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💡 Push, pull, repeat
Give a slinky a sharp push-pull and a bunched-up region races along it — while each coil just jiggles in place. Sound moves through air the very same way.
Activity 10.5 — Let us observe
In this Activity, we will observe how a disturbance travels along a slinky as a model for a sound wave.
Activity 10.5 - Slinky Wave
Step 1
Push and pull one end of a slinky
↓
Step 2
A disturbance travels along it
↓
Step 3
The marked turn only oscillates in place
↓
The disturbance moves but the coils do not
- Lay a slinky flat and mark one turn with a marker.
- Ask a friend to hold one end; keep the slinky slightly stretched.
- Give your end a sharp push toward your friend and pull it back (Fig. 10.8).
- Now push and pull repeatedly and watch the marked turn.
What we observe
A disturbance travels along the slinky, but the
marked turn only oscillates in place
— the disturbance moves, the coils do not travel with it.
What are compressions and rarefactions?
- When a piston pushes forward, it crowds the air into a high-density region — a compression (C) .
- When it moves back, the air spreads out into a low-density region — a rarefaction (R) .
- These pass forward through the medium while particles only oscillate about their mean positions.
What is a sound wave?
Sound Wave
A series of alternating compressions and rarefactions moving through a medium without the particles flowing along.
- A sound wave is a series of alternating compressions and rarefactions travelling through a medium.
- It moves without the actual flow of the particles of the medium.
- The direction it travels is the direction of propagation of the wave.
Why is sound a longitudinal mechanical wave?
- In a longitudinal wave , particles vibrate parallel to the direction of propagation (Fig. 10.12).
- Sound needs a material medium to travel.
- Waves that need a medium are mechanical waves — so sound is a mechanical wave.
ⓘ Note
- Particles of the medium do not travel with the wave.
- They only vibrate about their mean positions.
Longitudinal vs Transverse Waves
Longitudinal wave
Transverse wave
Particles vibrate
parallel
to propagation
Particles vibrate
perpendicular
to propagation
Compressions and rarefactions
Crests and troughs
Example: sound
Example: light (can travel through vacuum)
🧵 Threads of Curiosity
- What makes sudden loud sounds like firecrackers or thunder?
- Heated gases expand very rapidly in a short time.
- This creates a sudden disturbance in the air density.
- It travels outward and reaches us as a loud pulse.
- A supersonic aircraft flies faster than the speed of sound.
- It produces a similar loud pulse called a sonic boom .
📚 Ready to Go Beyond
- A real medium is not confined inside a tube.
- Its vibrating particles collide with neighbours in all directions.
- A small source sends compressions and rarefactions everywhere.
- These spread out as spherical waves (Fig. 10.10).
- Reaching a listener, they are perceived as sound.
📚 Ready to Go Beyond
- Mechanical waves are of two types.
- Longitudinal waves — particles vibrate parallel to propagation.
- Sound is a longitudinal wave.
- Transverse waves — particles vibrate perpendicular to propagation (Fig. 10.13).
- Earthquakes produce seismic waves through the Earth.
- These can be longitudinal or transverse.
- The longitudinal seismic waves are detected first by seismographs.
- Not all waves are mechanical — not all need a medium.
- Light is a transverse wave and travels through vacuum.
- That is why sunlight and starlight reach the Earth.
⏸▶ Pause and Ponder
- 4. Assertion (A): Compressions and rarefactions move through the medium.
- Reason (R): Individual particles move forward with the wave.
- A is true — the density disturbance does travel.
- R is false — particles only oscillate about their mean positions.
- Answer: (iii) A is true, but R is false.
NCERT Question 1 — Which observation best supports the
Which observation best supports the idea that sound is a mechanical wave? (i) Sound shows reflection (ii) Sound needs a medium to propagate (iii) Sound has frequency (iv) Sound carries energy
View Answer →
📚 Ready to Go Beyond
- Particles in a medium are never truly at rest.
- They are always vibrating randomly due to thermal energy .
- A passing sound wave temporarily increases this vibration.
- After the wave passes, the particles return to their usual random motion.
✅ Test Yourself
-
What is a compression?
Show Answer
A region of air with higher density than the average. -
What is a rarefaction?
Show Answer
A region of air with lower density than the average. -
Why is sound called a longitudinal wave?
Show Answer
Its particles vibrate parallel to the direction of propagation. -
What is a mechanical wave?
Show Answer
A wave that needs a material medium to propagate.
Important Definitions
- Compression — a region of higher density in a sound wave.
- Rarefaction — a region of lower density in a sound wave.
- Longitudinal wave — a wave in which particles vibrate parallel to the direction of propagation.
- Mechanical wave — a wave that needs a material medium to travel.
