LIGHT AND SOUND
LESSON 1
Summary
This lesson introduces the concepts of longitudinal and transverse waves. Students see several
demonstrations of waves and characterize them by transverse and longitudinal behaviour. This lesson also
introduces the Sunken Treasure theme of the Sound and Light unit — a continuous story line throughout
the lessons.
Engineering Connection
Engineers use sound and light waves for many reasons in our society. Sonar, reading glasses, light bulbs,
stereo equipment, and lasers all rely on either sound and/or light waves. Engineers must have an excellent
understanding of how sound and light waves work in order to apply them to new technology. Naval
engineers design ships to travel on ocean waves.
Learning Objectives
After this lesson, students should be able to:
• Define what a wave is and give examples of waves.
• Explain the difference between longitudinal and transverse waves.
• Describe an ocean wave as having characteristics of both longitudinal and transverse waves.
Introduction/Motivation
Angie and Harmon are elementary school kids on summer vacation with their parents in the Bahamas. One
afternoon they head out into the North Atlantic Ocean to try surfing. Angie catches a big wave right away
and rides it in toward the beach until a shiny golden object from the sea floor distracts her and causes her
to wipe out. When Harmon paddles over to check on her, Angie tells him what she saw. They paddle back
and dive down to look for it. Harmon comes up with an old gold coin. How did it get there? To understand
how the coin arrived at its resting place on the sea floor, you need to understand waves.
A wave is a change (a variation) that travels through a substance (or medium). You can often see the
change, such as the increased height of a traveling ocean, but what is important to understand is that the
medium itself does not travel with the wave. This can be confusing, so let's think about an example.
A good example of a wave is the type of "wave" performed in a football — or any athletic stadium — by
the fans. A person stands up, raises her arms, then the person next to her stands up and raises her arms,
the next person stands up, etc. In this way, a wave travels around the stadium, while all the people that
made the wave only travel up and down (stand up and sit down). So, the change (or variation) is traveling
all the way around the stadium, while the people (or medium) really stay in the same spot.
The "football wave" is an example of a transverse wave. In a transverse wave, the medium vibrates in the
opposite direction than the wave travels. In the football wave, the wave traveled right or left while the
people (the medium) moved up and down. The wave moved perpendicular to the movement of the
medium.
Another example of a transverse wave is a string tied to a doorknob. When the free end of the string is
pulled up suddenly from the floor, a transverse wave moves down the length of the string towards the
doorknob. Note that the string does not actually move towards the doorknob, but the wave does.
A different type of wave is a longitudinal wave. In a longitudinal wave, the particles oscillate back
and forth in the same direction that the wave travels. A Slinky can create a longitudinal wave. If two
students stretched a slinky between them and one of the students suddenly push her end of the
slinky towards the other student, a longitudinal wave is created. When the student pushes her end
toward the other student, the slinky coils bunch together; the bunching moves towards the other
student and back again. In this case, the medium is the coils of the slinky, while the wave is the
region of coils close together that moves from one student to the other. When the coils bunch close
together and then move farther apart they are moving left and right, just as the wave itself moves
left and right. In a longitudinal wave, the medium oscillates in the same direction that the wave
travels.
The type of wave that most students have experience with is an ocean wave, or a water wave. Do
you think ocean waves are transverse waves or longitudinal waves? Visually speaking, most
students will say transverse wave. This is correct, but ocean waves also show longitudinal wave
behaviour. The medium (water molecules) moves up and down (the crest and troughs of the wave)
as well as left and right, but the water does not actually travel with the wave. So, ocean waves are
both longitudinal and transverse waves.
A naval engineer is someone who designs ships to travel on the ocean. One thing that influences
the shape and size of a ship is the waves that it needs to travel through. Why are the fronts of most
ships pointed? (Answer: They are pointed so that the ship can separate the water and waves in front
of the boat, allowing it to glide right through the water.) Why are the decks of many ships slightly
curved upward? (Answer: To make the waves that crash on the deck run off.) Acting as naval
engineers, what else would you have to think about when designing a ship?
Vocabulary/Definitions
Longitudinal wave: A wave whose particles oscillate in the same direction as the wave travels.
Oscillate: To vibrate back and forth.
Transverse wave: A wave whose particles oscillate perpendicular to the direction that the wave travels.
Wave: A traveling disturbance in a medium.
Lesson Closure
Now that we know about transverse and longitudinal waves, do you know what caused the gold coin to
come to rest where it did on the ocean floor? That's right: the transverse and longitudinal motion of water
waves pushed it there over time. What does this mean? Well, somewhere further out to sea, there is
probably a sunken treasure ship with more treasures to find! Actually, it means that waves move all sorts
of treasures (and other debris) around in the ocean. Have you ever read a book or watched a movie about
someone stranded on an island who puts a note in a bottle, plugs the bottle and the sends it out to sea?
Well, their hope is that the waves carry the bottle to another location that is populated, which might lead
to their eventual rescue. Who knows? With the wonder of waves, it could happen.
Homework
• What is a wave?
• What are the two types of waves?
• Define each type of wave?
• In which type of wave does the medium oscillate in the same direction that the
wave travels?
• What is a real-life example of this type of wave?
• In which type of wave does the medium oscillate opposite to the direction that
the wave travels?
• What are some real life examples of these types of waves?
• Which kind of wave is an ocean wave?