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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.
In this Activity, we will observe how a disturbance travels along a slinky as a model for a sound wave.
- 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.
- 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.
- 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.
- 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.
The particles of the medium do not travel with the wave. They just vibrate about their mean positions.
What makes sudden loud sounds like firecrackers or a thunderclap? When gases heat and expand very rapidly, they create a sudden disturbance in air density that travels outward as a loud pulse. A supersonic aircraft flying faster than sound produces a similar loud pulse called a sonic boom .
When the medium is not confined to a tube, vibrating particles collide in all directions, so a small source sends out compressions and rarefactions as spherical waves spreading in every direction, perceived as sound when they reach a listener. Earthquakes also send out seismic waves, which can be longitudinal or transverse.
4. Assertion (A): Compressions and rarefactions move through the medium. Reason (R): Individual particles move forward with the wave. Answer: (iii) A is true but R is false — particles only oscillate about their mean positions.
NCERT Question 1 — Which observation best supports the
- 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.
