Additional Material
Read the text and get its central idea. Describe the nature of sound. In what do sounds vary?
Sound Waves
We have learned that heat is due to the vibration of the molecules of a substance. The more violently the molecules of an object vibrate, the greater the intensity of heat possessed by that object. Sound waves, however, are produced by the vibratory motion of the object as a whole, such as the prong of a tuning fork, a stretched vocal chord, or a tightened string. Sound waves reach our ears and stimulate the auditory nerves so that we have the sensation of sound. How many sound waves must reach our ears per second for us to be aware of a sound? Individuals vary greatly in the ability to hear a sound. Between 16 and 16,000 vibrations per second has often been given as an answer to the above question. Some persons cannot hear as few as 16, others can not hear as many as 16,000. Many individuals can hear sounds that
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have a frequency as high as 19 or 20 thousand vibrations per second. Accordingly, the frequency range of 16 to 20,000 vibrations per second is considered the audio-frequency range.
The sounds vary in loudness, pitch, and quality. The sound waves may be reflected to produce echoes. They may also be amplified or absorbed as we please. Some sounds are harsh and discordant, and their intensity measured in decibels may be almost painful. Other sounds are pleasing, and seem to act as a sedative for everwrought nerves and weary muscles. Above the range of frequencies which the normal human ear can hear is a wide range of inaudible sound vibrations called ultrasonic waves. Their frequency may vary from 15,000 to as high as 500,000,000 vibrations per second.
Read the texts and explain what is meant by a) velocity of sound,
b) the intensity of sound.
Velocity of Sound. The velocity of sound depends on the density and the elasticity of the medium. The greater the elasticity and the less the density, the greater is the velocity. The relation between the velocity, the density, and the elasticity of the medium is expressed by the formula
v=the
velocity of sound; E=the
modulus of elasticity of the medium; p=the
density of the medium.
The Intensity of Sound. When sound waves spread out in every direction from a source of sound, the intensity varies inversely as the square of the distance from the source. In this case, the sound waves spread out as spheres. The same amount of energy is transmitted across every spherical surface having its centre at the source of sound. The larger the surface of these spheres, the smaller the energy that goes through each square centimetre of surface. The surfaces of these spheres increase as the squares of their radii increase. Hence, the energy that passes through unit area decreases as the squares of the radii increase.
UNIT 12
Text. Light
The Nature of Light. Scientists have always been puzzled by the nature of light. In the seventeenth century there were two schools of thought concerning it. Isaac Newton regarded light as a stream
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of corpuscles or tiny particles traveling in straight lines, whilst the Dutch physicist Huygens held that light consisted of waves in a substance called the "ether", which he supposed filled the whole of space including that between the atoms of matter and which could not be removed even from a vacuum.
As time went on and more became known about the behaviour of light, Huygens' wave theory came to be accepted as the better one. At present, however, we have evidence to suppose that light consists of streams of tiny wave-like packets of energy called "quanta", which travel at a speed of 186,000 miles per second.
Atoms emit light at high temperatures produced by chemical reaction in a flame, by the electrical heating of thin tungsten wire in the ordinary electric lamp or by the bombardment of gas molecules by electrons in a discharge lamp tube.
Sources of Light. The sun is the chief source of light and heat, but there are many artificial sources. Any body when heated to a sufficient high temperature becomes a source of light. As the temperature of a body is raised, the body emits invisible radiations. When it becomes red-hot, visible radiations begin to be emitted. The higher the temperature, the greater is the amount of both heat and light waves that are emitted, but the percentage of visible radiations becomes larger as the temperature of the source of radiations is increased. For this reason, the modern tungsten lamp is much more efficient than the old carbon incandescent lamp. Tungsten has a very high melting point, and when it is surrounded by nitrogen or when it is in a vacuum, it can be heated to a high temperature and its efficiency thus made large.
Listen and read words and word combinations to be remembered:
stream [stri:m] поток artificial [,a:ti'fijal] искусственный
behaviour [bi'heiva] поведение efficiency [i'fijansi] эффек-
emit [i'mit] излучать тивность
evidence ['evidans] основание, incandescent [,inkaen'desnt]
очевидность раскаленный до бела
tungsten ['tAngsten] вольфрам fill [fil ] наполнять
tube [tju:b] трубка invisible [in'vizabl] невидимый
nitrogen ['naitridjen] азот discharge [dis'tja:dj] разряжать surround [se'raund] окружать
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as time went on так как время проходило
to come to be accepted быть признанным
at present в настоящее время
wave-like packets волновые пакеты
for this reason на этом основании
Models