9.01 Waves

Waves

Waves are repeating disturbances that transfer energy through matter or space. A wave will travel as long as it has energy to carry. A good example of waves is dropping a rock in the lake.

• The energy from the rock causes ripples to appear on the surface of the lake.
• As the energy is gradually transferred through the water, the ripples move farther and farther away from the drop zone.
• Only the energy travels along with the waves. As the energy spreads out and is transferred away from the waves, the waves will shrink and disappear.
• These ripple waves you see are mechanical waves.

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Mechanical Waves

Mechanical waves require a medium to travel. Waves, such as ocean waves, seismic waves, and sound waves are examples of mechanical waves. Mechanical waves are displacements of matter; therefore, they can occupy the same space at the same time.

A medium is matter through which a wave travels. A medium can be:

• a gas,
• a liquid,
• a solid, or
• a combination of states.

Not all waves require a medium. Electromagnetic waves do not require a medium. Radio waves, ultraviolet, and x-ray are all examples of electromagnetic waves.

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Longitudinal Waves

In this lesson, we will discuss two types of waves:

• longitudinal waves
• transverse waves

In a longitudinal wave, the matter in the medium moves back and forth along the same direction that the wave travels. These waves are also called compressional waves.

A slinky can demonstrate longitudinal waves. Place the slinky on a flat surface, hold one end, then push and pull on the slinky. This movement causes the wave. Watch the video Longitudinal Wave (0:12) to see this example in action. PBS login information.

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Transverse Waves

In a transverse wave, the particles in the medium move back and forth at right angles to the direction that the wave travels. Examples of transverse waves include:

• Light waves
• A ripple on a lake
• A wave on a string

Watch Transverse Waves (0:12) to see the motion of a transverse wave. PBS login information.

In the photo below, a man exercises using ropes. As he moves the ropes up and down, they create a transverse wave.

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Examples

There are lots of examples of waves in the natural world, including:

• Water waves are a mixture of longitudinal and transverse waves. As the wind blows across the surface of a body of water, water waves form.


• Seismic waves are caused by forces inside the Earth's crust. The crust can move, bend, or break due to these waves. Like water waves, seismic waves are both longitudinal and transverse. They occur in rocks and other materials inside the Earth.


• Sound waves are longitudinal waves. We will discuss them in more detail later in another lesson in this unit.

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