Unit 5. Interference and Diffraction.

Active vocabulary

1. Acoustic impedance акустичний імпеданс,

[ə'kuːstɪk ɪm'piːd(ə)ns] акустичний опір (повний)

2. Brightness ['braɪtnɪs] яскравість

3. Constructive interference конструктивна інтерференція

[kən'strʌktɪvˌɪntə'fɪərəns]

4. Corona [kə'rəunə] сонячна корона (при повному затемненні), кільце (навколо місяця)

5. (To) deflect [dɪ'flekt] відхиляти(ся)

6. Destructive interference деструктивна інтерференція

[dɪs'trʌktɪvˌɪntə'fɪərəns]

7. Diffraction [dɪ'frækʃ(ə)n] дифракція

8. Diffraction grating [dɪ'frækʃn'greɪtɪŋ] дифракційна ґратка

9. (To) disperse [dɪs'pɜːs] розсіювати

10. Fringe [frɪnʤ] край, межа, грань

11. Huygens–Fresnel principle принцип Гюйгенса-Френеля

['haɪgənz fre(ɪ)'nel'prɪnsəpl]

12. Interference [ɪntə'fɪərəns] інтерференція

13. Monochromatic light монохроматичне світло

[ˌmɔnəkrə'mætɪk'laɪt]

14. Naked eye ['neɪkɪdˌaɪ] неозброєне око

15. Secondary wave ['sekəndərɪˌweɪv] вторинна хвиля

16. Silver lining ['sɪlvə'laɪnɪŋ] срібляста кромка (хмари)

17. Speckle pattern ['spek(ə)lˌpætn] спекл-структура

18. Spectrometer [spek'trɔmɪtə] спектрометр

19. Superposition [ˌs(j)uːpəpə'zɪʃ(ə)n] суперпозиція, накладення

Reading

Exercise 52. Read, translate the text below.

Diffraction

Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word ‘diffraction’ and was the first to record accurate observations of the phenomenon in 1665. The diffraction phenomenon is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. Similar effects occur when light waves travel through a medium with a varying refractive index or a sound wave through one with varying acoustic impedance. Diffraction occurs with all waves, including sound waves, water waves, and electromagnetic waves such as visible light, x-rays and radio waves.

Thus, diffraction is the slight bending of light as it passes around the edge of an object. The amount of bending depends on the relative size of the wavelength of light to the size of the opening. If the opening is much larger than the light's wavelength, the bending will be almost unnoticeable. However, if the two are closer in size or equal, the amount of bending is considerable, and easily seen with the naked eye.

I

Fig. 6.12.

n the atmosphere, diffracted light is actually bent around atmospheric particles – most commonly, the atmospheric particles are tiny water droplets found in clouds. Diffracted light can produce fringes of light, dark or colored bands. An optical effect that results from the diffraction of light is the silver lining sometimes found around the edges of clouds or coronas surrounding the sun or moon. Figure 6.12 shows how light (from the sun or the moon) is bent around small droplets in the cloud.

While diffraction occurs whenever propagating waves encounter such changes, its effects are generally most pronounced for waves where the wavelength is on the order of the size of the diffracting objects. If the obstructing object provides multiple openings, a complex pattern of varying intensity can result. This is due to the superposition, or interference, of different parts of a wave that traveled to the observer by different paths.

Optical effects resulting from diffraction are produced through the interference of light waves. To visualize this, imagine light waves as water waves. If water waves were incident upon a float residing on the water surface, the float would bounce up and down in response to the incident waves, producing waves of its own. As these waves spread outward in all directions from the float, they interact with other water waves. If the crests of two waves combine, an amplified wave is produced (constructive interference). However, if a crest of one wave and a trough of another wave combine, they cancel each other out to produce no vertical displacement (destructive interference).

This concept also applies to light waves. When sunlight (or moonlight) encounters a cloud droplet, light waves are altered and interact with one another in a similar manner as the water waves described above. If there is constructive interference (the crests of two light waves combining), the light will appear brighter. If there is destructive interference (the trough of one light wave meeting the crest of another), the light will either appear darker or disappear entirely.

The effects of diffraction are often seen in everyday life. The most striking examples of diffraction are those involving light; for example, the closely spaced tracks on a CD or DVD act as a diffraction grating to form the familiar rainbow pattern seen when looking at a disk. This principle can be extended to engineer a grating with a structure such that it will produce any diffraction pattern desired; the hologram on a credit card is an example. Diffraction in the atmosphere by small particles can cause a bright ring to be visible around a bright light source like the sun or the moon. A shadow of a solid object, using light from a compact source, shows small fringes near its edges. The speckle pattern which is observed when laser light falls on an optically rough surface is also a diffraction phenomenon. All these effects are a consequence of the fact that light propagates as a wave.

Diffraction can occur with any kind of wave. Ocean waves diffract around jetties and other obstacles. Sound waves can diffract around objects, which is why one can still hear someone calling even when hiding behind a tree. Diffraction can also be a concern in some technical applications; it sets a fundamental limit to the resolution of a camera, telescope, or microscope.

Diffraction arises because of the way in which waves propagate; this is described by the Huygens–Fresnel principle. The propagation of a wave can be visualized by considering every point on a wavefront as a point source for a secondary radial wave. The subsequent propagation and addition of all these radial waves form the new wavefront. When waves are added together, their sum is determined by the relative phases as well as the amplitudes of the individual waves, an effect which is often known as wave interference. The summed amplitude of the waves can have any value between zero and the sum of the individual amplitudes. Hence, diffraction patterns usually have a series of maxima and minima (http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/opt/mch/diff.rxml).

Exercise 53. Match the lines in A to their definitions in B, and give their Ukrainian equivalents.

A	B	Ukrainian Equivalent
1. Diffraction grating	a) The variation of wave amplitude that occurs when waves of the same or different frequency come together.
2. Monochro-matic light	b) The ratio of the velocity of light in a vacuum to its velocity in a specified medium.
3. Destructive interference	c) Unassisted vision, without a telescope, microscope or other device.
4. Diffraction	d) A plate of glass or metal ruled with very close parallel lines, producing a spectrum by diffraction and interference of light.
5. Construc-tive interference	e) The ratio of the pressure over an imaginary surface in a sound wave to the rate of particle flow across the surface.
6. Spectro-meter	f) Any bending of wave by means other than reflection or refraction.
7. Acoustic impedance	g) Light of one color.
8. Refractive index	h) When the crest of one wave overlaps the crest of another, there individual effects add together.
9. Interfe-rence	i) When the crest of one wave overlaps the trough of another, their individual effects are reduced.
10. Naked eye	j) A spectroscope equipped with scales for measuring wavelengths or indexes of refraction.

Exercise 54. In the text above find words to complete the following groups of synonyms.

1. meet, collide, _________

2. create, make, generate, _________

3. obstruction, hindrance, barrier, ____________

4. repeated, frequent, manifold, _____________

5. devise, create, contrive, _____________

6. follow (from), ensue, _____________

7. changing, fluctuating, _____________

8. border, brim, brink, boundary, ____________

Exercise 55. Answer the questions to the text.

1. What is diffraction?

2. Who devised the term ‘diffraction’?

3. What examples of light diffraction can you give?

4. What is interference? What example of interference can you give?

5. What types of interference do you know?

6. What does the Huygens–Fresnel principle state?

Exercise 56. A) Fill in the blanks with the words and word combinations from the box.

colours monochromatic light Huygens interference prisms diffraction grating wave nature Thomas Young