The wave travels, not the medium
Definition
Wave
A travelling disturbance that transfers energy from one place to another without transferring matter.
Definition
Medium
The substance, such as air, water or steel, through which a wave travels.
- A wave transfers energy from one place to another, but the medium it travels through does not travel with it.
- Each particle of the medium oscillates about its rest position and passes the disturbance on to the next particle.
- Once the wave has passed, every particle is back where it started, so there is no overall movement of the material.
- A Mexican wave at a football ground works in the same way, because the wave races round the stadium while each fan only stands up and sits down in their own seat.
- Water particles and air particles are far too small to watch, so the evidence comes from objects that move along with them.
Evidence from water ripples
- A cork, a leaf or a small piece of paper floating on water moves exactly as the surface beneath it moves.
- As ripples pass, the floating object bobs up and down, with a slight back-and-forth movement.
- The floating object is not carried along with the ripples, and once they have passed it is still in about the same place.
- If the water itself travelled with the ripples, the floating object would be swept across to the far side.
- A duck sitting on a pond rises and falls as ripples from a thrown stone pass beneath it, but it does not drift towards the bank.
- This shows that the ripple, and the energy it carries, travels across the water while the water stays where it is.
Practical
A floating cork in a ripple tank
- Aim: to show that ripples travel across the water while the water itself stays in place.
- Apparatus: a ripple tank or large shallow tray of water, a dipper and motor or a pencil to tap the surface, a small cork or a few pieces of paper confetti, a ruler fixed to the side of the tank and a mobile phone to film from the side.
- Method:
- Let the water settle completely, then place the cork gently near the middle of the tank.
- Note the cork's starting position against the ruler on the side of the tank.
- Start the dipper, or tap the surface steadily at one end, so that a train of ripples travels past the cork.
- Film the cork from the side so that you can watch both the ripples and the cork.
- Stop the ripples and compare the cork's final position with its starting position.
- Results: the ripples travel the full length of the tank, but the cork only rises and falls with a small back-and-forth motion and ends up in almost the same place.
- Conclusion: the energy of the wave travels across the tank, but the water does not.
- Watch out: a draught or a tilted tank can make the cork drift for reasons that have nothing to do with the ripples, so close the windows and check the tank is level.
- Safety: keep electrical equipment away from the water and mop up any spills straight away.
Evidence from sound in air
- Air particles are invisible, so the evidence comes from the effect sound has on something light that sits in the air.
- A candle flame placed in front of a loudspeaker playing a loud, low note flickers backwards and forwards, but it is not blown away from the speaker.
- The flame moves back and forth because the air around it vibrates back and forth as compressions and rarefactions pass through it.
- If the air were flowing away from the loudspeaker, the flame would lean steadily away from it, as it does in a draught.
- Wind is a flow of air from one place to another, while a sound wave is a vibration of air in place, which is why loud music does not blow your hair back.
- A ribbon tied to a stretched slinky shows the same thing for any longitudinal wave, because it moves back and forth as compressions pass but stays at the same point on the spring.
Practical
A candle flame by a loudspeaker
- Aim: to show that a sound wave travels through the air while the air itself only vibrates back and forth.
- Apparatus: a loudspeaker, a signal generator, a candle in a stable holder on a heatproof mat, and matches or a lighter.
- Method:
- Place the lit candle about 5 cm5\ \text{cm}5 cm in front of the loudspeaker cone, in a room with no draughts.
- Connect the loudspeaker to the signal generator and set a low frequency of about 20 Hz20\ \text{Hz}20 Hz to 50 Hz50\ \text{Hz}50 Hz.
- Turn up the amplitude slowly until the sound is clearly audible, and watch the flame closely or film it in slow motion.
- Switch off the signal generator and compare the flame with how it looked before.
- Results: the flame flickers rapidly towards and away from the speaker, but it does not lean steadily away or blow out.
- Conclusion: the air near the speaker vibrates about a fixed position as the sound passes, so the sound wave travels but the air does not flow.
- Watch out: at high frequencies the flickering is too fast to see, so keep the frequency low.
- Safety: tie back long hair, keep the candle on a heatproof mat away from paper, and never leave the flame unattended.
Common Mistake
- Do not say that the particles of the medium stay completely still, because they do move, but only back and forth about their rest positions.
- Do not confuse a sound wave with wind, because wind carries air from place to place while sound does not.
Exam technique
Describing the evidence
- For each type of wave, give an observation followed by the conclusion you draw from it.
- Name the object you watch, such as a cork on water or a candle flame near a loudspeaker.
- Describe its motion precisely, as up and down or back and forth about a fixed position.
- Finish by stating that the wave, and the energy it carries, travels while the water or air does not.
Self review
- What does a wave transfer from one place to another?
- How does a cork on water move as ripples pass it?
- What does the motion of the cork show about the water?
- Why does a candle flame flicker in front of a loudspeaker instead of being blown away?
- What is the difference between the movement of air in wind and in a sound wave?
