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Bohrs Atomic Model

Bohrs Atomic Model Scientist Neils Bohr gave some corrections to Rutherford's atomic model in 1913. This corrected model is called the Bohr's atomic model. The main postulates of this model are - a) The electron moves around the nucleus in a circular motion, about an axis. This circular motion is called an energy level or orbit. They are also known as the shell or permanent orbit or principal energy level. Electrons do not absorb or emit any energy when they revolve these fixed orbits. If we express the permanent orbit as n, then n = 1, 2, 3, 4, etc.  In other words, if n = 1, the principal energy level is K,  when n = 2, L is the principal energy level,  when n = 3, M is the principal energy level,  when n = 4, N is the principal energy level, etc. b) According to Bohr's model, the angular momentum of an electron in an energy level is mvr =  nh / 2𝜋 . Here, m = the mass of the electron (9.11×10 -31  kg) r = the radius of the orbit that the electron follows v = the veloc

Waves and Sound


Chapter - 7: Waves and Sound

👉 All Chapter Contents 👈 

Simple Harmonic Motion

What is simple harmonic motion?

The harmonic motion of constant amplitude in which the acceleration is proportional and oppositely directed to the displacement of the body from a position of equilibrium is called the simple harmonic motion.

Why the vibrating motion of tuning fork is called oscillatory motion?

We know, if a vibratory body moves in one direction on the first half of its period and moves in the opposite direction during the remaining time then the motion of the body is called oscillatory motion. During vibration, the tuning fork moves in one direction in the first half of its period and moves in the opposite direction during the remaining time. And so the motion of tuning fork is oscillatory.

The motion of swing is an oscillatory motion?

If a vibrating body moves in one direction on the first half of its period and moves in the opposite direction during the remaining time then the motion of the body is called oscillatory motion. The swing moves in one direction in the first half of its period and moves in one opposite direction during the remaining time then the motion of the body is called oscillatory motion. And so the motion of the swing is oscillatory.

The motion of the wave of water in the pond is periodic?

When waves are formed in water it displays not only transitional motion but also moves in a upward and downward direction. In other words, the water particles gain periodic motion. Crests and troughs are formed simultaneously in the water, which propagates as a transverse wave. Thus, waves in pond water are periodic motion.


What is wave?

The disturbance or variation that transfers energy progressively from point to point in a medium is called a wave. The medium through which the wave travels may experience some local oscillations as the wave passes, but the particles in the medium do not travel with the wave.

What is mechanical wave?

Mechanical wave is the wave that requires a medium to transport their energy from one location to another. The sound wave is an example fo a mechanical wave.

Characteristics of Waves

  • For mechanical wave a medium is necessary. Waves can be created in water, a wave can be transmitted in a spring, a wave can be created in a rope. The sound we hear is also a wave and its medium is air.

  • When a wave propagates through a medium, the particles of the medium oscillate (vibrate or go up-down) about it own position but are not displaced permanently with the wave.

  • Energy can be transferred from one place to another through waves. The more the energy, the more the amplitude  of the wave. 

  • Every wave has a velocity; this velocity depends on the nature of the medium. In air, the velocity of sound is 330 m/s, in water this velocity is 1439 m/s. The velocity of a wave in a tight rope (under tension) will be more than the velocity of the wave in a loose rope.

  • Reflection or refraction occurs for waves. At the time of travelling from one medium to another medium, if part of a wave returns back to the first medium to the second medium, it is reflection. If we hear external sound when we are submerged under water it is due to refraction.

  • Among all the characteristics of waves, the most important is superposition, even though this is not observed often by our eyes in our daily lives. Let us consider that two different places reach one place. When one wave is trying to pull the medium up, another wave is then trying to pull it down. These are the subjects of superposition, which will give you a deep insight into waves, and you will be able to be understand it very clearly. Two waves, can reinforce one another or can destroy each other.

Types of Wave

There are two types of wave. They are -

a) Longitudinal wave

b) Transverse wave

a) Longitudinal wave: When a wave propagates through a medium, the direction of vibration and the direction of the velocity of wave is the same, this type of wave is called longitudinal wave. Example of longitudinal wave sound wave, spring wave, etc.

b) Transverse wave: When a wave propagates through a medium, the direction of vibration is perpendicular to the direction of the velocity of the wave, this type of wave is called transverse wave. Example of transverse wave is water wave.

Wave related quantities

What is the frequency?

Frequency is the number of cycles per second in a wave.

What is the period?

The period is the time taken for one complete cycle of vibration to pass a given point.

What is the amplitude?

Amplitude is the maximum height of the wave in any direction from the equilibrium position.

What is phase?

Phase is a particular point in time on the cycle of the waveform, measured as an angle in degrees.

What do you understand by the phase of a wave?

The phase is an important characteristics of any wave. Phase specifies the location of a point within a wave cycle of a repetitive waveform. It might represent the direction of velocity, displacement, velocity or acceleration at any instance. In a transverse wave, the crests and the troughs are always in the same phase.

Sound Wave

To produce sound waves a source is needed, a medium to send it through, and any type of receiver to receive that sound.

There are many sources of sound around us.

How to produce sound

The voice of males is deep and that of the females is high pitched - Explain.

The most familiar source is our throat; sound is created by the vibration of the vocal cord when air flows through it in our throat. We will feel the vibration if we tough our throat during conversation. 

The voice of a male is deep and the voice of female and children is sharp. When we make a sound then air comes out through throat from our lungs. In our throat there is a wind pipe through which air enters the lungs and come out from the lungs. 

To produce sound larynx is situated upon it. There are two membranes which work as valves, called vocal cords. These vibrate during the flow of air and produce sound.

The vocal cords of males become stiff with the increasing of age, whereas for females it remains soft. This is why males produce sound of low frequency and females produce sound of high frequency. Due to this, the voice of males is deep and that of the females is high pitched.

Sound can be produced by vibrating the thin diaphragm.

If we hit a school bell, it starts vibrating and sound is produced. If we grip the bell then the vibration will stop and the sound will stop instantly. After producing sound by vibration, we need a medium to transmit it from one place to another. 

Sound can also be sent through liquids and solids but we are used to hearing sound using air as a medium. 

Sound wave is which type of wave?

Sound wave is a longitudinal wave.

What is hearing range?

The hearing range describes the range of frequencies that can be heard by humans or other animals. The range of human hearing is 20 Hz to 20,000 Hz.

Characteristics of Sound Wave

Sound is a mechanical wave, because sound wave is produced due to the vibration of a body, and for its transmission an elastic medium is needed. This is a longitudinal wave, because the direction of propagation of the wave and the direction of vibration is the same. The velocity of sound depends on the property of the medium. In gaseous mediums its velocity depends on the temperature and humidity of the medium. The intensity of sound is proportional to the square of the amplitude of the wave. Therefore, if the amplitude of a wave is high, the intensity of the sound will be high and if the amplitude is low, the intensity will be low.

Why some sounds are harmonic while others are noise?

When the vibration is periodic the sounds are harmonic. When many waves superpose on one another they appear to be melodious. For example, harmonium, flute, guitar, etc. On the other hand, when symmetry is absent between the sound waves generated then the sound appears to be noisy, annoying and harsh. For example, chaos in Bazar, the sound of moving vehicles, etc.


Since sound is one type of wave, it can have reflection. If someone speaks in a big empty building it creates a reverberating sound. This is due to reflection. Within the building the distance is not enough to hear the sound separately. When we hear something the feeling persist for around 0.1 s, so to hear two sounds separately, it needs separation of 0.1 s differences between two sounds. Velocity of sound is 330 m/s, therefore to create a difference of 0.1 s, sound has to travel at least 33 m.

If we stand in front of a big wal, a building, or a high hill at least half of this distance (16.5 m), the sound will take 0.1 s to come back after being reflected and we will hear the reflection of sound or echo.

Bats have eyes and pretty good eye sight, yet they use echo during flight. While flying a bat uses its throat to produce sound, if there is an obstacle in the front, the sound returns after reflection, and the bat can estimate the distance from the without being obstructed. We cannot hear the sound produced by the bat; because the sound is ultrasound i.e. frequency of the sound is beyond our hearing range. Bats can produce sound of frequency 100 kHz or more.

What is an echo?

The repetition of sound produced by the reflection of sound waves from a wall, mountain, or other obstructing surface and arrives at the listener with a delay after the direct sound is called an echo.

Variation of Velocity of Sound

In air, velocity of sound is proportional to the square root of temperature. 

i.e. v ∝√T

Here, the temperature is not Celsius temperature. Temperature in the Kelvin scale.

The velocity of sound does not depend on the air pressure. But it is inversely proportional to the square root of the density of air, So if there is water vapor in the air, its density decreases, hence velocity increases.

Sound is a mechanical wave. It depends on the elasticity of the medium. The nature of liquid and solid is different from air and naturally the velocity of sound is different there. In liquid the velocity of sound is more than that in air and in solid velocity of sound is even more than in liquid.

Velocity of sound in different medium

Medium m/s
Air 330
Water 1,493
Iron 5,130
Diamond 12,000

What is the velocity of sound wave in water medium at 20℃?

The velocity of the sound wave in water medium at 20℃ is 1450 ms-1.

If we hit a hollow steel bar on one side why do we hear sound twice on the other side?

Hollow steel bar is filled with air. If we hit one side of the steel bar, the sound wave propagates in both air and solid media. The sound wave moves faster in a solid medium. As a result, the first sound reaches us rapidly. After an interval, the sound reaches us through the air medium. And so we can hear the sound twice.

Uses of Sound

We speak, listen to songs, doctors hear the heart beat, engineers hear the sound of instruments, etc. 

There are other applications of sound, which you may not have heard about. Previously we could not see from outside the fetus that grows in the womb of a pregnant mother; which is now possible with the help of a process called ultrasonography.

Three dimensional Seismic Surveys

Seismic survey is done to explore whether gas or oil is present below the earth. To do this, a small blast is done a little below the earth. Sound of the blast hits the different layers of the soil and returns after reflection. the reflected wave is detected by a special type of receiver named a geophone. Analyzing all information a perfect three dimensional picture of the soil is formed, from which gas or oil is present or not is determined. Since we know the position of the source of sound and the geophone, the distance of the different layers of soil can be calculated accurately from the time required for the sound to reach from source to geophone.

Ultrasound cleaner

Ultrasound cleaner is used when we need to clean small instruments perfectly in the laboratory. The smal instruments are immersed in a liquid, and subjected to the ultrasound wave, the vibrations of which remove the dirt.

Musical Sound

We hear different kinds of sound, some of these sounds are sweet and some of them are annoying. The sweet sounds are the sounds of musical instruments.

In a tuning fork a perfect vibration is created. But in a musical sound there is not only a single wave, many waves superpose on one another to make the sound melodious.

What is musical sound?

The sound waves that maintain intensity, pitch and timber are called musical sounds. As a result, these sounds are harmonic.

What is pitch?

The characteristics of musical sound by which the sound of the same intensity is sometimes heard dull and sometimes sharp is called pitch.

What is the intensity of sound?

Intensity is the measure of how loudly a sound can be heard. Therefore, the amount of energy that flows per sound per unit area is called the intensity.

Different Types of Musical Instruments

Different types of musical instruments that are used to produce melodious sound, can be divided into three groups:

Musical instrument made of wire: Ektara (Monochord), Violin, Sitar

Musical instruments based on air flow: Flute, Harmonium

Musical instrument made of percussion: Dhol, Tabla

Now a days melodious sounds are produced using electronics in a completely different way.

Sound Pollution

Sound is essential in our life, but excessive sound makes our life intolerable. Those who live in cities, especially those who live besides a road observe that the sound produced from engines of bus, car, truck and of continuous horns goes beyond our tolerance limit very often.

Most of the time we are used to living in such types of pollution for a long time. If we fortunate enough to go to a quiet place where there is no sound pollution then suddenly we can realize the importance of life without sound pollution.

What is sound pollution?

The sound generated from various sources that go beyond our tolerance limit, causes irritation and is injurious to health is called sound pollution.

Bad impact of Sound Pollution

Due to sound pollution, we are losing our hearing capability. To intensify the problem many of us listen to songs using headphones in the ears.

How to reduce Sound Pollution?

To reduce sound pollution the first step is to make laws against it. The second step is to make public awareness. This issue has to be clarified to all, using horns while in cars as little as possible, installation of sound absorbing machine in the industry, making less uses of loud speaker, using vehicles of less sound, etc. In addition, steps be taken of plantation of trees in the empty places in cities.

Question Bank

Simple Harmonic motion

  • What is simple harmonic motion?

Secondary Physics


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