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Introduction to Waves in Physics - Wave Motion, Types of Waves and Wave Terminology - Class 11 Notes



Comprehensive Class 11 Physics notes about Introduction to Waves in Physics where we covering wave motion essentials including types of waves, wave terminology, and fundamental properties. Students will learn about mechanical, electromagnetic, and matter waves with detailed explanations of amplitude, frequency, wavelength, and wave equations. Ideal for CBSE Class 11 curriculum and competitive exam preparation for NEET and JEE Main physics sections.


Introduction to Wave Motion

Wave is the propagation of disturbance in a medium without the actual transfer of matter. When a wave travels through a medium, the particles of the medium oscillate about their mean positions, but the medium as a whole does not move from one place to another.

Characteristics of Waves

  • Waves transfer energy and momentum from one point to another without transferring matter
  • Different types of waves exist: sound waves, light waves, water waves, seismic waves, and electromagnetic waves
  • Mechanical waves require a material medium for propagation, while electromagnetic waves can travel through vacuum

What is a medium?

A medium in wave motion is the substance or material that allows a wave to travel from one place to another. It is important to note that, the medium does not become the wave itself but just acts as the transporter of the wave’s energy.

Key Points about Medium in Wave Motion

  • A medium is the material or substance through which a wave propagates.

  • It acts as the carrier of the wave but does not create the wave itself.

  • The particles of the medium do not travel with the wave; instead, they usually vibrate about their mean positions, transferring the disturbance (energy, not matter) along the medium.

  • Different types of waves require different media:

    • Mechanical waves (like sound, water waves, or waves in a string) need a medium such as air, water, or a solid. They cannot travel without it.
    • Electromagnetic waves (like light, radio waves) do not require a medium and can travel through a vacuum.
  • Examples of wave media:

    • Slinky coils act as the medium for slinky waves.
    • Water acts as the medium for ocean waves.
    • Air acts as the medium for sound waves traveling in a room.

A medium for a wave is like a messenger: it does not create the message, and it is not the message itself. It only carries the wave (energy) from one location to another.

Understanding Wave Motion Through Examples

String Wave Experiment

Take a string and tie one end to a rigid support like a wall, while holding the other end in your hand. When you move your hand up and down, a disturbance is created in the string which moves from one end (your hand) to the other end (the wall).

Key Observations:

  • The disturbance travels as a transverse wave along the string
  • Particles of the string move perpendicular to the direction of wave propagation
  • The actual string material doesn't flow from your hand to the wall
  • Only the wave pattern travels along the string length

wave motionin a string

This demonstrates that the wave carries the disturbance without transporting the medium itself.

Water Wave Example

Consider throwing a pebble into calm water of a lake. The disturbance created spreads outward in concentric circles with constant speed until reaching the boundary.

Important Observation: The water in the lake is not flowing away from the disturbance point. Instead, water particles oscillate up and down in the same location while the disturbance pattern moves outward.

wave motionin a string

Cork Experiment

If a cork is placed on the water surface during wave propagation, it oscillates up and down but does not move away from its original position. This demonstrates that the medium (water) is not flowing, but rather the disturbance is being transferred.

Wave Motion in Elastic Medium

Fundamental Principle

When a disturbance is created at any point in an elastic medium, it advances through the medium with constant speed, but the medium as a whole does not move. Only the disturbance pattern propagates.

Particle Motion Analysis

When disturbance occurs at a point in an elastic medium :

  • The particle at that point begins simple harmonic motion about its mean position
  • Due to elasticity and inertia, neighbouring particles also start oscillating
  • Each particle executes simple harmonic motion about its respective mean position
  • The disturbance propagates with a definite speed characteristic of the medium

Wave Speed Relationship

The wave motion involves transmission of energy and momentum from one point to another without matter transfer. This is fundamentally different from material particle motion where the object physically moves from one location to another.

Key Learning Points

Essential Concepts to remember for CBSE Class 11

  1. Wave Definition: Propagation of disturbance without matter transfer
  2. Energy Transfer: Waves carry energy and momentum across distances
  3. Medium Behavior: Particles oscillate about mean positions during wave passage
  4. Speed Characteristics: Wave speed depends on medium properties, not on amplitude
  5. Practical Applications: Understanding wave principles helps explain sound, light, and water phenomena

Types of Waves

Classification of Waves

Physics recognizes two distinct types of waves based on their fundamental characteristics and propagation requirements.

Mechanical Waves

Definition and Characteristics

Mechanical waves are waves that can be produced and propagated only in a material medium. These waves cannot exist without matter and require the presence of particles to transfer energy from one point to another.

Common Examples of Mechanical Waves

  • Water waves: Ripples on water surface requiring water molecules as medium
  • Sound waves: Vibrations in air, liquids, or solids that need matter particles for propagation
  • String waves: Transverse vibrations along stretched strings or ropes
  • Seismic waves: Earthquake waves traveling through Earth's crust and interior

Key Properties

  • Always require a material medium for existence
  • Cannot propagate through vacuum
  • Speed depends on properties of the medium (density, elasticity)
  • Energy transfer occurs through particle oscillations

Electromagnetic Waves

Definition and Unique Features

Electromagnetic waves do not require any material medium for their production or propagation. These remarkable waves can travel through vacuum as well as through material substances.

Universal Speed Characteristic

All electromagnetic waves travel in vacuum with the same fundamental speed of 3 × 10⁸ m s⁻¹ (approximately 300,000 kilometres per second). This speed, known as the speed of light, represents one of the most important constants in physics.

Examples of Electromagnetic Waves

  • Light waves: Visible radiation that enables sight
  • Radio waves: Used for communication and broadcasting
  • Microwaves: Used in cooking and satellite communication
  • X-rays: Used in medical imaging
  • Gamma rays: High-energy radiation from nuclear processes

Important Applications

  • Enable space communication since they can travel through vacuum
  • Form the electromagnetic spectrum covering vast frequency range
  • Essential for modern technology including wireless communication

summary of types of waves and its properties

Figure: MInd map on types of waves and its properties for quick revision

Wave Terminology - Essential Definitions

These terms given below form the foundation for all wave-related calculations and understanding various wave phenomena.

Amplitude (A)

Amplitude represents the maximum displacement of a vibrating particle on either side of its mean position. This is one of the most important characteristics of any wave motion.

Key Points about Amplitude:

  • Amplitude is maximum at the source and decreases as the wave travels away from it
  • Unit: metre (m)
  • Symbol: Usually denoted by A
  • Determines the energy carried by the wave

Frequency (f or ν)

Frequency is defined as the number of oscillations per second performed by the particles of the medium. It represents how often the wave pattern repeats itself.

Important Characteristics:

  • Though amplitude varies at different particles, frequencies remain the same for all particles
  • All particles are forced to vibrate by the same source
  • Unit: hertz (Hz)
  • Symbol: Usually denoted by n or ν (Greek letter nu)

Phase

Phase describes the state of motion of a particle in the medium at a given point and given time. This concept helps understand the relative motion of different particles.

Phase Information Includes:

  • Position of the particle at that specific moment
  • Direction of motion at that point and time
  • Complete description of particle's instantaneous state

Time Period (T)

Time period of a wave is the time taken for particles of the medium to complete one full oscillation. It represents the duration of one complete wave cycle.

Key Features:

  • Unit: second (s)
  • Symbol: Denoted by T
  • Related to frequency by the relationship: \(T = \frac{1}{f}\)

Wavelength (λ)

Wavelength is the minimum distance between two particles of the medium which are in the same phase of vibration. This defines the spatial extent of one complete wave pattern.

Alternative Definition: Wavelength can also be defined as the distance traversed by the disturbance in one time period (T).

Specifications:

  • Unit: metre (m)
  • Symbol: Usually denoted by λ (Greek letter lambda)

Wave Velocity (v)

Wave velocity represents the speed with which the wave propagates through the medium. This is fundamentally different from particle velocity.

Mathematical Relationship: If v is the wave velocity, λ is the wavelength, and T is the time period:

\(\lambda = v \cdot T = v \cdot \frac{1}{f}\)

Therefore: \(v = f\lambda\)

Where \(f = \frac{1}{T}\) is the frequency.

Wave Vector (Propagation Constant)

Wave vector or propagation constant is 2π times the number of waves contained in unit length. This quantity helps in mathematical analysis of wave motion.

Mathematical Expression: \(k = \frac{2\pi}{\lambda}\)

Properties:

  • Unit: rad m⁻¹ or simply m⁻¹
  • Also known as angular wave number
  • Essential for advanced wave equations and analysis

Important Relationships for Students

Fundamental Wave Equation: \(v = f\lambda\)

Frequency-Period Relationship: \(f = \frac{1}{T}\)

Wave Vector Relationship: \(k = \frac{2\pi}{\lambda}\)

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