometric ace accuracy because each small surface defect is compared with its own image. 2) An object can be compared with itself as it was during earlier exposure. Thus the dimensions of the object during two or more conditions can be compared. 3) When the measuring object is to be compared with a master³(эталон) the latter can be substituted by its holographic image which might rake possible a saving of time and space.

1400 п. зн.

11. Translate Text 1C in writing using a dictionary (time limit 25 min.):

TEXT 1C ELEMENTS OF THE THEORY OF INTERFERENCE

There are two general methods of obtaining beams from a single beam of light, and these provide a basis for classify­ing the arrangements used to produce interference. In one the beam is divided by passing light through apertures placed side by side. This method, which is called division of wave - front, is useful only with sufficiently small sources. Alternatively the beam is divided at one or more partially reflecting surfa­ces at each of which part of the light being reflected and part transmitted. This method called division of amplitude can be used with extended sources and so effectс mау be of greater intensity than with division of wave front. In either case, it is convenient to consider separately the effects which re­sult from the superposition of two beams (two-beam interfe­rence), and those resulting from the superposition of more than two beams (multiple-beam-interference). Historically, interfe­rence phenomena have been the means of establishing the wave, nature of light and today they have important practical uses, for example in spectroscopy and metrology.

1100 п. зн.

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