Chapter 1 of 8

All 8 chapters
  1. 1Kinds of waves, readunderstood
  2. 2Measuring waves, readunderstood
  3. 3Waves on water, readunderstood
  4. 4Sound and ultrasound, readunderstood
  5. 5Reflection and refraction, readunderstood
  6. 6Lenses, readunderstood
  7. 7The electromagnetic spectrum, readunderstood
  8. 8Waves in communication and health, readunderstood

S4.10.1 Chapter 1

Kinds of waves

Mei flicks one end of a long rope, and a bump runs to the other end. What actually moved along the rope?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Compare transverse and longitudinal waves with a rope and a spring
  • Explain that waves carry energy, not matter

A wave is a disturbance that travels from one place to another and carries energy with it. A rope, a spring, the surface of a pond and the air can all carry waves.

Tie a ribbon to the middle of a long rope. Mei holds one end and flicks it up and down once. A bump runs along the rope. When the bump reaches the ribbon, the ribbon rises. Then it comes down again.

After the bump has passed, the ribbon is where it was before. The bump went on to the far end. The rope did not.

Draw it first

A bump runs past a ribbon

  1. Three pictures of a rope, one under the other, with a ribbon tied to it. A dashed line runs down through the ribbon. Row 1, labelled 0 s: a bump near the left end, labelled pulse, and the ribbon at rest, labelled ribbon. Row 2, labelled 1 s: the bump is under the ribbon and the ribbon is raised. Row 3, labelled 2 s: the bump has moved to the right and the ribbon is back at rest on the same dashed line. An arrow at the foot is labelled wave travels.0 spulseribbonwave travels

    1 At 0 s the bump is near Mei and the ribbon is at rest.

  2. Three pictures of a rope, one under the other, with a ribbon tied to it. A dashed line runs down through the ribbon. Row 1, labelled 0 s: a bump near the left end, labelled pulse, and the ribbon at rest, labelled ribbon. Row 2, labelled 1 s: the bump is under the ribbon and the ribbon is raised. Row 3, labelled 2 s: the bump has moved to the right and the ribbon is back at rest on the same dashed line. An arrow at the foot is labelled wave travels.0 s1 spulseribbonwave travels

    2 At 1 s the bump is under the ribbon, and the ribbon is at its highest.

  3. Three pictures of a rope, one under the other, with a ribbon tied to it. A dashed line runs down through the ribbon. Row 1, labelled 0 s: a bump near the left end, labelled pulse, and the ribbon at rest, labelled ribbon. Row 2, labelled 1 s: the bump is under the ribbon and the ribbon is raised. Row 3, labelled 2 s: the bump has moved to the right and the ribbon is back at rest on the same dashed line. An arrow at the foot is labelled wave travels.0 s1 s2 spulseribbonwave travels

    3 At 2 s the bump has gone on. The ribbon is back at rest, on the same dashed line. It has not moved along the rope.

The bump carried energy from Mei's hand to the far end. At the far end it can shake a hand tied there. So a wave carries energy from place to place. It does not carry the material it travels through.

Now let Mei keep flicking, up and down, again and again. A steady train of bumps and dips runs along the rope. The highest point of a wave is a crest. The lowest point is a trough. The dashed line is where the rope lay at rest.

A graph of a wave with the axes distance along the rope and sideways move. A smooth wave rises above a dashed line labelled rest line, then falls below it, then rises again. A dot at the top of the first hill is labelled crest. A dot at the bottom of the first dip is labelled trough.cresttroughsideways movedistance along the roperest line
A wave on a rope. The picture is a snapshot: each piece of rope is at its own height.

A piece of the rope moves up and down. The wave travels along the rope. These two directions are at right angles, which is 90°. A wave like this is a transverse wave.

A dot with a double headed arrow running up and down through it, labelled vibration. Below, a long arrow pointing right, labelled wave travels. The two arrows are at right angles.vibrationwave travels
In a transverse wave the vibration is across the way the wave travels.

Light is a transverse wave. So are the ripples on a pond, to a good first approximation. Strictly, each bit of water moves in a small circle. We treat ripples as transverse in this book.

Now take a long soft spring and push one end sharply towards the other end, along its length. The coils bunch up, and the bunch runs along the spring. Where coils are pressed together there is a compression. Where they are spread apart there is a rarefaction.

Two rows of upright lines, which are layers of a spring. In the top row the layers bunch together in two places and spread out in others. The bunched place is labelled compression and the spread place is labelled rarefaction. One thick layer has a short double headed arrow above it labelled vibration. In the lower row, labelled a moment later, the bunches have moved to the right. A dashed line joins a compression to its new place. The thick layer is almost where it was. An arrow at the foot is labelled wave travels.vibrationcompressionrarefactiona moment laterwave travels
A longitudinal wave on a spring. The compressions move to the right. The thick layer only shakes to and fro.

Look at the thick layer. It moves a little to the right and a little to the left, along the way the wave travels. This is a longitudinal wave. The vibration is along the direction of travel.

A dot with a double headed arrow running left and right through it, labelled vibration. Below, a long arrow pointing right, labelled wave travels. The two arrows lie on the same line.vibrationwave travels
In a longitudinal wave the vibration is along the way the wave travels.

Sound in air is a longitudinal wave. A loudspeaker pushes the air layers together and pulls them apart. The compressions and rarefactions run out through the air. In the air, as on the spring, each layer only shakes to and fro.

See it three ways

As a picture

A table with the columns Kind of wave and Vibration. Row 1: transverse, across. Row 2: longitudinal, along.Kind of waveVibrationtransverseacrosslongitudinalalong

In words

Across means at right angles to the way the wave travels. Along means on the same line as the way the wave travels.

In symbols

Transverse: vibration at 90° to travel. Longitudinal: vibration at 0° to travel, along the same line.

A table with the columns Transverse and Longitudinal. Row 1: rope, spring. Row 2: light, sound. Row 3: ripples, a pushed row.TransverseLongitudinalropespringlightsoundripplesa pushed row
Examples of the two kinds. The pushed row is the row of buttons in the second Try it.

A rope, a spring, water and air are all a medium: the material a wave travels through. Waves on a rope or a spring, ripples and sound all need a medium. Light does not. Sunlight crosses empty space to reach us, so light needs no medium.

Now think of a cork floating on a pond. Ripples pass it. The cork bobs up and down, and when the ripples have gone it is about where it was. The ripples carried energy past it. They did not carry the water or the cork along with them.

Three pictures of the surface of a pond, one under the other, with a cork on it. A dashed line runs down through the cork. Row 1, labelled 0.0 s: a ripple near the left, labelled ripple, and the cork at rest, labelled cork. Row 2, labelled 0.5 s: the ripple is under the cork and the cork is raised. Row 3, labelled 1.0 s: the ripple has moved to the right and the cork is back at rest on the same dashed line. An arrow at the foot is labelled wave travels.0.0 s0.5 s1.0 sripplecorkwave travels
A floating cork bobs as a ripple passes and stays where it is. Energy goes by, water does not.

How fast does a wave travel? The speed of a wave is the distance its pattern covers divided by the time it takes. You met this rule for moving things in the speed book: speed = distance ÷ time.

A bump that runs 10 m along a rope in 4 s has a speed of 10 m ÷ 4 s = 2.5 m/s. Waves in different materials have different speeds. The later chapters measure some.

Try it

Send a bump down a rope

You need: a long rope or string, about 5 m, a ribbon, a scrap of cloth or a piece of paper, a friend who can hold the far end, a post or a chair to tie the far end to, if you have no friend.

Be safe: Do this on flat ground, away from people walking past. Flick the rope sideways and low, not towards anyone's face.

  1. Tie the ribbon to the middle of the rope with a loose knot. Lay the rope on a flat floor, or hold it just off the ground.
  2. Hold one end. Your friend holds the other end, or ties it to the post. Pull the rope almost straight, not tight.
  3. Flick your end sideways once, quickly. Watch the bump run along the rope.
  4. Watch the ribbon as the bump reaches it. Then watch where the ribbon is after the bump has gone.
  5. Now flick your end again and again, at a steady pace. Watch how the ribbon moves.

What you will see: The ribbon moves sideways as the bump passes and then goes back to its place. It does not travel to the far end. The bump did.

If you cannot try it, this is what happens.
Three pictures of a rope with a ribbon, one under the other, with a dashed line down through the ribbon. Row start: a bump near the left end, labelled pulse, and the ribbon labelled ribbon at rest. Row under: the bump is under the ribbon and the ribbon is raised. Row past: the bump has gone on to the right and the ribbon is back at rest on the same dashed line. An arrow at the foot is labelled wave travels.startunderpastpulseribbonwave travels

Try it

A pushed row of buttons

You need: seven flat buttons of the same size, a smooth table.

Be safe: Keep the buttons away from young children, who can swallow them. Slide them along the table, not towards anyone.

  1. Put six buttons in a straight line on the table, touching each other.
  2. Slide the seventh button along the table so that it hits the end of the line, not hard.
  3. Watch which buttons move.

What you will see: The button at the far end of the line jumps away. The buttons in the middle hardly move. The push went along the row and carried energy to the last button. This is a longitudinal pulse.

If you cannot try it, this is what happens.
A table with the columns Button and What happens. Row 1: hits the end, stops. Row 2: middle ones, stay. Row 3: far end, jumps away.ButtonWhat happenshits the endstopsmiddle onesstayfar endjumps away

A wrong idea many people have

A wave carries water, or rope, or air, from one place to another.

Why it feels right: You see a bump run along a rope and ripples spread over a pond. It looks as if the stuff itself is moving.

The test that breaks it: Float a cork or tie a ribbon on. After the wave has gone by, the cork or ribbon is where it was. A wave that carried the water along would take it too.

What is true: A wave carries energy and a pattern from place to place. The material only moves a little to and fro, or up and down, and stays near where it started.

A wrong idea many people have

Sound is a wave like a rope wave, with the air moving up and down.

Why it feels right: Pictures of sound waves are drawn as a wavy line, which looks just like a rope wave.

The test that breaks it: Look at the spring. The layers move along the line of travel, not across it. Air layers do the same, so sound is not a rope type wave.

What is true: Sound is a longitudinal wave: the air shakes to and fro along the way the sound travels. The wavy line in a picture is a graph, not the path of the air.

A wrong idea many people have

Every kind of wave needs a material to travel through.

Why it feels right: Rope waves, ripples and sound all need rope, water or air, so it seems to be a rule for all waves.

The test that breaks it: The Sun is far off across empty space, and its light reaches us. If light needed a material, it could not cross the empty space between the Sun and the Earth.

What is true: Rope waves, ripples and sound need a medium. Light does not. Light crosses a vacuum, which is why we can see the Sun and the stars.

Key words

wave
a disturbance that travels and carries energy.
crest
the highest point of a wave.
trough
the lowest point of a wave.
transverse
a wave whose vibration is at right angles to the way it travels.
compression
a place in a longitudinal wave where the material is pressed together.
rarefaction
a place in a longitudinal wave where the material is spread apart.
longitudinal
a wave whose vibration is along the way it travels.

Back to your guess

The answer is B: The bump. Each piece of rope only moved up and down a little.

A wave is a disturbance that travels. Each piece of the rope moved up and down and stayed near its place. What travelled from Mei to the far end was the bump, and with it energy. The rope itself did not travel.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. A ribbon is tied to the middle of a rope. A bump runs along the rope and past the ribbon. What does the ribbon do?

    Tap your answer.

    1. Yes, that is right. The bump travels along the rope. Each piece of the rope moves up and down as the bump passes, and then comes back to rest in its place.

    2. Not this one. It is a natural thing to think, because you see the bump travel. But the bump is a disturbance. The ribbon is tied to the rope, which does not travel along. Try another.

    3. Not this one. The bump is made of the rope itself moving. As the bump passes, the piece of rope with the ribbon rises, so the ribbon rises too. Try another.

    4. Not this one. Each piece of rope moves across the rope, not along it. After the bump passes, the ribbon is back where it began. Try another.

    See the answer

    It moves up, then comes back down, in the same place along the rope.

    The bump travels along the rope. Each piece of the rope moves up and down as the bump passes, and then comes back to rest in its place.

    • If you chose B: It is a natural thing to think, because you see the bump travel. But the bump is a disturbance. The ribbon is tied to the rope, which does not travel along.
    • If you chose C: The bump is made of the rope itself moving. As the bump passes, the piece of rope with the ribbon rises, so the ribbon rises too.
    • If you chose D: Each piece of rope moves across the rope, not along it. After the bump passes, the ribbon is back where it began.

  2. The picture shows a dot that vibrates and the way a wave travels. Which kind of wave is it?

    A dot with a double headed arrow running left and right through it, labelled vibration. Below, a long arrow pointing right, labelled wave travels.vibrationwave travels
    Tap your answer.

    1. Not this one. In a transverse wave the vibration is at right angles to the way the wave travels. Here the two arrows are on one line. Try another.

    2. Not this one. The dot only shakes to and fro about its place. It does not go along with the wave. What travels is the disturbance. Try another.

    3. Yes, that is right. When the vibration is along the way the wave travels, the wave is longitudinal. Sound in air and pushes along a spring are like this.

    See the answer

    A longitudinal wave

    When the vibration is along the way the wave travels, the wave is longitudinal. Sound in air and pushes along a spring are like this.

    • If you chose A: In a transverse wave the vibration is at right angles to the way the wave travels. Here the two arrows are on one line.
    • If you chose B: The dot only shakes to and fro about its place. It does not go along with the wave. What travels is the disturbance.

  3. Which of these is a longitudinal wave?

    Tap your answer.

    1. Not this one. Light is a transverse wave. Its vibration is at right angles to the way it travels. Try another.

    2. Not this one. A sideways flick makes a transverse wave. The rope moves across the way the wave goes. Try another.

    3. Not this one. A ripple looks like a transverse wave, because the water goes up and down as it passes. We treat ripples as transverse. Try another.

    4. Yes, that is right. The air layers shake to and fro along the way the sound goes, so they make compressions and rarefactions. That is a longitudinal wave.

    See the answer

    Sound travelling through the air

    The air layers shake to and fro along the way the sound goes, so they make compressions and rarefactions. That is a longitudinal wave.

    • If you chose A: Light is a transverse wave. Its vibration is at right angles to the way it travels.
    • If you chose B: A sideways flick makes a transverse wave. The rope moves across the way the wave goes.
    • If you chose C: A ripple looks like a transverse wave, because the water goes up and down as it passes. We treat ripples as transverse.

  4. A wave travels along a rope from Mei to Ravi. What does the wave carry from Mei to Ravi?

    Tap your answer.

    1. Not this one. The pieces of rope stay near their places. If they travelled, the rope would end up in a heap at Ravi's end. Try another.

    2. Yes, that is right. A wave carries energy from place to place. The material it travels through moves only a little and stays near its place.

    3. Not this one. Only the energy travels. The rope moves up and down and stays where it was. Try another.

    See the answer

    Energy

    A wave carries energy from place to place. The material it travels through moves only a little and stays near its place.

    • If you chose A: The pieces of rope stay near their places. If they travelled, the rope would end up in a heap at Ravi's end.
    • If you chose C: Only the energy travels. The rope moves up and down and stays where it was.

  5. A cork floats on a pond. A line of ripples passes it. When the ripples have gone, where is the cork?

    Tap your answer.

    1. Not this one. Ripples carry energy, not water. A cork carried to the shore would mean the ripples had pushed the water there too. Try another.

    2. Yes, that is right. The water under the cork moves up and down as each ripple passes. The ripple moves on across the pond and the cork stays about where it was.

    3. Not this one. Nothing pulls the cork back to the start. It just bobs as the water goes up and down under it. Try another.

    See the answer

    About where it was, after bobbing up and down.

    The water under the cork moves up and down as each ripple passes. The ripple moves on across the pond and the cork stays about where it was.

    • If you chose A: Ripples carry energy, not water. A cork carried to the shore would mean the ripples had pushed the water there too.
    • If you chose C: Nothing pulls the cork back to the start. It just bobs as the water goes up and down under it.

  6. The picture shows a spring with a wave. In which part of the spring are the coils pressed closest together?

    A row of upright lines, which are layers of a spring. They bunch together in two places and spread apart in others. An arrow at the foot points right and is labelled wave travels.wave travels
    Tap your answer.

    1. Not this one. A rarefaction is where the coils are spread apart. The word means "made thinner". Try another.

    2. Not this one. Crests and troughs belong to a transverse wave. A longitudinal wave has compressions and rarefactions. Try another.

    3. Yes, that is right. Coils pressed together make a compression. Coils spread apart make a rarefaction.

    4. Not this one. The coils are pressed together where a compression is. A compression is not tied to a fixed place. It travels along the spring. Try another.

    See the answer

    In a compression

    Coils pressed together make a compression. Coils spread apart make a rarefaction.

    • If you chose A: A rarefaction is where the coils are spread apart. The word means "made thinner".
    • If you chose B: Crests and troughs belong to a transverse wave. A longitudinal wave has compressions and rarefactions.
    • If you chose D: The coils are pressed together where a compression is. A compression is not tied to a fixed place. It travels along the spring.

  7. Which kind of wave does not need a material to travel through?

    Tap your answer.

    1. Not this one. Sound needs air, water or a solid. Nothing is there to shake in a vacuum, so sound cannot cross one. Go to S3.9.6. Try another.

    2. Not this one. A rope wave is the rope moving, so it needs the rope. Try another.

    3. Not this one. Ripples are the water surface moving, so they need the water. Try another.

    4. Yes, that is right. Light can cross empty space. That is how sunlight reaches the Earth. Sound, rope waves and ripples all need a medium.

    See the answer

    Light

    Light can cross empty space. That is how sunlight reaches the Earth. Sound, rope waves and ripples all need a medium.

    • If you chose A: Sound needs air, water or a solid. Nothing is there to shake in a vacuum, so sound cannot cross one. Go to S3.9.6.
    • If you chose B: A rope wave is the rope moving, so it needs the rope.
    • If you chose C: Ripples are the water surface moving, so they need the water.

  8. A bump takes 2 s to run 6 m along a rope. How fast does it travel?

    See the answer

    3 m/s

    Speed = distance ÷ time = 6 m ÷ 2 s = 3 m/s.

Keep it

Copy this into your exercise book.

A wave carries energy, not material. Transverse: vibration at 90° to travel (rope, light). Longitudinal: vibration along travel (sound, spring). Light needs no medium.

Draw: A rope with a ribbon, a bump in three positions, and the ribbon staying on one dashed line. Beside it, a spring with one compression.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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