Building directly on Lesson 16, learners investigate what actually happens when sound leaves its source and travels to a listener. Students discover that sound moves as a longitudinal wave (a traveling pattern of compression and rarefaction) and that it requires a medium of particles to travel through. Through demonstrations and discussion, the lesson establishes that sound moves fastest through solids, slower through liquids, and slowest through gases, and cannot travel through a vacuum at all. The lesson concludes by turning to light: students compare sound waves to light waves and discover a key difference, light does not need a medium and travels incomparably faster, which is why we see lightning before we hear thunder.

Key Ideas

• Sound travels as a longitudinal wave. The particles of the medium push back and forth in the same direction the wave travels.
• Sound requires a medium (a solid, liquid, or gas) to travel through. Without particles there is nothing to carry the wave.
• Sound travels fastest through solids, slower through liquids, and slowest through gases because the closer the particles, the faster energy passes between them.
• In a vacuum, like outer space, there are no particles, so sound cannot travel at all. Explosions in space are completely silent.
• Light also travels in waves, but it is a transverse wave and does not need a medium. Light can travel through empty space.
• Light travels incomparably faster than sound, about 186,000 miles per second versus about 1,100 feet per second, which is why we see lightning before we hear thunder.

Vocabulary

• Medium: The material (solid, liquid, or gas) that sound travels through.
• Longitudinal wave: A wave in which particles move back and forth in the same direction as the wave travels.
• Compression: A zone in a sound wave where particles are pushed close together.
• Rarefaction: A zone in a sound wave where particles are spread apart.
• Vacuum: Space that contains no particles. Sound cannot travel through it; light can.

Supplies for Live Class

• Soundwaves Printable or paper
• Homemade Harmonica from previous class
• Slinky (optional)

Discussion

• Why can't you hear someone talking in outer space, even if they were right next to you?
• What do you think would happen to sound if the particles in the air were much farther apart? What about much closer together?
• Why does sound travel differently through a solid, liquid, and gas?
• 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?
• Why can light travel through space when sound cannot? What does that tell us about the two types of waves?
• Imagine you were standing on the Moon during a meteor impact. What would you see and what would you hear? Why?
• Why do we see lightning before we hear thunder, even though they happen at almost exactly the same time?
• If sound traveled faster than light, how would thunderstorms look different to us?
• Movies often show huge explosions in space with loud “BOOM!” sounds. Why isn't that scientifically accurate?
• 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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