Words and word combinations to be remembered

a transverse wave – поперечная волна	pattern – образец, система	stretch – тянуться, растягиваться
disturbance – возмущение, пертурбация	a longitudinal wave – продольная волна	refract – преломлять
crest – гребень	rarefaction – разрежение	displace – перемещать, двигать
trough – подошва волны	compression – сжатие	to spread – распространяться
rest – покой	upwards and downwards – вверх и вниз	snapshot – фотография

Ex. 1. Translate from Russian into English:

Звуковая волна, световая волна, микроволны, поперечная волна, продольная волна, вершина волны, подошва волны, амплитуда, цикл, длина волны, сжатие, разрежение, перпендикулярный, параллельный, горизонтальный, вертикальный, перемещать, двигаться, вибрировать, над, под.

Ex. 2. Fill in the blanks:

1. … are just a few of the examples of our daily encounters with waves.

2. … a wave in which the particles of the medium are displaced in a direction perpendicular to the direction of energy transport.

3. … is the distance from rest to crest.

4. … is a point on a medium through which a longitudinal wave is travelling that has the minimum density.

5. … is simply the length of one complete wave cycle.

6. … cannot travel trough a vacuum.

7. … a point on a medium through which a longitudinal wave is travelling that has the maximum density.

8. … can be reflected and refracted.

9. … could be above or below the rest position at any given moment in time.

10. … can be created in a rope if the rope is stretched out horizontally and the end is vibrated back-and-forth in a vertical direction.

Ex. 3. Are these sentences true or false?

1. There is the transmission of energy by wave motion. 2. One of the most important phenomena in nature is the transmission of energy from one point to another by wave motion. 3. Transverse waves are not easily produced. 4. Sound waves are excellent illustrations of transverse waves. 5. Each particle of the rope imparts its upward or downward motion to its neighbours. 6. Sound waves are due to varying electric and magnetic forces. 7. The vibrations of the particles in sound waves are not in the same direction as that along which the sound travels. 8. Once a disturbance is introduced into the string, the particles of the string begin to vibrate upwards and downwards. 9. When you drop a stone into water, the surface of it is covered with longitudinal waves. 10. The displacement of the particles causes regions of high density and of low density to be formed.

Ex. 4. Say a few words about:

• whether the following waves are transverse or longitudinal: water waves, the waves travelling along the string, in a rope. Draw sketches to illustrate your answer.

• the difference between transverse and longitudinal waves.

Unit 10. Sources of light

There are different kinds of light sources, some of them are caused by high temperatures, others by some other factors.

Every source of light is known to have a luminous intensity, which is measurable. In general, the higher is the temperature of the source, the greater is the luminous intensity of the light it emits.

Incandescence is the emission of light caused by high temperatures. To produce light by incandescence, we maintain the object we are using as a source at a high temperature relative to say room temperature. Under these conditions a substance becomes white or bright-red hot and emits light. Certain substances emit light without becoming incandescent; we consider them to be luminescent.

Luminescence is the emission of light from a body from any cause other than high temperature. We consider fluorescence and phosphorescence to be particular cases of luminescence. A candle flame is an example of incandescence. The light emitted by a firefly is an example of luminescence. Certain substances emit visible light when ultraviolet light shines on them. We know them to be fluorescent materials. Experiment shows that ultraviolet radiation of this sort does not increase the temperature of fluorescent substances appreciably above room temperature. Light from fluorescent sources is not incandescent.

Almost immediately after an ordinary light bulb has been lit it is uncomfortably hot to the touch. As far as an ordinary light bulb is concerned, it is an incandescent source. A lighted fluorescent lamp is unlikely to be too hot to the touch. It is known to be a luminescent light source. Ultraviolet light shining on zinc sulfide causes it to emit a green light. We know it to be a fluorescent source.

As we have already stated, every object which emits light has a measurable luminous intensity. The sun certainly has greater luminous intensity than an ordinary electric bulb. To measure luminous intensity we have to define a unit. The unit is the standard candle (or more briefly, the candela). A source which has a luminous intensity of 25 candles is 25 times more intense than the candela. We define a standard candle as 1/60 of the luminous intensity of the light emitted by 1 cm² of an incandescent platinum metal surface maintained at a fixed temperature 1763°C. One cm² of such a surface has a luminous intensity of 60 (standard) candles.

A source of light which is small compared to the area it illuminates is called a point source. An electric bulb illuminating a large auditorium is a point source. A small candle flame has less luminous intensity than the bulb of a large street lamp. A surface placed at equal distances from each of these sources is certain to receive more illumination from the street lamp. The illumination of a surface is called illuminance and it is the amount of light falling on unit area of the surface per second. The derived unit of illumination in SI system of units is the lux (lumen per square metre). Illuminance is directly proportional to the luminous intensity of the source. However, it depends not only on the luminous intensity of the source but also on the distance of the illuminated area from the source of light. Illuminance is inversely proportional to the square of the distance from the source.