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Sound needs something to travel through

Waves, sound and light · level 3, lesson 2 of 6

A question to start

In films, spaceships explode with a huge bang. But astronauts say space is completely silent. Who is right?

The idea

Sound is squashes and stretches passed from particle to particle. So it needs particles to travel through: a gas, a liquid or a solid. Scientists call the stuff a wave travels through its medium.

Space is almost empty: there are almost no particles at all. So sound cannot travel through it. A real explosion in space would make no sound you could hear.

Light is different. It can cross empty space, which is how sunlight reaches us.

air in: you hear itair out: silence

Sound travels through solids and liquids too, often better than through air. Put your ear on a table and tap the other end: you hear the tap loud and clear.

Worked examples

1. The bell in a jar. A ringing bell sits in a glass jar. As a pump takes the air out, the ringing gets fainter and fainter, although you can still see the hammer hitting the bell.

2. Spacewalks. Astronauts outside a spacecraft talk by radio, which uses waves that can cross empty space.

3. Whales call to each other through the sea. Their sounds can travel a very long way through water.

Try it

  1. Why can't sound travel through space?
  2. What is a medium?
  3. Name a solid, a liquid and a gas that sound can travel through.
  4. How do astronauts on a spacewalk talk to each other?
Answers
  1. Space has almost no particles to pass the squashes and stretches along.
  2. The stuff a wave travels through.
  3. For example: wood, water, air.
  4. By radio.

Common mistake

"Mars is in space, so it is silent." Mars has air, so sound does travel there, though more quietly than on Earth. Only the near-empty space between the planets is silent.

Where it leads