# What Is Wave motion, Classification of Waves | Waves and Their Types

In this section, you will read about waves and their types. At the end you will also get to solve important MCQs on waves

## Wave Motion

A wave motion is a process of transferring energy and momentum from one point to another without any actual movement of matter between these points. The particles of the medium only vibrate about their mean positions and do not leave their original respective positions.

The most common form of wave motion with which we are familiar is the waves on the surface of water. If we put a piece of cork on water and then throw a piece of stone in it. We will see the formation of ripples, travelling on the surface of water in concentric circles , but the cork piece moves only in an up and down direction, and it does not travel along with the waves.

Thus in a wave motion, disturbance travels through some medium, but the medium does not travel along with the disturbance.

For formation of a wave, three conditions are required. They are:

(1) Source: It is a vibrating body and it is necessary to create the disturbance.

(ii) Medium: An elastic medium through which the wave transmits. It is also called propagating medium.

(iii)Particles of the medium: These particles take part in the process of onward transmission of the disturbance by executing successive similar vibrations in the source about their respective mean position. Such wave (disturbance) progression along with energy transmission is called a wave-motion.

## Classification of waves.

The waves are broadly categorized into two types. They are:

### 1. Mechanical waves (Elastic waves):

Waves which propagate in materialistic elastic medium like solid, liquid or gas are called mechanical waves. There are two essential features- elasticity and inertia for the existence of the mechanical waves.

#### Types of Mechanical waves

The mechanical waves are further classified into Transverse mechanical waves and Longitudinal mechanical waves. They are discussed below:

##### (a)Transverse mechanical wave :

If in an elastic medium wave propagates (transmits) along the perpendicular direction of the particles vibration then the wave is called transverse mechanical wave.

The features of a transverse mechanical wave are discussed below:

1. Transverse mechanical wave can be generated in solids and upper surfaces of the liquids.
2. Transverse mechanical wave cannot be generated through gases and inside liquids due to lack of rigidity.
3. The transverse wave propagates in the form of crests and troughs.
4. The distance between two adjacent crests or troughs is called wavelength and it is represented by λ.
5. Example of a transverse mechanical wave is the wave formed on the surface of water.
##### (b) Longitudinal mechanical wave :

If in an elastic medium, a wave propagates (transmits) along the direction of particles vibration, then the wave is called longitudinal mechanical wave.

The features of a longitudinal mechanical wave are discussed below:

1. Longitudinal wave can be generated (produced) in all medium solids, liquids and gases.
2. A longitudinal mechanical wave such wave propagates in the form of compressions and rarefactions. In compression the pressure and the density of the medium is maximum, while in rarefaction the pressure and the density of the medium is minimum.
3. The distance between two adjacent compressions or rarefactions is called wavelength and it is represented by λ.
4. Example of longitudinal mechanical wave is a sound wave.

### 2. Non-mechanical waves (Electromagnetic waves)

Waves whose propagation (transmission) does not need any elastic medium and which is generated by the mutual oscillations of electric and magnetic fields perpendicular to each other. Such waves are called non-mechanical waves (electromagnetic waves).

The electromagnetic waves propagate in a perpendicular direction to each electric field and magnetic field and it travels in vacuum with velocity of light 3x10m/s or 3 lakh km/sec. The examples of electromagnetic waves are radio waves, ultra violet rays, X-rays, y-rays, thermal radiations etc.

In the early days light was also assumed to be an elastic wave and a hypothetical medium called ether was supposed to be its medium of propagation. But this theory of ether was disproved by Michelson and Morley experiment.

Some of the important terms related to a wave motion are discussed below:

### 1. Wavelength

Wavelength of a wave is the length of one wave. It is equal to the distance travelled by a wave during the time, any one particle of the medium completes one vibration about its mean position.

In simpler words, the wavelength of a wave is equal to the distance between two adjacent troughs or crests in case of a transverse wave, or distance between two adjacent compressions or rarefactions in case of a longitudinal wave.

Wavelength is denoted by λ, and has the same units as that of length.

### 2. Frequency

The number of vibrations completed by a particle in one second is called frequency.

Since one vibration is equivalent to one wavelength, therefore frequency can also be defined as the number of complete wavelengths traversed by the wave in one second.

Wavelength of a wave is denoted by f or v, and its SI unit is Hertz.

### 3. Time Period

The time taken by a particle to complete one vibration about its mean position is called its Time period.

As one vibration is equivalent to one wavelength, therefore time period of a wave is equal to time taken by the wave to travel a distance equal to one wavelength.

Time period of a wave is denoted by T and its unit is Second.

### 4. Amplitude

The maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium position, is called amplitude.

## Reation between velocity, frequency and wavelength of a wave

Suppose,

f = frequency of the wave

T = time period of the wave

λ = wavelength of the wave

v = velocity of the wave.

Since we know that velocity = distance/time taken .. (1)

In one complete vibration of the particle, distance travelled = λ, and time taken = T

Therefore from (1), we can write v = λ/T …. (2)

Since 1/T = f, therefore from (2), we can write v = λ f

Hence, the velocity of a wave is the product of its wavelength and frequency. This relation is true for both transverse as well as longitudinal waves.

## Some Common Waves

### 1. Sound Waves

1. Sound waves enable us to hear.
2. Sound wave is a longitudinal wave. It travels in the form of compressions and rarefactions.
3. The velocity of a sound wave or simple sound in air at room temperature and normal pressure is roughly 332 m/s.
4. Our ear is sensitive to only those vibrations of sound, whose frequency lies between 20 hertz to 20000 hertz. This range is called audible range.
5. The sound waves whose frequencies are greater than 20000 hertz, they are called ultrasonic vibration or Ultra sonic waves. It cannot be heard by human ear. Bats can hear Ultrasonic waves.
6. The sound waves which have frequencies less than the 20 hertz are called infrasonic waves.
7. Any object moving with a speed greater than the speed of sound is said to move with a supersonic speed.
8. Speed of sound is maximum in Steel, and minimum in vacuum.
9. Speed of sound is maximum in solids minimum in gases.
10. The speed of sound increases with the increase of temperature of the medium.
11. The speed of sound is more in humid air than in dry air because the density of humid air is less than the density of the dry air.
12. The repetition of sound due to reflection of sound waves is called an echo.
13. The sensation of a frequency is commonly referred to as the pitch of a sound. Male voice has less pitch and female voice has more pitch.
14. Sound waves can be used to detect and locate objects especially underwater objects. The devices which employs Sound waves to detect and locate objects is known as SONAR, which stands for Sound Navigation and Ranging.

## MCQs On Waves | Questions On Wave Motion

Now, it is time to solve MCQs on Waves.

1. Steel
2. Air
3. Vacuum
4. Water

Steel

### Speed of sound is minimum in:

1. Steel
2. Air
3. Vacuum
4. Water

Vacuum. Speed of sound in vacuum is 0 m/s, as it can not travel through vacuum

### To measure the speed of an approaching car, a police officer throws:

1. light waves on it.
2. micro waves on it.
4. ultra high frequency on it.

1. Ultra Sonic
2. Infra Sonic
3. Audible
4. Sub Sonic

Ultra Sonic

### Light is a/an :

1. Matter wave
2. Mechanical wave
3. Probability wave
4. Electromagnetic wave

Electromagnetic wave

1. vacuum
2. Solid
3. Liquid
4. Gas

Vacuum

### Waves that are required for wireless communication over long distances are known as:

1. Micro waves
3. Infra Red waves
4. Ultra Violet waves

### Heat from the sun reaches earth by the process of :

1. Conduction
2. Convection
4. All of the above

### The parameter of a wave, which is independent of other three is:

1. Amplitude
2. Velocity
3. Wavelength
4. Frequency

Amplitude. Since velocity, wavelength and freqency are not independent of each other. They are related to each other through v = λ f

### Which of the following waves cannot travel through vacuum:

1. Light
2. Electromagnetic waves
3. Sound waves
4. Heat waves

Sound Waves

### The frequency of ultrasonic waves is:

1. Above 20 kHz.
2. Above 2000 kHz
3. below 20 kHz
4. below 02 kHz

Above 20 kHz

### Which of the following is not shown by the longitudinal waves?

1. Reflection
2. Refraction
3. Polarisation
4. Diffraction

polarisation. Polarization is the phenomenon observed only for transverse waves. Polarization is the property of waves to acquire one-sidedness with respect to the direction of propagation.

This was all about waves and their types. We are sure that you will find this section about waves and their types very useful, and are sure that you might have also solved the MCQs on waves also.

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