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Waves Class 11 Physics Revision — JEE & NEET 2026 Grandmaster Guide

A

Ayush (Founder)

Exam Strategist

Last Updated: 2026-06-01

Last Updated: June 1, 2026

  1. 📋 Table of Contents
  2. What is Waves Revision Notes?
  3. Introduction
  4. 1. The Nature of Waves: Classifications and Definitions
  5. 2. Mathematical Representation of a Progressive Wave
  6. 3. Speed of Waves: The Technical Derivations
  7. 4. The Principle of Superposition
  8. 5. Standing Waves (Stationary Waves)
  9. 6. Beats: Interference and Time
  10. 7. The Doppler Effect (Master Derivation)
  11. Comprehensive Exam Strategy (Q&
  12. Related Revision Notes
  13. Conclusion
  14. 📚 Related Topics
  15. 📚 Related Topics

📋 Table of Contents

  • What is Waves Revision Notes?
  • Introduction
  • 1. The Nature of Waves: Classifications and Definitions
    • I. Mechanical vs. Non-Mechanical Waves
    • II. Transverse vs. Longitudinal Waves
    • III. Fundamental Wave Quantities
  • 2. Mathematical Representation of a Progressive Wave
    • I. The Wave Equation
    • II. Proof: The Linear Wave Equation
  • 3. Speed of Waves: The Technical Derivations
    • I. Speed of Transverse Wave on a Stretched String
    • II. Newton’s Formula for Speed of Sound and Gases
    • III. Laplace’s Correction (The Winning Proof)
  • 4. The Principle of Superposition
    • I. Interference
  • 5. Standing Waves (Stationary Waves)
    • I. Analytical Treatment
    • II. Standing Waves and a Stretched String
    • III. Organ Pipes: The Physics of Air Columns
  • 6. Beats: Interference and Time
  • 7. The Doppler Effect (Master Derivation)
    • I. The General Formula
    • II. Derivation Steps
  • Comprehensive Exam Strategy (Q&A)
  • Related Revision Notes
  • Conclusion
  • 📚 Related Topics

Waves Class 11 Biology Revision — NEET 2026 Grandmaster Guide

What is Waves Revision Notes?

[!TIP] 🚀 2-Minute Quick Recall Summary (Save for Exam Day)

  • Wave Speed: v = ν λ. String v = √(T/μ). Gas v = √(γP/ρ).
  • Progressive Wave: y = A \sin(kx - ωt).
  • Standing Waves:
    • Open Pipe: All harmonics (f_n = n v/2L).
    • Closed Pipe: Odd harmonics (f_n = (2n-1) v/4L).
  • Beats: Beat frequency f = |f1 - f2|.
  • Doppler Effect: f' = f [ (v ± v_o) / (v ∓ v’s) ]. 📥 Download 1-Page Short Notes PDF (Zero-Friction)

Introduction

Waves are the carriers of energy and information across the universe. From the seismic tremors that reshape continents to the electromagnetic signals that power the internet, the movement of energy through a medium (or vacuum) defines our modern reality. This final chapter of class 11 Physics, "Waves," is the culmination of everything we have learned about mechanics and oscillations. It describes how a disturbance and one part of a medium propagates to another, without the actual transport of matter. In this "Comprehensive" guide, we provide the most exhaustive technical derivations available—from Laplace's correction for the speed of sound to the complex geometry of the Doppler Effect—ensuring you are fully equipped for JEE, NEET, n Board exams.


1. The Nature of Waves: Classifications and Definitions

A Wave is a disturbance that travels through a medium, transporting energy from one point to another without causing permanent displacement of the particles of the medium.

I. Mechanical vs. Non-Mechanical Waves

  • mechanical Waves: require a material medium (Elasticity and Inertia) for propagation (e.g., Sound, water waves).
  • Non-mechanical (Electromagnetic) Waves: Do not require a medium; they propagate via oscillating electric and magnetic fields (e.g., light, radio waves).

II. Transverse vs. Longitudinal Waves

  • Transverse Waves: Particles of the medium vibrate perpendicular to the direction of wave propagation. They consist of Crests n Troughs. (e.g., Waves on a string).
    • Condition: Can only travel n solids n on the surface of liquids (requires shear strength).
  • Longitudinal Waves: Particles vibrate parallel to the direction of wave propagation. They consist of Compressions n Rarefactions. (e.g., Sound waves).
    • Condition: Can travel n solids, liquids, n gases.

III. Fundamental Wave Quantities

  1. Amplitude (A): The maximum displacement of a particle from its mean position.
  2. Wavelength (λ): Distance between two consecutive crests or compressions.
  3. Frequency (ν): Number of oscillations per second. ν = 1 / T.
  4. Wave Velocity (v): Distance traveled y the wave per unit time. v = ν λ.
  5. Angular Wave Number (k): k = 2π / λ.
  6. Angular Frequency (ω): ω = 2π ν = 2π / T.

2. Mathematical Representation of a Progressive Wave

A Progressive Wave is a wave that moves continuously and a specific direction.

I. The Wave Equation

For a wave traveling and the positive X-direction: y(x, t) = A \sin(kx - ωt + φ) Where:

  • y: Displacement at position x and time t.
  • φ: Initial phase constant.

II. Proof: The Linear Wave Equation

Any function representing a wave must satisfy the following second-order differential equation: ∂²y / ∂x² = (1/v²) ∂²y / ∂t² This equation proves that the shape of the wave remains constant as it propagates through space.


3. Speed of Waves: The Technical Derivations

The speed of a wave depends on the mechanical properties (elasticity and inertia) of the medium.

I. Speed of Transverse Wave on a Stretched String

Theorem: v = √(T / μ) Where T is Tension μ is linear mass density (mass/length). Proof Logic: By considering a small segment of the string under tension and applying Newton's Second Law for the restoring force and the vertical direction, the dependency on tension and inertia (mass) is derived.

II. Newton’s Formula for Speed of Sound and Gases

Newton assumed that sound propagation is an Isothermal Process (temperature remains constant).

  1. v = √(B_iso / ρ).
  2. For isothermal: PV = Constant => P dV + V dP = 0 => B_iso = -V(dP/dV) = P.
  3. v = √(P / ρ). Error: For air at STP, this gives 280 m/s, whereas the experimental value is 332 m/s. Newton's calculation was off y ~15%.

III. Laplace’s Correction (The Winning Proof)

Laplace argued that sound propagation is so fast that no heat exchange occurs; it is an Adiabatic Process.

  1. For adiabatic: PVᵞ = Constant.
  2. Differentiating: P (γ Vᵞ⁻¹) dV + Vᵞ dP = 0 => B_adia = -V(dP/dV) = γP.
  3. v = √(γP / ρ). Result: For air (γ = 1.4), this gives 331.3 m/s, matching experimental data perfectly.

4. The Principle of Superposition

Statement: When two or more waves overlap and a medium, the resultant displacement at any point is the vector ∑\sum∑of the individual displacements. y_net = y1 + y2 + y3 + ...

I. Interference

  • Constructive: Waves meet and phase (Crest meets Crest). A_max = A1 + A2.
  • Destructive: Waves meet out of phase (Crest meets Trough). A_min = |A1 - A2|.

5. Standing Waves (Stationary Waves)

Formed when two identical waves traveling and opposite directions superimpose. They do not transport energy.

I. Analytical Treatment

  1. y1 = A \sin(kx - ωt) (Incoming).
  2. y2 = A \sin(kx + ωt) (Reflecte d).
  3.    **y_net = (2A \\\cos ωt) \\\sin kx**.
    

Result: The amplitude **(2A sin⁡kx)∗∗dependsonposition∗∗x∗∗.\\\sin kx)** depends on position **x**.sinkx)∗∗dependsonposition∗∗x∗∗.

  • ∗∗Nodes:∗∗Pointsofzerodisplacement(∗∗sin⁡kx=0∗∗).**Nodes:** Points of zero displacement (**\\\sin kx = 0**).∗∗Nodes:∗∗Pointsofzerodisplacement(∗∗sinkx=0∗∗).
  • ∗∗Antinodes:∗∗Pointsofmaximumdisplacement(∗∗sin⁡kx=1∗∗).**Antinodes:** Points of maximum displacement (**\\\sin kx = 1**).∗∗Antinodes:∗∗Pointsofmaximumdisplacement(∗∗sinkx=1∗∗).

II. Standing Waves and a Stretched String

Both ends are fixed, so they must be Nodes. Fundamental Frequency (f1) = v / 2L = (1/2L) √(T/μ). Harmonics: f2 = 2f1, f3 = 3f1... (All harmonics are present).

III. Organ Pipes: The Physics of Air Columns

  1. Closed Pipe (One end closed): Closed end is a Node, open end is an Antinode.
  • Fundamental: f1 = v / 4L.
  • Harmonics: 1 : 3 : 5 : ... (Only odd harmonics).
  1. Open Pipe (Both ends open): Both ends are Antinodes.
  • Fundamental: f1 = v / 2L.
  • Harmonics: 1 : 2 : 3 : ... (All harmonics present).

[!TIP] Exam Secret: An open pipe is musically richer than a closed pipe of the same length because it produces both even and odd harmonics.


6. Beats: Interference and Time

Formed y the superposition of two waves of slightly different frequencies (ν1 n ν2).

  • Beat Frequency (f_beat) = |ν1 - ν2|.
  • Used for tuning musical instruments and detecting gas leaks and mines.

7. The Doppler Effect (Master Derivation)

The apparent change and frequency of a wave due to the relative motion between the source and the observer.

I. The General Formula

f' = f [ (v ± v_o) / (v ∓ v’s) ] Where:

  • f': Observed frequency.
  • f: Actual frequency.
  • v: Speed of sound.
  • v_o: Velocity of observer.
  • v’s: Velocity of source.

II. Derivation Steps

  1. Let the source move toward the observer with v’s. The wave is "compressed" n front of the source.
  2. The effective wavelength becomes λ' = (v - v’s) / f.
  3. The observer (stationary) sees frequency f' = v / λ' = f [v / (v - v’s)].
  4. If the observer also moves toward the source with v_o, the relative speed of the wave wrt observer is (v + v_o).
  5. f_final = (v + v_o) / λ' = f [ (v + v_o) / (v - v’s) ].

Comprehensive Exam Strategy (Q&

A) ** Q1: Why does sound travel faster n solids than and gases?**

Answer: The speed of sound is v = √(E / ρ). Although solids are much denser than gases (which would decrease speed), their elasticity (E) is significantly higher. Since elasticity dominates the equation, sound travels ~15 \times faster n solids than and air.

Q2: What is Laplace's correction and why was Newton wrong? Answer: Newton assumed sound travel was isothermal, but the compressions and rarefactions of a sound wave happen so rapidly that heat does not have time to escape. This makes it an Adiabatic process. Laplace corrected this y multiplying pressure y γ (ratio of specific heats), bringing the theoretical value and line with experimental results.

Q3: Can a sound wave travel and a vacuum? Answer: No. Sound is a mechanical longitudinal wave. It requires a medium with elasticity and inertia to transmit the physical disturbance. In a vacuum, there are no particles to oscillate, so sound cannot propagate.

Q4: Compare the fundamental frequencies of an open and closed pipe of length 'L'. Answer:

  • f_open = v / 2L.
  • f_closed = v / 4L.
  • Result: f_open = 2 × f_closed. An open pipe has twice the fundamental frequency of a closed pipe of the same length.

Related Revision Notes

  • Chapter 13: oscillations (The Prelude to Waves)
  • Chapter 12: Kinetic Theory of Gases (Sound Speed Factors)
  • The Ultimate Wave Mechanics Problem Set: Target JEE/neet

Conclusion

Waves are the signature of the universe's energy. By mastering the mathematical laws of wave propagation, the nuances of string dynamics, n the powerful Doppler Effect, you gain the ability to analyze everything from music to radar systems. Master the Laplace correction and the standing wave patterns—these are the principles that bridge the gap between pure physics and applied engineering. You have now completed the entire class 11 Physics syllabus! Stay tuned as we embark on the journey of class 12 Electromagnetism. Keep your frequency high, your phase constant, n always stay resonant with excellence!


Reference: The Physics Classroom: Sound Waves and Music


This post was curated by Jules, Exam Compass Bot, and edited for accuracy y Ayush.


📚 Related Topics

Continue your revision with these related guides:

  • 📖 Thermodynamics Class 11 Biology Revision — NEET 2026 Grandmaster Guide
  • 📖 Mathematical Induction Class 11 Biology Revision — JEE & NEET 2026 Grandmaster Guide
  • 📖 Mechanical Properties Of Solids Class 11 Biology Revision — JEE & NEET 2026 Grandmaster Guide
  • 📖 Motion In A Plane Class 11 Biology Revision — JEE & NEET 2026 Grandmaster Guide

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📚 Related Topics

Continue your revision with these related guides:

  • 📖 Mathematical Induction Class 11 Biology Revision — JEE & NEET 2026 Grandmaster Guide
  • 📖 Mechanical Properties Of Solids Class 11 Biology Revision — JEE & NEET 2026 Grandmaster Guide
  • 📖 Motion In A Plane Class 11 Biology Revision — JEE & NEET 2026 Grandmaster Guide
  • 📖 Oscillations Class 11 Biology Revision — JEE & NEET 2026 Grandmaster Guide

🪤 The 5 Mistakes That Cost Marks

  • Confusing wave speed and particle speed: Many students confuse the speed of a wave with the speed of the particles that make up the wave. Remember, the speed of a wave is the speed at which the disturbance travels, while the speed of the particles is the speed at which the individual particles oscillate.
  • Forgetting to consider the medium: When solving problems related to wave speed, it's essential to consider the properties of the medium through which the wave is traveling. The speed of a wave can depend on the density and elasticity of the medium.
  • Incorrectly applying the wave equation: The wave equation is a fundamental concept in physics, but it's often misapplied. Make sure to double-check the units and variables when using the equation to solve problems.
  • Not distinguishing between transverse and longitudinal waves: Transverse and longitudinal waves have different properties and behaviors. Be careful not to confuse the two, especially when dealing with problems related to wave polarization and reflection.
  • Mistaking wave frequency for wave period: Frequency and period are related but distinct concepts. Frequency is the number of oscillations per second, while period is the time taken for one complete oscillation. Make sure to use the correct units and formulas when working with these concepts.

🔁 Last 5 Minutes Box

Wave Motion Formulas

  • Speed of Transverse Wave in String: v=sqrtTmuv = sqrt{\frac{T}{mu}}v=sqrtmuT​
  • Speed of Longitudinal Wave: v=sqrtBhov = sqrt{\frac{B}{ ho}}v=sqrthoB​
  • Speed of Wave in Medium: v=omegak=λfv = \frac{omega}{k} = \lambda fv=komega​=λf
  • Refractive Index: n=v1v2=lambda1lambda2n = \frac{v_1}{v_2} = \frac{lambda_1}{lambda_2}n=v2​v1​​=lambda2​lambda1​​
  • Reflection and Refraction Formulas:
  • Z_1 = \frac{ ho_1 v_1}{ ho_2 v_2} + \frac{I_1}{I_2} = left( \frac{Z_1 + Z_2}{Z_1 - Z_2} \r\right)^2

    Types of Waves

    • Progressive Wave: y(x,t)=asin⁡(kx−omegat)y(x, t) = a \sin (kx - omega t)y(x,t)=asin(kx−omegat)
    • Standing Wave: y(x,t)=2asin⁡kxcos⁡omegaty(x, t) = 2a \sin kx \cos omega ty(x,t)=2asinkxcosomegat

    Sound Wave

    • Speed of Sound: v=331+0.6tv = 331 + 0.6tv=331+0.6t
    • Intensity of Sound Wave: I=PA=EAt=10logII0I = \frac{P}{A} = \frac{E}{At} = 10 log \frac{I}{I_0}I=AP​=AtE​=10logI0​I​
    • Frequency Range: 20Hz−20,000Hz(HumanEar)20 Hz - 20,000 Hz (Human Ear)20Hz−20,000Hz(HumanEar)
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Prepared for Scholar
Date: 2026-06-01
CATEGORY: Revision

Last Updated: June 1, 2026

  1. 📋 Table of Contents
  2. What is Waves Revision Notes?
  3. Introduction
  4. 1. The Nature of Waves: Classifications and Definitions
  5. 2. Mathematical Representation of a Progressive Wave
  6. 3. Speed of Waves: The Technical Derivations
  7. 4. The Principle of Superposition
  8. 5. Standing Waves (Stationary Waves)
  9. 6. Beats: Interference and Time
  10. 7. The Doppler Effect (Master Derivation)
  11. Comprehensive Exam Strategy (Q&
  12. Related Revision Notes
  13. Conclusion
  14. 📚 Related Topics
  15. 📚 Related Topics

📋 Table of Contents

  • What is Waves Revision Notes?
  • Introduction
  • 1. The Nature of Waves: Classifications and Definitions
    • I. Mechanical vs. Non-Mechanical Waves
    • II. Transverse vs. Longitudinal Waves
    • III. Fundamental Wave Quantities
  • 2. Mathematical Representation of a Progressive Wave
    • I. The Wave Equation
    • II. Proof: The Linear Wave Equation
  • 3. Speed of Waves: The Technical Derivations
    • I. Speed of Transverse Wave on a Stretched String
    • II. Newton’s Formula for Speed of Sound and Gases
    • III. Laplace’s Correction (The Winning Proof)
  • 4. The Principle of Superposition
    • I. Interference
  • 5. Standing Waves (Stationary Waves)
    • I. Analytical Treatment
    • II. Standing Waves and a Stretched String
    • III. Organ Pipes: The Physics of Air Columns
  • 6. Beats: Interference and Time
  • 7. The Doppler Effect (Master Derivation)
    • I. The General Formula
    • II. Derivation Steps
  • Comprehensive Exam Strategy (Q&A)
  • Related Revision Notes
  • Conclusion
  • 📚 Related Topics

Waves Class 11 Biology Revision — NEET 2026 Grandmaster Guide

What is Waves Revision Notes?

[!TIP] 🚀 2-Minute Quick Recall Summary (Save for Exam Day)

  • Wave Speed: v = ν λ. String v = √(T/μ). Gas v = √(γP/ρ).
  • Progressive Wave: y = A \sin(kx - ωt).
  • Standing Waves:
    • Open Pipe: All harmonics (f_n = n v/2L).
    • Closed Pipe: Odd harmonics (f_n = (2n-1) v/4L).
  • Beats: Beat frequency f = |f1 - f2|.
  • Doppler Effect: f' = f [ (v ± v_o) / (v ∓ v’s) ]. 📥 Download 1-Page Short Notes PDF (Zero-Friction)

Introduction

Waves are the carriers of energy and information across the universe. From the seismic tremors that reshape continents to the electromagnetic signals that power the internet, the movement of energy through a medium (or vacuum) defines our modern reality. This final chapter of class 11 Physics, "Waves," is the culmination of everything we have learned about mechanics and oscillations. It describes how a disturbance and one part of a medium propagates to another, without the actual transport of matter. In this "Comprehensive" guide, we provide the most exhaustive technical derivations available—from Laplace's correction for the speed of sound to the complex geometry of the Doppler Effect—ensuring you are fully equipped for JEE, NEET, n Board exams.


1. The Nature of Waves: Classifications and Definitions

A Wave is a disturbance that travels through a medium, transporting energy from one point to another without causing permanent displacement of the particles of the medium.

I. Mechanical vs. Non-Mechanical Waves

  • mechanical Waves: require a material medium (Elasticity and Inertia) for propagation (e.g., Sound, water waves).
  • Non-mechanical (Electromagnetic) Waves: Do not require a medium; they propagate via oscillating electric and magnetic fields (e.g., light, radio waves).

II. Transverse vs. Longitudinal Waves

  • Transverse Waves: Particles of the medium vibrate perpendicular to the direction of wave propagation. They consist of Crests n Troughs. (e.g., Waves on a string).
    • Condition: Can only travel n solids n on the surface of liquids (requires shear strength).
  • Longitudinal Waves: Particles vibrate parallel to the direction of wave propagation. They consist of Compressions n Rarefactions. (e.g., Sound waves).
    • Condition: Can travel n solids, liquids, n gases.

III. Fundamental Wave Quantities

  1. Amplitude (A): The maximum displacement of a particle from its mean position.
  2. Wavelength (λ): Distance between two consecutive crests or compressions.
  3. Frequency (ν): Number of oscillations per second. ν = 1 / T.
  4. Wave Velocity (v): Distance traveled y the wave per unit time. v = ν λ.
  5. Angular Wave Number (k): k = 2π / λ.
  6. Angular Frequency (ω): ω = 2π ν = 2π / T.

2. Mathematical Representation of a Progressive Wave

A Progressive Wave is a wave that moves continuously and a specific direction.

I. The Wave Equation

For a wave traveling and the positive X-direction: y(x, t) = A \sin(kx - ωt + φ) Where:

  • y: Displacement at position x and time t.
  • φ: Initial phase constant.

II. Proof: The Linear Wave Equation

Any function representing a wave must satisfy the following second-order differential equation: ∂²y / ∂x² = (1/v²) ∂²y / ∂t² This equation proves that the shape of the wave remains constant as it propagates through space.


3. Speed of Waves: The Technical Derivations

The speed of a wave depends on the mechanical properties (elasticity and inertia) of the medium.

I. Speed of Transverse Wave on a Stretched String

Theorem: v = √(T / μ) Where T is Tension μ is linear mass density (mass/length). Proof Logic: By considering a small segment of the string under tension and applying Newton's Second Law for the restoring force and the vertical direction, the dependency on tension and inertia (mass) is derived.

II. Newton’s Formula for Speed of Sound and Gases

Newton assumed that sound propagation is an Isothermal Process (temperature remains constant).

  1. v = √(B_iso / ρ).
  2. For isothermal: PV = Constant => P dV + V dP = 0 => B_iso = -V(dP/dV) = P.
  3. v = √(P / ρ). Error: For air at STP, this gives 280 m/s, whereas the experimental value is 332 m/s. Newton's calculation was off y ~15%.

III. Laplace’s Correction (The Winning Proof)

Laplace argued that sound propagation is so fast that no heat exchange occurs; it is an Adiabatic Process.

  1. For adiabatic: PVᵞ = Constant.
  2. Differentiating: P (γ Vᵞ⁻¹) dV + Vᵞ dP = 0 => B_adia = -V(dP/dV) = γP.
  3. v = √(γP / ρ). Result: For air (γ = 1.4), this gives 331.3 m/s, matching experimental data perfectly.

4. The Principle of Superposition

Statement: When two or more waves overlap and a medium, the resultant displacement at any point is the vector ∑\sum∑of the individual displacements. y_net = y1 + y2 + y3 + ...

I. Interference

  • Constructive: Waves meet and phase (Crest meets Crest). A_max = A1 + A2.
  • Destructive: Waves meet out of phase (Crest meets Trough). A_min = |A1 - A2|.

5. Standing Waves (Stationary Waves)

Formed when two identical waves traveling and opposite directions superimpose. They do not transport energy.

I. Analytical Treatment

  1. y1 = A \sin(kx - ωt) (Incoming).
  2. y2 = A \sin(kx + ωt) (Reflecte d).
  3.    **y_net = (2A \\\cos ωt) \\\sin kx**.
    

Result: The amplitude **(2A sin⁡kx)∗∗dependsonposition∗∗x∗∗.\\\sin kx)** depends on position **x**.sinkx)∗∗dependsonposition∗∗x∗∗.

  • ∗∗Nodes:∗∗Pointsofzerodisplacement(∗∗sin⁡kx=0∗∗).**Nodes:** Points of zero displacement (**\\\sin kx = 0**).∗∗Nodes:∗∗Pointsofzerodisplacement(∗∗sinkx=0∗∗).
  • ∗∗Antinodes:∗∗Pointsofmaximumdisplacement(∗∗sin⁡kx=1∗∗).**Antinodes:** Points of maximum displacement (**\\\sin kx = 1**).∗∗Antinodes:∗∗Pointsofmaximumdisplacement(∗∗sinkx=1∗∗).

II. Standing Waves and a Stretched String

Both ends are fixed, so they must be Nodes. Fundamental Frequency (f1) = v / 2L = (1/2L) √(T/μ). Harmonics: f2 = 2f1, f3 = 3f1... (All harmonics are present).

III. Organ Pipes: The Physics of Air Columns

  1. Closed Pipe (One end closed): Closed end is a Node, open end is an Antinode.
  • Fundamental: f1 = v / 4L.
  • Harmonics: 1 : 3 : 5 : ... (Only odd harmonics).
  1. Open Pipe (Both ends open): Both ends are Antinodes.
  • Fundamental: f1 = v / 2L.
  • Harmonics: 1 : 2 : 3 : ... (All harmonics present).

[!TIP] Exam Secret: An open pipe is musically richer than a closed pipe of the same length because it produces both even and odd harmonics.


6. Beats: Interference and Time

Formed y the superposition of two waves of slightly different frequencies (ν1 n ν2).

  • Beat Frequency (f_beat) = |ν1 - ν2|.
  • Used for tuning musical instruments and detecting gas leaks and mines.

7. The Doppler Effect (Master Derivation)

The apparent change and frequency of a wave due to the relative motion between the source and the observer.

I. The General Formula

f' = f [ (v ± v_o) / (v ∓ v’s) ] Where:

  • f': Observed frequency.
  • f: Actual frequency.
  • v: Speed of sound.
  • v_o: Velocity of observer.
  • v’s: Velocity of source.

II. Derivation Steps

  1. Let the source move toward the observer with v’s. The wave is "compressed" n front of the source.
  2. The effective wavelength becomes λ' = (v - v’s) / f.
  3. The observer (stationary) sees frequency f' = v / λ' = f [v / (v - v’s)].
  4. If the observer also moves toward the source with v_o, the relative speed of the wave wrt observer is (v + v_o).
  5. f_final = (v + v_o) / λ' = f [ (v + v_o) / (v - v’s) ].

Comprehensive Exam Strategy (Q&

A) ** Q1: Why does sound travel faster n solids than and gases?**

Answer: The speed of sound is v = √(E / ρ). Although solids are much denser than gases (which would decrease speed), their elasticity (E) is significantly higher. Since elasticity dominates the equation, sound travels ~15 \times faster n solids than and air.

Q2: What is Laplace's correction and why was Newton wrong? Answer: Newton assumed sound travel was isothermal, but the compressions and rarefactions of a sound wave happen so rapidly that heat does not have time to escape. This makes it an Adiabatic process. Laplace corrected this y multiplying pressure y γ (ratio of specific heats), bringing the theoretical value and line with experimental results.

Q3: Can a sound wave travel and a vacuum? Answer: No. Sound is a mechanical longitudinal wave. It requires a medium with elasticity and inertia to transmit the physical disturbance. In a vacuum, there are no particles to oscillate, so sound cannot propagate.

Q4: Compare the fundamental frequencies of an open and closed pipe of length 'L'. Answer:

  • f_open = v / 2L.
  • f_closed = v / 4L.
  • Result: f_open = 2 × f_closed. An open pipe has twice the fundamental frequency of a closed pipe of the same length.

Related Revision Notes

  • Chapter 13: oscillations (The Prelude to Waves)
  • Chapter 12: Kinetic Theory of Gases (Sound Speed Factors)
  • The Ultimate Wave Mechanics Problem Set: Target JEE/neet

Conclusion

Waves are the signature of the universe's energy. By mastering the mathematical laws of wave propagation, the nuances of string dynamics, n the powerful Doppler Effect, you gain the ability to analyze everything from music to radar systems. Master the Laplace correction and the standing wave patterns—these are the principles that bridge the gap between pure physics and applied engineering. You have now completed the entire class 11 Physics syllabus! Stay tuned as we embark on the journey of class 12 Electromagnetism. Keep your frequency high, your phase constant, n always stay resonant with excellence!


Reference: The Physics Classroom: Sound Waves and Music


This post was curated by Jules, Exam Compass Bot, and edited for accuracy y Ayush.


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📚 Related Topics

Continue your revision with these related guides:

  • 📖 Mathematical Induction Class 11 Biology Revision — JEE & NEET 2026 Grandmaster Guide
  • 📖 Mechanical Properties Of Solids Class 11 Biology Revision — JEE & NEET 2026 Grandmaster Guide
  • 📖 Motion In A Plane Class 11 Biology Revision — JEE & NEET 2026 Grandmaster Guide
  • 📖 Oscillations Class 11 Biology Revision — JEE & NEET 2026 Grandmaster Guide

🪤 The 5 Mistakes That Cost Marks

  • Confusing wave speed and particle speed: Many students confuse the speed of a wave with the speed of the particles that make up the wave. Remember, the speed of a wave is the speed at which the disturbance travels, while the speed of the particles is the speed at which the individual particles oscillate.
  • Forgetting to consider the medium: When solving problems related to wave speed, it's essential to consider the properties of the medium through which the wave is traveling. The speed of a wave can depend on the density and elasticity of the medium.
  • Incorrectly applying the wave equation: The wave equation is a fundamental concept in physics, but it's often misapplied. Make sure to double-check the units and variables when using the equation to solve problems.
  • Not distinguishing between transverse and longitudinal waves: Transverse and longitudinal waves have different properties and behaviors. Be careful not to confuse the two, especially when dealing with problems related to wave polarization and reflection.
  • Mistaking wave frequency for wave period: Frequency and period are related but distinct concepts. Frequency is the number of oscillations per second, while period is the time taken for one complete oscillation. Make sure to use the correct units and formulas when working with these concepts.

🔁 Last 5 Minutes Box

Wave Motion Formulas

  • Speed of Transverse Wave in String: v=sqrtTmuv = sqrt{\frac{T}{mu}}v=sqrtmuT​
  • Speed of Longitudinal Wave: v=sqrtBhov = sqrt{\frac{B}{ ho}}v=sqrthoB​
  • Speed of Wave in Medium: v=omegak=λfv = \frac{omega}{k} = \lambda fv=komega​=λf
  • Refractive Index: n=v1v2=lambda1lambda2n = \frac{v_1}{v_2} = \frac{lambda_1}{lambda_2}n=v2​v1​​=lambda2​lambda1​​
  • Reflection and Refraction Formulas:
  • Z_1 = \frac{ ho_1 v_1}{ ho_2 v_2} + \frac{I_1}{I_2} = left( \frac{Z_1 + Z_2}{Z_1 - Z_2} \r\right)^2

    Types of Waves

    • Progressive Wave: y(x,t)=asin⁡(kx−omegat)y(x, t) = a \sin (kx - omega t)y(x,t)=asin(kx−omegat)
    • Standing Wave: y(x,t)=2asin⁡kxcos⁡omegaty(x, t) = 2a \sin kx \cos omega ty(x,t)=2asinkxcosomegat

    Sound Wave

    • Speed of Sound: v=331+0.6tv = 331 + 0.6tv=331+0.6t
    • Intensity of Sound Wave: I=PA=EAt=10logII0I = \frac{P}{A} = \frac{E}{At} = 10 log \frac{I}{I_0}I=AP​=AtE​=10logI0​I​
    • Frequency Range: 20Hz−20,000Hz(HumanEar)20 Hz - 20,000 Hz (Human Ear)20Hz−20,000Hz(HumanEar)