Chapter 7 of 8

All 8 chapters
  1. 1Reflection, readunderstood
  2. 2Refraction, readunderstood
  3. 3Lenses and the eye, readunderstood
  4. 4The colours of light, readunderstood
  5. 5Why things have colour, readunderstood
  6. 6Sound as a wave, readunderstood
  7. 7The speed of sound, readunderstood
  8. 8Pitch, loudness and hearing, readunderstood

S3.9.7 Chapter 7

The speed of sound

Why do I see lightning first and hear the thunder later?

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

  • Explain why we see lightning before we hear thunder
  • Estimate a storm's distance by counting from flash to thunder

Tariq watches a storm far away. A flash lights up the sky. A few seconds later he hears a rumble. The flash and the thunder began at the same moment. Why did they reach him at different times?

The speed of sound in air is about 340 metres each second. We write about 340 m/s. It changes a little with the temperature.

Light is far faster. The speed of light is about 300 000 kilometres each second. That is more than 7 times round the Earth in one second.

Draw it first

Draw a flash, light and sound

  1. In the first picture a zigzag of lightning is labelled flash, and a person on the right is labelled you. In the second picture a long arrow runs from the flash to the person, labelled light: almost at once. In the third picture a second arrow runs below it, labelled sound: much later. Labels: flash, you, light: almost at once, sound: much later.flashyou

    1 Draw the flash on one side and yourself on the other.

  2. In the first picture a zigzag of lightning is labelled flash, and a person on the right is labelled you. In the second picture a long arrow runs from the flash to the person, labelled light: almost at once. In the third picture a second arrow runs below it, labelled sound: much later. Labels: flash, you, light: almost at once, sound: much later.flashyoulight: almost at once

    2 Draw an arrow for the light. It arrives almost at once, so you see the flash.

  3. In the first picture a zigzag of lightning is labelled flash, and a person on the right is labelled you. In the second picture a long arrow runs from the flash to the person, labelled light: almost at once. In the third picture a second arrow runs below it, labelled sound: much later. Labels: flash, you, light: almost at once, sound: much later.flashyoulight: almost at oncesound: much later

    3 Draw an arrow for the sound. It is slower, so it arrives much later, and you hear the thunder.

Sound is not the same speed in everything. It travels about 340 m/s in air. It is faster in water, at about 1500 m/s. It is faster still in a solid, such as iron, at about 5000 m/s or more.

A bar chart called speed (m/s). The bar for air reaches 340. The bar for water reaches 1500. The bar for iron reaches 5000. The scale goes from 0 to 5000 in steps of 1000. Labels: speed (m/s), air, water, iron, 0, 1000, 2000, 3000, 4000, 5000.010002000300040005000airwaterironspeed (m/s)
Sound goes fastest in a solid and slowest in air. In a solid the particles are held tightly to each other. Each one passes the push on quickly.

Now you can work out how far away a storm is. The sound goes about 340 metres every second. So distance = speed × time.

A zigzag of lightning, labelled flash, on the left. A person, labelled you, on the right. An arrow runs between them, labelled 6 s above it and distance ? m below it. Labels: flash, you, 6 s, distance ? m.flashyou6 sdistance ? m
Tariq counts 6 seconds from the flash to the thunder. How far away is the storm?

Worked example

Tariq counts 6 s from the flash to the thunder. How far away is the lightning?

  1. Distance = speed × time. The speed of sound is about 340 m/s.
  2. 340 × 6 = 2040 m.
  3. There are 1000 m in 1 km. So the storm is about 2 km away.

Answer: 2040 m

There is a quicker way. In 3 seconds, sound goes about 1 kilometre, because 3 × 340 is about 1000. So divide the seconds by 3 to get the kilometres.

See it three ways

As a picture

A table with two columns, seconds and km. 3 seconds, 1 km. 6 seconds, 2 km. 9 seconds, 3 km. 12 seconds, 4 km. Labels: seconds, km.secondskm316293124

In words

Count the seconds from the flash to the thunder. Every 3 seconds is about 1 kilometre of distance.

In symbols

kilometres ≈ seconds ÷ 3.

Worked example

Kofi counts 10 s from the flash to the thunder. How far away is the storm in kilometres?

  1. Distance = speed × time. The speed of sound is about 340 m/s.
  2. 340 × 10 = 3400 m.
  3. 3400 m ÷ 1000 = 3.4 km.

Answer: 3.4 km

Try it

Learn to count seconds, and make a storm ruler

You need: a friend with a watch or a clock that shows seconds, paper and a pencil.

Be safe: Count a real storm only from indoors or from inside a closed vehicle. Stay away from windows, water and metal. If you can hear thunder, stay inside.

  1. Say "one thousand and one, one thousand and two" aloud. Each count should take one second.
  2. Ask your friend to time ten counts. Adjust your speed until ten counts take ten seconds.
  3. Now divide each of these counts by 3: 3, 6, 9, 12 and 15. Write the kilometres next to each.
  4. Next time there is a storm, count only from indoors or a closed vehicle. Count from the flash to the thunder, and use your ruler.

What you will see: Each count of 3 seconds adds about 1 kilometre. If the gap grows from one flash to the next, the storm is moving away. If it shrinks, the storm is coming closer.

If you cannot try it, this is what happens.
A graph of seconds counted across, from 0 to 15 in steps of 3, against kilometres away up, from 0 to 5 in steps of 1. The points lie on a straight line: 0 seconds is 0 km, 3 seconds is 1 km, 6 seconds is 2 km, 9 seconds is 3 km, 12 seconds is 4 km, 15 seconds is 5 km. Labels: seconds counted, km away, 0, 3, 6, 9, 12, 15, 1, 2, 3, 4, 5.03691215012345seconds countedkm away

A wrong idea many people have

The thunder happens later than the lightning.

Why it feels right: You always see the flash first and hear the rumble afterwards.

The test that breaks it: Ask a friend to hit two pots together far away across a field, and watch. You see the pots meet. A moment later, you hear the clang. The clang was made when the pots met.

What is true: The flash and the thunder start together. Light arrives almost at once, and sound arrives later.

Key words

speed of sound
how far sound goes in one second. In air it is about 340 m/s.
speed of light
how far light goes in one second. It is about 300 000 km.

Back to your guess

The answer is A: Light travels far faster than sound, so the flash reaches you almost at once.

The flash and the thunder start together. Light is far faster than sound, so the flash reaches you almost at once. The sound is left behind and arrives later.

Check yourself

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

  1. Why do we see lightning before we hear thunder?

    Tap your answer.

    1. Not this one. They start together. Sound takes longer to reach you. Go to S3.9.7. Try another.

    2. Yes, that is right. Light goes about 300 000 km each second and sound about 340 m each second. The flash arrives almost at once, and the thunder arrives later.

    3. Not this one. They go the same distance. Sound is much slower than light. Go to S3.9.7. Try another.

    See the answer

    Light travels far faster than sound.

    Light goes about 300 000 km each second and sound about 340 m each second. The flash arrives almost at once, and the thunder arrives later.

    • If you chose A: They start together. Sound takes longer to reach you. Go to S3.9.7.
    • If you chose C: They go the same distance. Sound is much slower than light. Go to S3.9.7.

  2. Ana counts 9 seconds from a flash to the thunder. About how far away is the storm?

    A zigzag of lightning, labelled flash, on the left. A person, labelled you, on the right. An arrow runs between them, labelled 9 s above it and distance ? km below it. Labels: flash, you, 9 s, distance ? km.flashyou9 sdistance ? km
    Tap your answer.

    1. Not this one. You used the seconds as kilometres. Divide the seconds by 3 to get kilometres. Go to S3.9.7. Try another.

    2. Not this one. You multiplied by 3. Sound goes 1 km in 3 s, so you divide by 3. Go to S3.9.7. Try another.

    3. Not this one. You took 3 away. The rule is to divide the seconds by 3. Go to S3.9.7. Try another.

    4. Yes, that is right. Sound goes about 1 km in 3 s. 9 ÷ 3 = 3 km.

    See the answer

    3 km

    Sound goes about 1 km in 3 s. 9 ÷ 3 = 3 km.

    • If you chose A: You used the seconds as kilometres. Divide the seconds by 3 to get kilometres. Go to S3.9.7.
    • If you chose B: You multiplied by 3. Sound goes 1 km in 3 s, so you divide by 3. Go to S3.9.7.
    • If you chose C: You took 3 away. The rule is to divide the seconds by 3. Go to S3.9.7.

  3. Sound goes 340 m each second in air. How far does it go in 5 s?

    Tap your answer.

    1. Not this one. You divided the speed by the time. To find a distance, multiply the speed by the time. Go to S3.9.7. Try another.

    2. Not this one. You added the numbers. A speed of 340 m/s means 340 metres every second, so multiply by the seconds. Go to S3.9.7. Try another.

    3. Yes, that is right. Distance = speed × time = 340 × 5 = 1700 m.

    See the answer

    1700 m

    Distance = speed × time = 340 × 5 = 1700 m.

    • If you chose A: You divided the speed by the time. To find a distance, multiply the speed by the time. Go to S3.9.7.
    • If you chose B: You added the numbers. A speed of 340 m/s means 340 metres every second, so multiply by the seconds. Go to S3.9.7.

  4. In which of these does sound travel fastest?

    Tap your answer.

    1. Yes, that is right. In a solid the particles are held tightly together, so the shake passes quickly. Sound goes about 5000 m/s in iron, 1500 m/s in water and 340 m/s in air.

    2. Not this one. Air is the slowest of the three. Its particles are only loosely joined, so the shake passes on slowly. Go to S3.9.7. Try another.

    3. Not this one. Water is faster than air, but a solid is faster still. Go to S3.9.7. Try another.

    See the answer

    Iron.

    In a solid the particles are held tightly together, so the shake passes quickly. Sound goes about 5000 m/s in iron, 1500 m/s in water and 340 m/s in air.

    • If you chose B: Air is the slowest of the three. Its particles are only loosely joined, so the shake passes on slowly. Go to S3.9.7.
    • If you chose C: Water is faster than air, but a solid is faster still. Go to S3.9.7.

  5. Joseph is outside and hears thunder. What should he do?

    Tap your answer.

    1. Not this one. If you can hear thunder, lightning can reach you. Count only from a safe place. Go to S3.9.7. Try another.

    2. Yes, that is right. If you can hear thunder, lightning can reach you. Go inside at once. You can count from there.

    3. Not this one. A tall tree can be struck. Never shelter under one in a storm. Go to S3.9.7. Try another.

    See the answer

    Go indoors, or into a closed vehicle.

    If you can hear thunder, lightning can reach you. Go inside at once. You can count from there.

    • If you chose A: If you can hear thunder, lightning can reach you. Count only from a safe place. Go to S3.9.7.
    • If you chose C: A tall tree can be struck. Never shelter under one in a storm. Go to S3.9.7.

  6. From an earlier chapter

    Review: why can sound not cross empty space?

    Tap your answer.

    1. Not this one. Cold air still carries sound. What is missing in space is the particles. Go to S3.9.6. Try another.

    2. Not this one. Distance is not the reason. A sound cannot even start to travel with nothing to shake. Go to S3.9.6. Try another.

    3. Yes, that is right. A sound wave is squashed and stretched particles. Empty space has almost none, so sound cannot cross it.

    See the answer

    There are no particles to pass the shake along.

    A sound wave is squashed and stretched particles. Empty space has almost none, so sound cannot cross it.

    • If you chose A: Cold air still carries sound. What is missing in space is the particles. Go to S3.9.6.
    • If you chose B: Distance is not the reason. A sound cannot even start to travel with nothing to shake. Go to S3.9.6.

Keep it

Copy this into your exercise book.

Light is far faster than sound, so we see lightning first. Sound goes about 340 m/s. Divide the seconds from flash to thunder by 3 to find the kilometres.

Draw: A flash on the left and you on the right. A short arrow for light and a long wait for sound. Write seconds ÷ 3 = km.

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

No internet tomorrow? Save the whole book as one file (720 KB). It opens on any phone.