Diffraction

Let us have a look at Fig. 19 introduced to illustrate the optical phenomenon known as diffraction. When light is allowed to pass through a small opening like that shown in the figure, it is supposed not to spread out into the region XY. but to proceed in straight lines and produce a sharp image on the screen MN. The light, however, does not proceed in exactly straight lines but spreads out somewhat into the region XY. In other words, light appears to bend round the corners of an obstacle in a way similar to that of water waves bending round the corners of an object. This spreading of a wave motion onto the geometric shadow of an object is called diffraction. This effect is large in the case of water waves, rather large in the case of sound waves, but small when we have to deal with light waves. What is then responsible for these differences in the degree of the diffraction in the above three cases? These differences are accounted for by the differences in wavelengths: the larger the wavelength» the larger the diffraction and vice versa.

Huygens' Principle. To account for the fact that light does not travel exactly in

straight lines, as well as for certain other reasons, Huygens considered that each vibrating particle in the wave front of any wave motion may be considered as ж secondary source of spherical wavelets (i.e. "small waves") which spread out from their sources with the same velocity as the primary waves. The surface that is tangent to all these secondary spherical wave fronts gives the new position of the primary wave front.

Diffraction by a Straight Edge. Suppose that light is diverging from a luminous point and that it passes over the edge of an opaque strip to a screen beyond it. Were the light propagated securely in straight lines, there would be uniform illumination on the screen above the edge while below it there would be complete darkness. However, this is not the case. The illumination neither becomes zero immediately below the edge, nor is it uniform immediately above it. In the former case it fades gradually, in the latter it shows a series of bright and dark bands. these bands correspond to maxima and minima in the illumination on the screen. The brilliant fringes (bands, stripes) thus produced run parallel to the edge of the diaphragm. The intensity of the fringes decreases as we proceed from the center point, and at a short distance from it the intensity of the illumination becomes uniform. It follows that the geometric shadow has no distinct demarcation line, for we see the light fade away gradually on one side and pass through a series of maxima and minima on the other.

Diffraction by Circular Aperture. The case of diffraction by a narrow wire as well as that of diffraction through a narrow slit will be considered elsewhere for want of place. We shall, however, take up one other striking illustration of diffraction to be observed when light from a luminous point is allowed to pass through a small circular aperture like a pinhole. When this aperture is viewed by means of a magnifying glass there appears a brilliant spot surrounded with a number of bright rings. The size and appearance of these rings are altered as the eye is moved farther from the opening or closer to it.

Any system of diffraction bands or rings is called a diffraction pattern.

Diffraction Grating. If a series of very fine equidistant parallel slits be ruled on a plate of glass with a fine diamond point we have what is known a diffraction grating. Where the diamond point has made a line in the glass, the light cannot pass through as effectively as it does in between those lines, where the surface has remained undisturbed and transparent. The plate of glass can then be likened to a picket fence, i.e. one made of equidistant wooden pegs. In effect, every strip the light can pass through is followed by one it cannot penetrate. These strips are very close together.

Now let us assume parallel light to be falling on this kind of grating in such a way that the direction of the ray is perpendicular to the planes of the grating. (Fig. 20) Then from all these slits wave fronts will start. These wavelets will destroy or reinforce

Fig. 18 Bending of light into the geometric shadow

Fig. 19 Diffraction grating. For reinforcement λ = d sin X

each Other according to whether they are in phase or out of phase at a given point. Beams of parallel light will appear from the slits. If these rays coming in the direction perpendicular to the plane of the gratings are brought to a focus on the screen PS by the lens ML, there will be formed a bright line at K. If the light is viewed in a direction making an angle X with the normal to the grating, parallel rays of light emerging from the slits in this direction will travel unequal distances to reach the screen. So when they are brought to a focus at N by the lens IM they may either reinforce or destroy each other. If the angle X is made such that a ray from one slit is one half wavelength behind the corresponding ray from the neighboring slit, these rays will be out of phase, destroying (neutralizing) each other, which will result in darkness at N. as the angle between the normal to the grating and the ray is increased, the rays from the lower slits will be more and more behind those from the upper slits and so when the path difference between corresponding rays from neighboring slits amounts to one wavelength of light, those rays will again be in phase and reinforce. By focusing them with the aid of the lens IM, we cause another bright image of the slit to be formed on the screen.

Diffraction underlies the design of various instruments, notably that of (he spectrometer.

The angle X the telescope of the spectrometer must be turned through to again obtain reinforcement after leaving the central bank К can be easily measured on the divided circle of the spectrometer. The number of rulings per millimeter of the grating being given by the maker, the distance d between the slits can be computed. For the first reinforcement

λ = d sin X.

Since all the quantities in this equation are known except X, there is no difficulty in calculating the wavelength of light. It may be added that this is a very accurate method of measuring it.

Nowadays, a number of techniques and several kinds of equipment based on interference and diffraction phenomena have found their way into research as well as industry, both civil and military.

Diffraction gratings of the rougher kind, using transmitted long-wave (infrared) radiation are ruled directly on glass, the number of lines, as a rule, being about 20 — 30 per millimeter.

Most up-to-date gratings are those operating in the shorter wavelength (visible) part of the spectrum; it should be noted that in this case it is reflected light, and not transmitted light, that made use of; rulings are made with a diamond point - not on glass itself, but on a thin aluminum film spread on a glass base, 300 - 600 lines per millimeter being quite commonplace. Cases are not infrequent when the number of lines amounts to twelve hundred per millimeter; gratings have been produced that number as many as twenty four hundred lines per millimeter, though this can only be achieved at the expense of precision.

I. State what parts of speech the following words belong to; translate them:

Science, scientific, scientist, scientifically; England, English, Englishman; cover. discover, rediscover, discovery, discoverer, discoverable; important, importance;

element, elementary; react, reaction, reactor, reactivity; electric, electrical, electricity;

mix, mixture, mixer; danger, dangerous; fame, famous, fameless, infamous; contain, container, containing; man, fisherman, nobleman, gentlemen, workman, newspapermen, postman.

II. Tell what word building elements take part in the production of the words below; try to translate them:

Electrical, electricity, technical, technique, transform, transformation, transformer, mechanism, mechanical, cylinder, cylindrical, system, systematical, systematically, electron.

III. Translate the following words, tell what parts of speech they belong to:

Improve, improvement;

easy, easier, ease, easiness;

use, useful, usefulness, co-user, usage, usable,

invent, inventor, invention, co-inventor, inventarization,

wonder, wonderful, wonderland, wonderment, wondering,

transmit, transmission, transmitter, transmittance, transmissive, transmissivity,

interest, interesting, disinterested, uninterested.

IV. Underline affixes that change the meaning of the words:

Physics, physical, physicist; fail, failure, failed, unfailed; known, unknown, knowledge; see, seen, unseen; believe, believer, belief, believing; build, builder, building, co-builder, rebuild, built; comfort, comfortable, discomfort; mystery, mysterious, unmysterious.

V. Form antonyms with the help of suffixes de-, in-, dis-, un-, im-, ex-:

Include, known, possible, cover, courage, visible, definite, familiar, form, explained.

VI. Answer the following questions:

1. What is called diffraction? 2. What is responsible for the difference in the degree of diffraction? 3. In which case is diffraction small (large, rather large)? 4. Who tried to account for the fact that light does not travel along straight lines? 5. What did Huygens do to give an exact explanation of diffraction? 6. What do you know about diffraction by a straight edge? 7. What are the characteristics of diffraction by circular aperture? 8. What is called a diffraction grating? 9. In what instruments is the phenomenon of diffraction used? 10. What kind of radiation is used in case of diffraction grating of a rougher kind? 11. What kind of light is made use of in case of the most up-to-date grating? 12. What is the utmost quantity of lines in modern gratings? 13. Is the largest number of lines per millimeter in gratings achieved at the expense of precision or not?

VII. Find and identify Passive constructions, translate the sentences:

1. Rays of light are allowed to pass through a small opening in the screen. 2. Light will not be supposed to spread out into the region XY. 3. This spreading of a wave motion into the geometric shadow of an object is called diffraction. 4. The telescope of the spectrometer must be turned at a small angle. 5. For the experiment we need what is known as a diffraction grating. 6. The plate of glass with a diffraction grating can thus be compared the a picket fence. 7. Every strip the light can pass through will be. followed by one it cannot penetrate. 8. We make another bright image of the lit to be formed on the screen. 9. It may be added that it is a very accurate method of measuring lengths.

VIII. Identify functions of Participles, translate the sentences:

1. Rays of light brought to a focus either reinforced or destroyed each other. 2. The surface of the glass plate remained undisturbed and transparent. 3. Corresponding rays from neighboring slits are in phase and reinforce each other. 4. If the rays coming in the direction perpendicular to the plane of the grating are brought to a focus oh the screen, there will be formed a bright line. 5. If light is viewed in a direction making an angle with the normal to the grating, parallel rays of light emerging from the slits in this direction will travel unequal distances to reach the screen. 6. The illuminated screen can be seen in the class room. 7. The illuminating source of light was high above our heads. 8. Light fading away gradually comes through the narrow slit. 9. Having considered the vibrating particles in the wave front of any wave motion Huygens came to know something new about the nature of light motion. 10. Being allowed to pass through a small circular aperture like a pinhole light produces diffraction pattern on the screen. 11. The brilliant fringes are running parallel to the edge of the diaphragm described. 12. The geometric shadow has no distinct demarcation line, the light fading away gradually.

IX. Translate the sentences, paying attention to the functions of Gerund and Particles:

1. Illuminating the screen through the pinhole helps to observe a diffraction pattern. 2. Having ruled a series of equidistant parallel slits with a fine diamond point on a plate of glass we have what is known as a diffraction grating. 3. After leaving the lens the rays of light are reflected from the polished surface of the looking glass. 4. Beams of parallel rays of light are appearing from the slits. 5. This spreading of a wave motion into the geometric shadow of an object is called diffraction. 6. Diverging from a luminous point light is propagating through different transparent plates. 7. By focusing the rays of light we cause another bright image of the slit to be formed on the screen. 8. There is no difficulty in calculating the wavelength of light. 9. It is considered to be a very accurate method of measuring different wavelengths. 10. Rays entering the glass plate with slits causes the production of diffraction pattern consisting in diffraction bands or rings. 11. Neutralizing of the rays that are out of phase results in darkness. 12. Testing optical instruments showed that their design was based on diffraction. 13, Producing grating with twenty four hundred lines per millimeter can only be achieved at the expense of precision.

X. Find and qualify Infinitives from their fuctions in the sentences:

1. Cases to be observed are quite frequent. 2. The distance between the slits can be computed. 3. We are to examine the experiment from every point of view. 4. It should be noted that we have to deal with reflected light in the case mentioned below. 5. This instrument seems to be finding the way into production. 6. The worker tried hard to understand the design of the optical instruments he had to use. 7. The interference and diffraction phenomena are to be studied by each student of our optical instrument making faculty: 8. The intensity of illumination seems to be increasing slowly. 9. Light cannot travel along exactly straight lines. 10. The effect is known to have been large in the case of the sound waves. 11. It is the distance between the slits that we have to calculate. 12. Every stripe the light ray is to pass through is followed by one it cannot. 13. It should be noted that in this case it is reflected light to be observed. 14. He knew the calculations to have been carried out by the students. 15. To explain the phenomenon the following illustrations are to be taken.

XI. Speak on interference and diffraction.

TEST ON INTERFERENCE

I. Answer the following questions:

1. Give definition of Interference.

2. Who was the first to understand the principles of this phenomenon?

3. What happens when two trains of waves with the same wavelength and amplitude of vibration and traveling in the same direction are superimposed?

4. What experiment is important to prove that light is a wave motion?

5. What is the simplest modification of the original interference experiment?

6. What instruments are based on interference?

7. What do you know about holography?

II. Give Russian equivalents	III. Give English equivalents:
alternate fringes odd (even) numbers a biprism an obtuse angle a spatial image ray displacement disturbance of light a diamond point	лазерный луч новейшие исследования последние достижения современные приемы амплитуда колебаний волновое движение проверка параллельности оптические приборы

IV. Find 3 sentences in the text INTERFERENCE, one with Participle, one with Gerund and one with Infinitive. State the function of the Non-Finite form. Translate the sentences.

TEST ON DIFFRACTION

I. Answer the following questions:

1. When do we observe the largest diffraction effect: with light, sound or water waves?

2. What is Huygens' principle?

3. What is the dependence between diffraction and wavelengths?

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