Lesson 18: Sound and Light Waves

Sound travels through matter as vibrations, while light can travel through empty space. Wave models and comparisons between materials help explain how each travels and why lightning is seen before thunder is heard.

Key Ideas

  • Sound travels through air as a wave. Air particles jiggle back and forth and pass the vibration along.
  • Sound needs material to travel through, such as air, water, or a solid. It cannot cross empty space.
  • Sound travels at different speeds in different materials. It travels faster through steel than through water or air.
  • Light travels in waves too, but it can cross empty space. That is how sunlight reaches Earth.
  • Light travels much faster than sound. We see lightning before we hear thunder.

Vocabulary

  • Medium: The material a wave travels through. Air is a medium for sound.
  • Longitudinal wave: A wave in which particles move back and forth along the direction the wave travels.
  • Compression: A part of a sound wave where particles are closer together.
  • Rarefaction: A part of a sound wave where particles are farther apart.
  • Vacuum: A space with very little matter in it, including very little air.

Discussion Questions

  1. Why can't you hear someone talking in outer space, even if they were right next to you?
  2. What do you think would happen to sound if the particles in the air were much farther apart? What about much closer together?
  3. Why does sound travel differently through a solid, liquid, and gas?
  4. If you put your ear against a table while someone taps the other end, how might the sound you hear be different from the sound traveling through the air?
  5. Why can light travel through space when sound cannot? What does that tell us about the two types of waves?
  6. Imagine you were standing on the Moon during a meteor impact. What would you see and what would you hear? Why?
  7. Why do we see lightning before we hear thunder, even though they happen at almost exactly the same time?
  8. If sound traveled faster than light, how would thunderstorms look different to us?
  9. Movies often show huge explosions in space with loud “BOOM!” sounds. Why isn't that scientifically accurate?
  10. Which would be more difficult to explain to someone who had never experienced Earth: why sound can't travel through space, or why light can? Why?

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