Instructions
Total Questions: 20 | Marks: 4 each | No Negative Marking
Waves & Sound – IIT JEE Notes (Set 9)
Basic Wave Concepts
Wave Definition
A wave is a disturbance that transfers energy from one place to another without transferring matter.
Wave Equation
v = fλ
Where v is wave speed, f is frequency, and λ is wavelength.
Types of Waves
Mechanical Waves
Require a medium to propagate. Example: sound waves.
Electromagnetic Waves
Do not require a medium and can travel in vacuum. Example: light waves.
Sound Waves
Nature
Sound waves are longitudinal waves consisting of compressions and rarefactions.
Speed of Sound
Depends on temperature and medium. In air, speed increases with temperature.
Frequency, Pitch and Loudness
Frequency
Number of oscillations per second. Unit is Hertz (Hz).
Pitch
Determined by frequency. Higher frequency means higher pitch.
Loudness
Depends on amplitude of the wave.
Doppler Effect
Concept
Apparent change in frequency due to relative motion between source and observer.
Key Insight
Frequency increases when source approaches and decreases when it moves away.
Reflection of Sound
Echo
Echo is the reflection of sound from a distant surface.
Condition
Minimum distance required for echo is about 17 meters for distinct hearing.
Superposition of Waves
Principle
When two waves overlap, resultant displacement is the sum of individual displacements.
Application
Used in interference and formation of standing waves.
Standing Waves
Formation
Formed by superposition of two waves traveling in opposite directions.
Nodes and Antinodes
Nodes: zero displacement points.
Antinodes: maximum displacement points.
Beats
Concept
Beats are periodic variations in intensity due to interference of two waves of slightly different frequencies.
Frequency
Beat frequency = |f₁ – f₂|
Resonance
Concept
Occurs when frequency of external force matches natural frequency of system.
Effect
Results in maximum amplitude of vibration.
Intensity of Sound
Relation
Intensity ∝ Amplitude²
Key Insight
Doubling amplitude increases intensity four times.
Range of Sound
Audible Range
20 Hz to 20 kHz.
Infrasonic
Below 20 Hz.
Ultrasonic
Above 20 kHz.
Wave Properties
Reflection
Wave bounces back from a surface.
Refraction
Change in direction when wave enters different medium.
Diffraction
Bending of waves around obstacles.
Important Exam Concepts
Conceptual Traps
Frequency remains constant during reflection and refraction. Speed and wavelength may change.
JEE Strategy
Focus on formulas, graph interpretation, and conceptual clarity. Practice numerical problems on Doppler effect and standing waves.