Lesson 18
How Sound and Light Travel in Waves
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.


Hands-On Activity: Sound Through the Table

Supply List
  • A sturdy wooden table
  • A hand for knocking
Instructions
  • Sit at a wooden table. Knock gently on the underside and listen to how it sounds through the air. Notice the volume and clarity.
  • Press one ear flat against the tabletop and close your other ear with your finger. Knock again with the same force.
  • Compare the two experiences. The knock heard through the solid wood should be noticeably louder and clearer than through the air.
  • Discuss why: molecules in a solid are packed closely together and pass vibrations from one to the next very efficiently, while air molecules are spread far apart and lose energy quickly over distance.
  • Extension: try the same experiment on different surfaces — a metal desk, a plastic table, a carpeted floor. Does the material matter?

Supplies for Live Class
Sources

Sources
L18_How_Sound_and_Light_Travel_in_Waves.pptx by Selene
https://wordwall.net/resource/114570318
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