Unit 3. Refraction.

Active vocabulary

1. Alter ['ɔːltə] змінювати(ся)

2. Angle of refractio ['æŋgl] [rɪ'frækʃən] кут заломлення

3. (To) bend [bend] згинати(ся)

4. Dispersion [dɪs'pɜːʃ(ə)n] дисперсія

5. (To) distort [dɪs'tɔːt] викривляти

6. Distorted image [dɪ'stɔːtɪd'ɪmɪʤ] викривлене зображення

7. Fata Morgana [ˌfɑːtəmɔː'gɑːnə] фата-моргана, марево

8. (To) interact [ˌɪntər'ækt] взаємодіяти

9. Line of sight [laɪn] [saɪt] лінія прямої видимості

10. (To) magnify ['mægnɪfaɪ] збільшувати

11. Medium (pl. media) ['miːdɪəm] ['miːdɪə] середовище (cередовища)

12. Mirage ['mɪrɑːʒ] міраж

13. Optical phenomenon (pl. phenomena) оптичне явище (явища)

['ɔptɪk(ə)l fɪ'nɔmɪnən],[fɪ'nɔmɪnə]

14. Prism [prɪz(ə)m] призма

15. Rainbow ['reɪnbəu] веселка, райдуга

16. (To) refract [rɪ'frækt] заломлювати

17. Refraction [rɪ'frækʃ(ə)n] заломлення, рефракція

18. Refractive index (indices) [ri'fræktɪvˌindeks] показник заломлення

19. Sound wave ['saundweɪv] звукова хвиля

20. (To) split ['splɪt] розщеплювати

Pre-reading task

Exercise 30. In pairs, discuss the following questions.

1. Where can you see a rainbow? Why are rainbows called rainbows?

What colours does a rainbow spectrum include?

2. What is a mirage? Have you ever heard about Fata Morgana? What is it?

Reading

Exercise 31. Read, translate the text and find the words which correspond with these descriptions.

1. an optical illusion in which atmospheric refraction by a layer of hot air distorts or inverts reflections of distant objects._____________

2. The bending of any wave, such as a light or sound wave, when it passes from one medium into another of different density. ____________

3. An arc of spectral colors, usually identified as red, orange, yellow, green, blue, etc, that appears in the sky opposite the sun as a result of the refractive dispersion of sunlight in drops of rain or mist. ____________

4. A surface forming a common boundary between two media. ________

5. A mirage consisting of multiple images, as of cliffs and buildings, that are distorted and magnified to resemble elaborate castles, often seen near the Straits of Messina. _____________

6. The ratio of the velocity of light in a vacuum to its velocity in a specified medium. _______________

Refraction

When electromagnetic radiation, in the form of visible light, travels from one substance or medium into another, the light waves may undergo a phenomenon known as refraction, which is manifested by a bending or change in direction of the light. Refraction occurs as light passes from one medium to another only when there is a difference in the index of refraction between the two materials. The effects of refraction are responsible for a variety of familiar phenomena, such as the apparent bending of an object that is partially submerged in water and the mirages observed on a hot, sandy desert. The refraction of visible light is also an important characteristic of lenses that enables them to focus a beam of light onto a single point.

Refraction is the bending of a wave as it crosses the boundary between two media at an angle. Refraction of light occurs at the interface between two media of different refractive indices, with n₂ > n₁. Since the phase velocity is lower in the second medium (v₂ < v₁), the angle of refraction θ₂ is less than the angle of incidence θ₁; that is, the ray in the higher-index medium is closer to the normal.

Refraction is the change in direction of a wave due to a change in its speed. Sound waves are refracted when parts of a wave front travel at different speeds. Sound wave refraction occurs in uneven winds or when sound is travelling through air of uneven temperature. On a cold day or night, when the layer of air near the ground is colder than the air above it, the speed of sound near the ground is reduced. The higher speeds of the wave fronts above cause a bending of the sound toward Earth. When it happens, sound can be heard over considerably longer distances.

Changes in the speed of light as it passes from one medium to another, or variations in the temperatures and densities of the same medium, cause refraction. When light rays enter a medium in which their speed decreases, as when passing from air into water, the rays bend toward the normal. When light rays enter a medium in which their speed increases, as when passing from water into air, the rays bend away from the normal.

R

Fig. 6.8.

efraction occurs when light waves travel from a medium with a given refractive index to a medium with another. At the boundary between the media, the wave's phase velocity is altered, it changes direction, and its wavelength increases or decreases but its frequency remains constant. Refraction can be seen when looking into a bowl of water. Air has a refractive index of about 1.0003, and water has a refractive index of about 1.33. If a person looks at a straight object, such as a pencil or straw, which is placed at a slant, partially in the water, the object appears to bend at the water's surface (Fig. 6.8). This is due to the bending of light rays as they move from the water to the air. Once the rays reach the eye, the eye traces them back as straight lines (lines of sight). The lines of sight intersect at a higher position than where the actual rays originated. This causes the pencil to appear higher and the water to appear shallower than it really is. The depth that the water appears to be when viewed from above is known as the apparent depth.

R

Fig. 6.9. A rainbow.

efraction is also responsible for rainbows (Fig. 6.9) and for the splitting of white light into a rainbow-spectrum as it passes through a glass prism. Glass has a higher refractive index than air and the different frequencies of light travel at different speeds (dispersion), causing them to be refracted at different angles, so that you can see them. The different frequencies correspond to different colors observed. While refraction allows for beautiful phenomena such as rainbows it may also produce peculiar optical phenomena, such as mirages and Fata Morgana.

T

Fig. 6.10. Fata Morgana

he appearance of distorted image that forms as the result of atmospheric refraction is called a mirage. A mirage and Fata Morgana (Fig. 6.10) are caused by the refraction of light in Earth’s atmosphere (from ‘Conceptual Physics’).

Exercise 32. Answer the questions to the text.

1. What is refraction? When does it occur?

2. What phenomena is refraction responsible for?

3. Why is the speed of sound reduced near the ground on a cold day?

4. What happens when you look at a pencil placed at slant in a glass of water?

5. What is the apparent depth?

6. What is a rainbow? How can this phenomenon be explained?

7. What is a mirage?

8. What are a mirage and Fata Morgana caused by?

Exercise 33. Match the words in columns A and B to make word combinations. Translate them into Ukrainian.

A B Word combination Translation

Visible temperature _______________ ______________

Sound prism _______________ ______________

Refractive light _______________ ______________

Uneven velocity _______________ ______________

Light image _______________ ______________

Straight wave _______________ ______________

Apparent line _______________ ______________

Glass phenomena _______________ ______________

Optical index _______________ ______________

Distorted ray _______________ ______________

Phase desert _______________ ______________

Sandy depth _______________ ______________

Exercise 34. Look for words in the text above to complete each of the groups of synonyms.

1. To change, vary, modify, __________.

2. Located, situated, arranged, ____________.

3. To go (by), to move, ___________, ___________.

4. To happen, take place, befall, ___________.

5. (At a) slope, (at an) angle, _________________.

6. To decrease, diminish, lower, lessen, _____________.

7. To induce, make, be the reason, ____________.

Exercise 35. Fill in the blanks in the text about a Fata Morgana with words and word-combinations from the box. Summarize the information in three or four sentences.

distant derived from unrecognizable inverted horizon optical right side up refracting distort bent deserts mirage layers

A Fata Morgana is an unusual and complex form of ________ that is seen in a narrow band right above the _________. It is an Italian phrase _________ the Latin for "fairy" and the sorceress Morgan le Fay, from a belief that these mirages, often seen in the Strait of Messina, were fairy castles in the air or false land created by her witchcraft to tempt sailors to their death.

Fata Morgana mirages ________ the object(s) which they are based on, often such that the object is completely ________. A Fata Morgana can be seen on land or at sea, in polar regions or in _______. This kind of mirage can involve almost any kind of _______ object, including boats, islands, and the coastline. A Fata Morgana is often rapidly changing. The mirage comprises several ________ (upside down) and erect (________) images that are stacked on top of one another. Fata Morgana mirages also show alternating compressed and stretched zones.

This ______ phenomenon occurs because rays of light are _____ when they pass through air _________ of different temperatures in a steep thermal inversion where an atmospheric duct has formed. In calm weather, a layer of significantly warmer air can rest over colder dense air, forming an atmospheric duct which acts like a __________ lens, producing a series of both inverted and erect images.