If you've ever been to the ocean, you've seen a wave. However, in physics, a wave is more than a swell in the sea. A wave is one of two ways in which energy is transfered. In the past, we have talked only of particles. If you throw a ball at a brick wall, the target might gain some kinetic energy, and the ball would lose some. The ball itself had to move over a distance. The ball is a moving particle. If you tie a rope to a brick wall, and then shake the rope, the target also gains energy. The rope, however, transfered the energy as a wave and didn't actually have to move toward the wall. In this lesson you will earn what a wave is as well as some basics about wave behavior. Then you will have the opportunity to learn more by studying light, which is really a type of wave.

Mechanical waves are waves that require what we call a "medium." Ocean waves, sound waves and even a wiggling rope are mechanical wave examples. By "medium" we mean that the wave travels through a substance. The sound travels through the air. The air would be the sound's medium. The properties of the medium often affect the behavior of the wave, as you will soon learn.

Electromagnetic waves do not require a medium. Light, X-Rays and radio waves are examples of electromagnetic waves. Light, for example, can travel through air or the vacuum of space without trouble. Electromagnetic waves are generally very fast- light moves at 299 792 458 m/s. That's fast.

The best way to learn waves in by learning about mechanical waves. There are three types of mechanical waves. The first type is the transverse wave. This is the vibration of the medium in a direction perpendicular to the direction of the wave. The vest way to explain it is for us to draw it.

The second type of wave is a longitudinal wave. The medium moves parallel to the direction of the wave. Sound waves are longitudinal.

The third type is the surface wave. These are waves like the waves of the ocean. Only the surface ripples, the rest of the medium does not. If you dive deep under the ocean, you wont be able to feel the waves high above you.

Waves tend to repeat themselves. This is the nature of the wave. There will be one wave pulse, then another, then another, in a rhythmic pattern. The time it takes for the wave to repeat itself is called the wave's period, and is given the symbol T. The inverse of the period, or 1/T, is called the frequency (f). This is the number of wave repeats in one second. You will need to know both of these to continue your study of waves.

Waves have several components. Study the diagram below very carefully. The wavelength of a wave is the distance between the crests of the wave. You will notice that this distance is the same as the distance between the troughs of the wave, so the wavelength can really be either. Wavelength is given the symbol Lambda (no, not Lambada, that's the forbidden dance), or . The distance between the equilibrium and the crest is called the amplitude, or wave height. The larger the amplitude, the more energy that the wave has.

Physicists like you and me love to make calculations. You do, don't you? For instance, you probably are dying to find out how to calculate the velocity of a wave. This shouldn't be hard for you. Velocity is measured in meters per second, or m/s. What part of a wave is measured in meters? The wavelength is, of course. And what part is measured in seconds? The period. So the velocity is measured as the wavelength over the period.


v = / T

Since the period is the same as 1 over the frequency, we can simplify this equation to:

v = f

The speed of a mechanical wave does not depend upon the amplitude of a wave. Really, the properties of the medium have the greatest effect on speed. A wave with a large amplitude moves at the same speed as a wave with a small amplitude, provided the mediums and wavelengths are the same.

When a wave changes mediums, it is a big deal. Say we tie one end of a rope to a brick wall, then send a wave down the rope. The wave tries to change from the medium of the rope to the slower, denser medium of the wall. It isn't likely that the wall will start to vibrate. Instead, the wave will bounce off of the wall, and return down the other side of the rope. This is called wave reflection. When a wave enters a slower medium, there will be a reflected wave which is 180 degrees out of phase. (180 degrees out of phase means that it is on the other side of the medium.)

If we tie a rope to a thread, and send a wave down the rope, it will definitely move the thread. It is changing to a lighter, faster medium. There will still be a reflection though. It will be small, but it will be present. It won't be out of phase though. When a wave changes to a faster medium, the reflected wave is in phase with the original wave.

We use the words "slow medium" to refer to a heavier, more dense medium. This is because a wave gets slower when entering a more rigid medium. Likewise, when a wave enters a less rigid medium, it enters a "fast medium."

Waves sometimes end up hitting each other. If we send a wave down each side of a rope, they will interact when they come together. There is a scientific rule that we follow for the interaction of two waves. It is called the principle of superposition, and it states that "the displacement of a medium caused by two waves is the sum of the displacements of the individual waves." What this means is that the two waves will combine into one bigger or smaller wave when they meet. This is called wave interference. If two waves are in phase, then they will make a single, big wave. The crest of this big wave is called the anti-node. If the two waves are out of phase, the resulting wave will be equal to the larger wave minus the smaller wave. If two waves that are out of phase meet, and they have the same displacement, then they will cancel out in the place where they join. This point is called a node. There will be waves up and down the medium, except at the node.

A standing wave is really a simle concept, but it sounds difficult. When the period of the wave is the same as the time it takes to cover distance to a node point and back, you have a standing wave. If you've ever heard of someone shattering a glass with the sound of their voice, they used a standing wave. In that particular case, the standing wave of sound causes something called resonance. This makes the glass vibrate exactly the same way the persons voice is, and then the vibration destroys the glass. A resonance occurs when one object causes another to vibrate with the same frequency.

To learn more about light, read the lessons on light. Light is an electromagnetic wave, but now that you know some wave basics, you can use it as an example to learn the many ways that waves can behave in the universe. Many properties of light waves also work with other types of waves.