PrepYodhaClass Notes · Physics
Physics · Chapter 08

Oscillations & Waves

A wave is one of the simplest yet most heavily tested ideas in physics — it carries energy from place to place without carrying matter along with it. The sections below build up from the basic idea of a wave, through mechanical and electromagnetic types, the quantities that describe them, and finish with sound and its everyday behaviours of reflection, refraction and diffraction.

〰️ 15 topics🎯 115+ points📝 self-test
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Topic 01

Waves: Basic Concept

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A wave is a travelling disturbance, and the single most important fact about it is what it does — and does not — carry.

⭐
Key Point
A wave is a disturbance which propagates energy from one place to another without the transport of matter.
What a wave really is
  • The key point to remember: waves transfer energy, not matter.
  • The arrow drawn along a wave shows its direction of propagation — the direction in which the energy travels.
  • Waves are broadly of two types: mechanical waves and non-mechanical (electromagnetic) waves.
📝 Quick self-test 2 MCQs · 2 fill-ups

A wave propagates energy from one place to another without the transport of:

  1. Energy
  2. Matter
  3. Force
  4. Frequency
✔ B. Matter — Waves transfer energy, not matter.

Waves are broadly divided into which two types?

  1. Long and short waves
  2. Mechanical and electromagnetic waves
  3. Sound and light waves
  4. Fast and slow waves
✔ B. Mechanical and electromagnetic waves — Waves are broadly mechanical waves and non-mechanical (electromagnetic) waves.

The key point about a wave is that it transfers energy, not .

✔ matter

The arrow drawn along a wave shows its direction of .

✔ propagation
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Topic 02

Mechanical Waves

Mechanical waves are the everyday waves of strings, water and sound — they cannot exist on their own and must have something to travel through.

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Key Point
The waves which require a material medium (solid, liquid or gas) for their propagation are called mechanical waves or elastic waves.
The defining condition
  • Mechanical waves are themselves of two types: longitudinal waves and transverse waves.
Longitudinal vs transverse — the key distinction
TypeParticle motionPattern seenExamples
Longitudinalparticles vibrate parallel to (along) the direction of propagationcompressions and rarefactionssound waves
Transverseparticles vibrate perpendicular to the direction of propagationcrests and troughswaves on a string, water-surface waves, light
  • In a longitudinal wave the particles of the medium vibrate in the direction of propagation, producing alternate compressions and rarefactions.
  • In a transverse wave the particles of the medium vibrate at right angles to the direction of propagation.
  • Waves on stretched strings and waves on the surface of water are transverse waves.
📝 Quick self-test 2 MCQs · 2 fill-ups

Mechanical waves require which of the following for their propagation?

  1. A vacuum
  2. A material medium
  3. A magnetic field
  4. Photons
✔ B. A material medium — Mechanical (elastic) waves require a material medium to propagate.

In a longitudinal wave, the particles of the medium vibrate:

  1. Perpendicular to propagation
  2. Parallel to propagation
  3. In circles
  4. Randomly
✔ B. Parallel to propagation — In a longitudinal wave, particles vibrate along (parallel to) the direction of propagation.

A transverse wave shows a pattern of crests and .

✔ troughs

Waves on stretched strings and on the surface of water are waves.

✔ transverse
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Topic 03

Non-mechanical Waves (Electromagnetic Waves)

Light, radio and X-rays belong to a very different family — they need nothing at all to travel through and can cross the empty vacuum of space.

⭐
Key Point
The waves which do not require any medium for their propagation — they can travel even through a vacuum — are called non-mechanical waves.
The defining condition
  • Examples: light waves, radio waves, X-rays, gamma rays and the like.
  • Light and heat (radiation) are examples of non-mechanical waves.
  • In fact, all electromagnetic waves are non-mechanical.
  • All electromagnetic waves are made up of photons.
  • The wavelength of electromagnetic waves ranges from about 10⁻¹⁴ m to 10⁴ m.
  • An electromagnetic wave transfers energy without any transfer of matter.
  • Remember: all electromagnetic waves are transverse in nature.
📝 Quick self-test 2 MCQs · 2 fill-ups

Non-mechanical (electromagnetic) waves are characterised by:

  1. Needing a solid medium
  2. Not requiring any medium
  3. Being longitudinal
  4. Being charged
✔ B. Not requiring any medium — Electromagnetic waves do not require any medium and can travel through a vacuum.

All electromagnetic waves are made up of:

  1. Electrons
  2. Protons
  3. Photons
  4. Ions
✔ C. Photons — All electromagnetic waves are made up of photons.

All electromagnetic waves are in nature.

✔ transverse

Light waves, radio waves, X-rays and rays are examples of non-mechanical waves.

✔ gamma
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Topic 04

Properties of Electromagnetic Waves

A short, high-yield list of features that examiners pull questions from directly.

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Key Point
They are electrically neutral (uncharged).
Five key properties
  • They propagate as transverse waves.
  • They travel with the velocity of light, c = 3 × 10⁸ m/s in vacuum.
  • They carry both energy and momentum.
  • Their existence was first predicted by Maxwell.
📝 Quick self-test 2 MCQs · 2 fill-ups

Electromagnetic waves travel with the velocity of light, which is:

  1. 3 × 10⁶ m/s
  2. 3 × 10⁸ m/s
  3. 3 × 10¹⁰ m/s
  4. 3 × 10⁵ m/s
✔ B. 3 × 10⁸ m/s — They travel at c = 3 × 10⁸ m/s in vacuum.

The existence of electromagnetic waves was first predicted by:

  1. Newton
  2. Maxwell
  3. Hertz
  4. Faraday
✔ B. Maxwell — Their existence was first predicted by Maxwell.

Electromagnetic waves are electrically (uncharged).

✔ neutral

Electromagnetic waves carry both energy and .

✔ momentum
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Topic 05

Quick Points (Exam Pointers)

A cluster of one-line facts that are very frequently asked.

⭐
Key Point
Light is a part of the electromagnetic spectrum.
Rapid pointers
  • Heat reaches us mainly as infrared radiation.
  • Electromagnetic waves can travel through vacuum — no medium is needed.
  • The speed of all electromagnetic waves in vacuum is c = 3 × 10⁸ m/s.
  • NOTE: mechanical waves need a medium (air, water, solids), while non-mechanical (electromagnetic) waves need no medium.
📝 Quick self-test 2 MCQs · 2 fill-ups

Heat reaches us mainly as which kind of radiation?

  1. Ultraviolet
  2. Infrared
  3. Gamma
  4. Microwave
✔ B. Infrared — Heat reaches us mainly as infrared radiation.

Which waves need a medium to travel?

  1. Electromagnetic waves
  2. Mechanical waves
  3. Light waves
  4. Radio waves
✔ B. Mechanical waves — Mechanical waves need a medium, while electromagnetic waves need none.

Light is a part of the spectrum.

✔ electromagnetic

The speed of all electromagnetic waves in vacuum is c = m/s.

✔ 3 × 10⁸
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Topic 06

Waves That Are NOT Electromagnetic

A common trap: several "rays" are actually streams of particles or mechanical waves, not electromagnetic waves — they require a material medium (or are moving particles).

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Key Point
Cathode rays — these are streams of electrons, not electromagnetic waves.
The non-electromagnetic list
  • Canal rays — streams of positive ions.
  • Alpha (α) rays — streams of helium nuclei (particles).
  • Beta (β) rays — streams of fast electrons (particles).
  • Sound waves — mechanical longitudinal waves.
  • Ultrasonic waves — mechanical sound waves above 20,000 Hz.
📝 Quick self-test 2 MCQs · 2 fill-ups

Cathode rays are actually:

  1. Electromagnetic waves
  2. Streams of electrons
  3. Streams of positive ions
  4. Sound waves
✔ B. Streams of electrons — Cathode rays are streams of electrons, not electromagnetic waves.

Which of these is NOT an electromagnetic wave?

  1. X-rays
  2. Gamma rays
  3. Alpha rays
  4. Radio waves
✔ C. Alpha rays — Alpha rays are streams of helium nuclei (particles), not electromagnetic waves.

Canal rays are streams of positive .

✔ ions

Sound waves are mechanical waves, not electromagnetic waves.

✔ longitudinal
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Topic 07

Some Important Electromagnetic Waves & Their Discoverers

Knowing who is associated with each radiation is a recurring static-GK question.

⭐
Key Point
Note: electromagnetic waves of wavelength roughly 10⁻³ m to 10⁻² m are called microwaves.
Wave / radiation and its discoverer
Wave / RadiationDiscoverer
α-RaysHenri Becquerel
X-RaysW. Röntgen
Ultraviolet raysJohann Ritter
Visible radiationNewton
Infrared raysHerschel
Short radio waves (Hertzian waves)Heinrich Hertz
Long radio wavesMarconi
📝 Quick self-test 2 MCQs · 2 fill-ups

X-rays were discovered by:

  1. Henri Becquerel
  2. W. Röntgen
  3. Johann Ritter
  4. Newton
✔ B. W. Röntgen — X-rays were discovered by W. Röntgen.

Infrared rays were discovered by:

  1. Herschel
  2. Marconi
  3. Hertz
  4. Newton
✔ A. Herschel — Infrared rays were discovered by Herschel.

Ultraviolet rays were discovered by Ritter.

✔ Johann

Electromagnetic waves of wavelength roughly 10⁻³ m to 10⁻² m are called .

✔ microwaves
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Topic 08

Amplitude

Amplitude measures how strong, or how "big", an oscillation is.

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Key Point
Amplitude is the maximum displacement of a vibrating particle from its equilibrium (mean) position, on either side.
Meaning of amplitude
  • Key point: amplitude shows how far the particle moves from its mean position — it is linked to the energy and loudness/brightness of the wave.
📝 Quick self-test 2 MCQs · 2 fill-ups

Amplitude is the maximum displacement of a vibrating particle from its:

  1. Crest
  2. Trough
  3. Equilibrium (mean) position
  4. End point
✔ C. Equilibrium (mean) position — Amplitude is the maximum displacement from the equilibrium (mean) position.

Amplitude is linked to which property of a wave?

  1. Speed
  2. Frequency
  3. Energy and loudness/brightness
  4. Wavelength
✔ C. Energy and loudness/brightness — Amplitude is linked to the energy and loudness/brightness of the wave.

Amplitude is the maximum of a vibrating particle from its mean position.

✔ displacement

Amplitude shows how far the particle moves from its position.

✔ mean
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Topic 09

Wavelength

Wavelength fixes the "size" of one complete wave and is tied to frequency through the wave speed.

⭐
Key Point
Wavelength is the distance between two nearest particles of the medium vibrating in the same phase.
Meaning of wavelength
  • It is denoted by the Greek letter lambda (λ) and is measured in metres.
  • In a transverse wave, the distance between two consecutive crests (or two consecutive troughs) equals one wavelength λ.
  • In a longitudinal wave, the distance between two consecutive compressions (or rarefactions) equals one wavelength λ.
  • The wave speed is given by velocity = frequency × wavelength, i.e. v = f × λ.
  • Remember: for a fixed speed, higher the frequency, shorter the wavelength (since λ = v / f).
📝 Quick self-test 2 MCQs · 2 fill-ups

Wavelength is the distance between two nearest particles vibrating in the:

  1. Opposite phase
  2. Same phase
  3. Different medium
  4. Same crest
✔ B. Same phase — Wavelength is the distance between two nearest particles vibrating in the same phase.

For a fixed wave speed, a higher frequency means a:

  1. Longer wavelength
  2. Shorter wavelength
  3. Same wavelength
  4. Zero wavelength
✔ B. Shorter wavelength — Since λ = v/f, higher frequency gives shorter wavelength.

Wavelength is denoted by the Greek letter .

✔ lambda

In a transverse wave, the distance between two consecutive crests equals one .

✔ wavelength
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Topic 10

Wave Formula & Units

The single master equation of wave motion, with the units of each quantity.

⭐
Key Point
Formula: v = f × λ, where v = velocity of the wave, f = frequency, and λ = wavelength.
The wave equation
Quantities and their SI units
QuantitySymbolSI Unit
Wavelengthλmetre (m)
Frequencyfhertz (Hz)
Velocityvmetre/second (m/s)
📝 Quick self-test 2 MCQs · 2 fill-ups

The master wave equation is:

  1. v = f + λ
  2. v = f × λ
  3. v = f / λ
  4. v = λ / f
✔ B. v = f × λ — The wave equation is v = f × λ.

The SI unit of frequency is:

  1. metre
  2. hertz
  3. metre/second
  4. second
✔ B. hertz — Frequency is measured in hertz (Hz).

In the wave equation v = f × λ, the symbol v stands for the of the wave.

✔ velocity

The SI unit of wavelength is the (m).

✔ metre
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Topic 11

Sound

Sound is the most familiar mechanical wave, and its behaviour under change of medium is a favourite exam point.

⭐
Key Point
Sound waves are mechanical longitudinal waves and require a medium for propagation.
Nature of sound
  • Sound cannot travel through a vacuum.
  • When sound passes from one medium to another, its speed and wavelength change but its frequency stays constant.
The three ranges of sound by frequency
Type of Sound WaveFrequency Range
Infrasonicbelow 20 Hz
Audible20 Hz – 20,000 Hz
Ultrasonicabove 20,000 Hz
  • The audible range for humans is about 20 Hz to 20,000 Hz; below it is infrasonic, above it is ultrasonic.
📝 Quick self-test 2 MCQs · 2 fill-ups

When sound passes from one medium to another, which quantity stays constant?

  1. Speed
  2. Wavelength
  3. Frequency
  4. Amplitude
✔ C. Frequency — Its speed and wavelength change but its frequency stays constant.

The audible range of sound for humans is:

  1. Below 20 Hz
  2. 20 Hz to 20,000 Hz
  3. Above 20,000 Hz
  4. 2 Hz to 200 Hz
✔ B. 20 Hz to 20,000 Hz — The audible range for humans is about 20 Hz to 20,000 Hz.

Sound waves are mechanical waves and require a medium for propagation.

✔ longitudinal

Sound cannot travel through a .

✔ vacuum
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Topic 12

Properties of Sound Wave

A checklist of what sound can and cannot do.

⭐
Key Point
Sound requires a material medium for its propagation.
Key properties of sound
  • It is a longitudinal wave — it travels as alternate compressions and rarefactions along the direction of propagation.
  • The speed of sound depends on the nature of the medium.
  • It carries energy.
  • It can produce the sensation of hearing.
  • It can be reflected, and it obeys all the laws of reflection.
  • It can produce echo and reverberation.
  • Note: sound travels fastest in solids, slower in liquids and slowest in gases (opposite to the order for light).
📝 Quick self-test 2 MCQs · 2 fill-ups

In which medium does sound travel fastest?

  1. Gases
  2. Liquids
  3. Solids
  4. Vacuum
✔ C. Solids — Sound travels fastest in solids, slower in liquids and slowest in gases.

Sound obeys which laws that light also obeys?

  1. Laws of reflection
  2. Laws of gravitation
  3. Laws of thermodynamics
  4. Ohm's law
✔ A. Laws of reflection — Sound obeys all the laws of reflection.

The speed of sound depends on the nature of the .

✔ medium

Sound can produce echo and .

✔ reverberation
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Topic 13

Reflection (of Sound)

When sound bounces off a hard surface it obeys the same reflection laws as light, and this gives rise to echoes and reverberation.

⭐
Key Point
The bouncing back of sound when it strikes a hard surface is called reflection of sound.
Reflection of sound
  • Sound obeys the laws of reflection: the angle of incidence (∠i) equals the angle of reflection (∠r), measured from the normal.
  • The working of a megaphone, sound boards and the ear trumpet is based on reflection of sound.
  • The repetition of sound due to reflection is called an echo.
  • The persistence of hearing in the human ear is about 1/10 of a second (0.1 s).
  • An echo is heard only when the time interval between the original and reflected sound is ≥ 0.1 second.
  • The minimum distance from a reflecting surface needed to hear an echo is about 17 m.
  • Sound-proof rooms are made with two layers of wall with vacuum in between.
  • Reverberation is the persistence of sound after the source has stopped, caused by multiple reflections of sound.
  • While designing an auditorium, proper care is taken of the absorption and reflection of sound.
  • The time taken by a reverberant sound to fall in intensity by a factor of 10⁶ is called the reverberation time.
📝 Quick self-test 2 MCQs · 2 fill-ups

The repetition of sound due to reflection is called:

  1. Reverberation
  2. Echo
  3. Diffraction
  4. Refraction
✔ B. Echo — The repetition of sound due to reflection is called an echo.

The persistence of sound after the source has stopped, caused by multiple reflections, is called:

  1. Echo
  2. Diffraction
  3. Reverberation
  4. Resonance
✔ C. Reverberation — Reverberation is the persistence of sound due to multiple reflections.

An echo is heard only when the time gap between original and reflected sound is at least second.

✔ 0.1

Sound-proof rooms are made with two layers of wall with in between.

✔ vacuum
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Topic 14

Refraction (of Sound)

Refraction is the bending of sound as it crosses from one medium into another, caused by a change in its speed.

⭐
Key Point
When a sound wave passes from one medium to another and bends from its original path, the phenomenon is called refraction.
Refraction of sound
  • It occurs because the speed of sound differs in the two media.
  • Key point: the change in direction happens at the boundary between the media.
  • If sound enters a slower medium, it bends towards the normal.
  • If sound enters a faster medium, it bends away from the normal.
  • For the standard diagram: medium 1 has velocity v₁, medium 2 has velocity v₂; if v₂ < v₁ then the angle of refraction r < i.
How direction changes between media
FromToEffect on Direction
Air (fast)Water (slow)bends towards the normal
Water (slow)Air (fast)bends away from the normal
Hot air (fast)Cold air (slow)bends towards the normal
Cold air (slow)Hot air (fast)bends away from the normal
📝 Quick self-test 2 MCQs · 2 fill-ups

Refraction of sound occurs because the speed of sound:

  1. Is constant everywhere
  2. Differs in the two media
  3. Increases with frequency
  4. Depends on amplitude
✔ B. Differs in the two media — Refraction occurs because the speed of sound differs in the two media.

When sound enters a slower medium, it bends:

  1. Away from the normal
  2. Towards the normal
  3. Straight through
  4. Backward
✔ B. Towards the normal — Entering a slower medium, sound bends towards the normal.

Refraction of sound is the bending of a sound wave as it passes from one to another.

✔ medium

If sound enters a faster medium, it bends from the normal.

✔ away
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Topic 15

Diffraction (of Sound)

Diffraction is why you can hear someone around a corner even when you cannot see them — sound bends around edges and through openings.

⭐
Key Point
The bending of sound waves around the edges of an obstacle or opening is called diffraction.
Diffraction of sound
  • Because of diffraction, sound spreads out and you can easily hear another person's voice even around a barrier.
  • In the standard diagram, incident plane waves pass through a small opening and emerge as spreading diffracted waves.
  • Condition for maximum diffraction: the size of the opening or obstacle should be of the same order as the wavelength of the sound.
  • More diffraction occurs at lower frequency (longer wavelength).
  • Real-life examples: we can hear a voice from behind a wall, and sound from loudspeakers spreads around corners.
📝 Quick self-test 2 MCQs · 2 fill-ups

Diffraction of sound is the:

  1. Bouncing of sound off a surface
  2. Bending of sound around edges of obstacles or openings
  3. Splitting of sound into frequencies
  4. Speeding up of sound
✔ B. Bending of sound around edges of obstacles or openings — Diffraction is the bending of sound waves around the edges of an obstacle or opening.

Diffraction of sound is greater at:

  1. Higher frequency
  2. Lower frequency (longer wavelength)
  3. Any frequency equally
  4. Zero frequency
✔ B. Lower frequency (longer wavelength) — More diffraction occurs at lower frequency (longer wavelength).

Maximum diffraction occurs when the size of the opening is of the same order as the of the sound.

✔ wavelength

Because of diffraction, we can hear a voice from behind a .

✔ wall
🎯
Recap

Quick Revision

⭐
Key Point
A wave carries energy, not matter, from one place to another.
  • Mechanical waves need a medium; electromagnetic waves need none and travel even through vacuum.
  • In a transverse wave particles move perpendicular to the wave (e.g. light); in a longitudinal wave particles move parallel to it (e.g. sound).
  • All electromagnetic waves are transverse and travel at c = 3 × 10⁸ m/s in vacuum.
  • Cathode rays, canal rays, alpha and beta rays are NOT electromagnetic — they are streams of particles; sound is a mechanical wave.
  • Amplitude is the maximum displacement from the mean position; wavelength λ is the distance between two nearest in-phase particles.
  • The master wave equation is v = f × λ; frequency is measured in hertz (Hz).
  • Sound is a mechanical longitudinal wave that cannot travel through vacuum; it travels fastest in solids, slowest in gases.
  • Human audible range is 20 Hz to 20,000 Hz; below is infrasonic, above is ultrasonic.
  • An echo needs a time gap of ≥ 0.1 s and a reflecting surface at least about 17 m away.
  • Reverberation is the persistence of sound due to multiple reflections after the source stops.
  • Sound entering a slower medium bends towards the normal; entering a faster medium it bends away.
  • Diffraction (bending around edges) is strongest when the obstacle/opening size matches the wavelength, so it is greater at low frequency.

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