07: Sound : Icse-x-physics

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#1-a
amplitude
Ans : Amplitude: The maximum displacement of the particle of medium on either side of its mean position is called the amplitude of wave. Its S.I. unit is metre (m).

#1-b
frequency
Ans : Frequency: The number of vibrations made by a particle of the medium in one second is called the frequency of the waves. It is also defined as the number of waves passing through a point in one second. Its S.I. unit is hertz (Hz).

#1-c
wavelength and
Ans : Wavelength: The distance travelled by the wave in one time period of vibration of particle of medium is called its wavelength. Its S.I. unit is metre (m).

#1-d
wave velocity
Ans : Wave velocity: The distance travelled by a wave in one second is called its wave velocity. Its S.I. unit is metre per second (ms^-1 ).

Qstn #2
A wave passes from one medium to another medium. Mention the property of the wave (i) which changes, (ii) which does not change.
Ans : 2 (i) Wavelength (or speed) of the wave changes, when it passes from one medium to another medium.
(ii) Frequency of a wave does not change when it passes from one medium to another medium.

Qstn #3
State two factors on which the speed of a wave travelling in a medium depends.
Ans : 3 Two factors on which the speed of a wave travelling in a medium depends are:
(i) Density: The speed of sound is inversely proportional to the square root of density of the gas.
(ii) Temperature: The speed of sound increases with the increase in temperature.

Qstn #4
State two differences between the light and sound waves.
Ans : 4 (i) The light waves can travel in vacuum while sound waves need a material medium for propagation.
(ii) The light waves are electromagnetic waves while sound waves are the mechanical waves.

Qstn #5
What is meant by an echo? What is the condition necessary for an echo to be heard distinctly?
Ans : 5 If a person stands at some distance from a wall or a hillside and produces a sharp sound, he hears two distinct sounds: one is original sound heard almost instantaneously and the other one is heard after reflection from the wall or hillside, which is called echo.
The condition for the echo: An echo is heard only if the distance between the person producing the sound and the rigid obstacle is long enough to allow the reflected sound to reach the person at least 0.1 second after the original sound is heard.

Qstn #6
A man is standing at a distance of 12 m from a cliff. Will he be able to hear a clear echo? Give a reason for your answer.
Ans : 6 t = 2d/V = 2 × 12/340 = 24/340 <0.1 seconds so the man will not be able to hear the echo. This is because the sensation of sound persists in our ears for about 0.1 second after the exciting stimulus ceases to act.

Qstn #7
State two applications of echo.
Ans : 7 The applications of echo:
(i) Dolphins detect their enemy and obstacles by emitting the ultrasonic waves and hearing their echo.
(ii) In medical science, the echo method of ultrasonic waves is used for imaging the human organs such as the liver, gall bladder, uterus, womb etc. This is called ultrasonography.

Qstn #8
Explain how the speed of sound can be determined by the method of echo.
Ans : 8 Sound is produced from a place at a known distance say, d at least 50 m from the reflecting surface. The time interval t in which the echo reaches the place from where the sound was produced, is noted by a stop watch having the least count 0.01 s. then the speed of sound is calculated by using the following relation
V = total distance travelled / time interval = 2d/t m/s

Qstn #9
State the use of echo by a bat, dolphin and fisherman.
Ans : 9 Bats, dolphin and fisherman detect their enemies or obstacles or position of fish by emitting/sending the ultrasonic waves and hearing/detecting the echo.