What Are Waves?
You may have seen a wave tumbling sand at the beach, or a wave in a pond after you toss a pebble into it. But did you know that waves are actually much more common than the ones you see in water?
Scientists have shown that there are light waves, sound waves, ocean waves, radiation waves, and many, many more kinds of waves. You might wonder, if there are so many different types of waves, what exactly is a wave?
A wave is a movement of energy through a medium.
For waves in an ocean, a lake, or your neighborhood pool, the medium is water. For sound, the medium can be air, water, even a wall inside a house. Light waves can travel though air, glass, water, and other mediums.
A wave is not movement of actual particles or material. That’s why when you are bobbing in the ocean, a wave can pass under you without you actually moving. Once the wave goes by, you are still in the same place. It also means that the energy of the wave can pass from one medium to another.
Sound is a Wave
Consider sound waves. Say a drummer strikes the top of a drum. Imagine the air particles inside the drum. When the drummer hits the drum, it pushes the drum skin downward, much faster than we can see. The drum skin is then pushed into the air particle right underneath it. Like bumper cars, that air particle bumps into another air particle. Its energy is transferred to the second particle. The second particle starts to move, and the first particle stops moving.
Particle after particle, bump after bump, sound waves are created. These waves travel from the drum into the air, where they reach your ears. Meanwhile, all the particles that bumped each other along the way are back at rest. We can’t see any of these waves forming. We just hope we like the music.
Energy Moves, Not Matter
Although the sound wave from the drum moves through the air, none of the air particles themselves really do. Instead, the particles move a very small distance, bump into one another and then return to their initial position. Only the energy travels, from the drum to the air to your ear.
This isn’t just true of sound waves. It happens with all waves! To picture this, imagine a duck floating on water as a boat goes by. When the waves created by the boat pass under the duck, the duck just bobs up and down, even as the waves keep moving away from the boat. If the actual water molecules were moving, then the duck would travel as well, since it floats on the water molecules.
Like the sound waves, that duck demonstrates that matter itself is not transferred in a wave. It’s just one molecule bumping into and pushing another molecule, on and on, until the wave passes under the duck and splashes onto shore.
Here’s another fun example: people doing a wave in a stadium. As the wave gets close, you stand up and lift your arms high into the air. As the wave moves past you, you lower your arms and sit back down. The wave has moved straight through you, yet you and all the people nearby are still in the same place you were before the wave passed by. In this case, you and everyone else in the stadium are the medium that the wave is passing through. You’re one of those particles that doesn’t move.
What does this tell us about waves, whether they are in the ocean, the air, a drum, or a stadium? Waves are disturbances in any medium that transports energy from one spot in the medium to another, without transporting the matter of the medium itself.
The Properties of Waves
Now that we know what waves are, we can talk about how they are described by scientists. The five ways to measure and describe waves are: amplitude, wavelength, period, frequency and velocity. Together, these define how a wave moves.
Amplitude is the height of a wave. Think of it as the distance from the duck’s rest position to the highest point it reaches when bobbing on the wave.
Wavelength is the distance between the peaks of waves. If the duck is at the top of a wave, the closest peak of the next wave is exactly one wavelength away. In the case of waves from a boat, the wavelength might be 2 meters.
Period is similar to frequency. It is the time, usually in seconds, that it takes for a wave to complete a full cycle. If we started counting when the duck reached the top of one wave up and down and count how long it takes for the duck to go from there to the top of the next wave, we would have the period.
Frequency is how many waves go past a point in some amount of time, usually one second.
Lastly: velocity. Velocity describes how fast a wave is moving. If we look at the peak of a wave when it is under the duck and then see that one second later it is two meters away from the duck, the wave’s velocity is two meters per second.
Waves are all around us, from sound waves to light waves to water to radio waves and beyond, they are all part of how the world works, and our experience and perception of that world. These aren’t the only types of waves, of course. Scientists have discovered that radiation, energy, earthquakes and much more also travel in waves.