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An Introduction to Waves Waves are energy that cause a medium vibrate in a periodic way. A common example is ocean waves, where the medium is water. Sound is also a wave, with the medium being air. The sound wave causes the air molecules shake back and forth as the wave passes through them. Note that the medium itself does not move in any net direction, it oscillates around a central point. Two Types of Waves There are two basic kinds of wave motion, longitudinal and transverse. Longitudinal waves propagate in the direction of the medium's vibration. Sound waves are longitudinal waves. A spring with a compression (longitudinal) wave moving through it. Transverse waves move at a 90-degree angle (perpendicular) to their direction of propagation. Transverse wave Examples of transverse waves are vibrating strings, ripples on the surface of water, and electromagnetic radiation (light). Basic Properties of Waves The image below defines some basic aspects of waves. The vertical axis represents the displacement of the wave from it's equilibrium position, and the horizontal axis is length. Amplitude is the maximum displacement of the wave. A wavelength is the distance between two common points on the wave. In the image above I used the amplitude as my two common points. For light, the units of wavelength are typically measured in nanometers, which are 10^-9 of a meter. The period of a wave is how much time it takes to complete a full wavelength. The frequency is how many times a wavelength completes a full cycle in one second. It is inversely related to the period by the equation: where f is the frequency in Hz (1/seconds), and T is the period in seconds. Hz is short for "Hertz." FM radio stations are signals measured in megahertz(MHz), or millions of Hz. The frequency is also related to the wavelength. If you pick a point on a wave (a crest, for example) and follow it over time, it appears to move with a certain speed. This is called, oddly enough, the wave speed. When surfers catch a wave and glide along top of it, they move at the wave speed. The wave speed depends on the frequency and the wavelength: here ws is wave speed in m/s, λ is the wavelength in meters, and f is the frequency in Hz. If the wave speed is constant then the frequency and wavelength are inversely related to each other. One interesting property of waves is that they can interfere with each other. Waves can add together, cancel each other out, or do a combination of the two. See some nice examples of this at http://www.kettering.edu/~drussell/Demos/superposition/superposition.html. Interference is very common. For example, the sound we hear when a musician plays the B note on a violin is actually millions of different sound waves interfering with each other. We hear the sum of waves as the unique sound of the violin. <<< Previous \| Next >>> Home

MIRA's Field Trips to the Stars Internet Education Program

Back to the MIRA Website | Field Trips to the Stars Home > Introduction > Waves

An Introduction to Waves

Waves are energy that cause a medium vibrate in a periodic way. A common example is ocean waves, where the medium is water. Sound is also a wave, with the medium being air. The sound wave causes the air molecules shake back and forth as the wave passes through them. Note that the medium itself does not move in any net direction, it oscillates around a central point.

Two Types of Waves

There are two basic kinds of wave motion, longitudinal and transverse. Longitudinal waves propagate in the direction of the medium's vibration. Sound waves are longitudinal waves.

A spring with a compression (longitudinal) wave moving through it.

Transverse waves move at a 90-degree angle (perpendicular) to their direction of propagation.

Transverse wave

Examples of transverse waves are vibrating strings, ripples on the surface of water, and electromagnetic radiation (light).

Basic Properties of Waves

The image below defines some basic aspects of waves. The vertical axis represents the displacement of the wave from it's equilibrium position, and the horizontal axis is length.

Amplitude is the maximum displacement of the wave.

A wavelength is the distance between two common points on the wave. In the image above I used the amplitude as my two common points. For light, the units of wavelength are typically measured in nanometers, which are 10^-9 of a meter.

The period of a wave is how much time it takes to complete a full wavelength.

The frequency is how many times a wavelength completes a full cycle in one second. It is inversely related to the period by the equation:

where f is the frequency in Hz (1/seconds), and T is the period in seconds. Hz is short for "Hertz." FM radio stations are signals measured in megahertz(MHz), or millions of Hz.

The frequency is also related to the wavelength. If you pick a point on a wave (a crest, for example) and follow it over time, it appears to move with a certain speed. This is called, oddly enough, the wave speed. When surfers catch a wave and glide along top of it, they move at the wave speed. The wave speed depends on the frequency and the wavelength:

here ws is wave speed in m/s, λ is the wavelength in meters, and f is the frequency in Hz. If the wave speed is constant then the frequency and wavelength are inversely related to each other.

One interesting property of waves is that they can interfere with each other. Waves can add together, cancel each other out, or do a combination of the two. See some nice examples of this at http://www.kettering.edu/~drussell/Demos/superposition/superposition.html. Interference is very common. For example, the sound we hear when a musician plays the B note on a violin is actually millions of different sound waves interfering with each other. We hear the sum of waves as the unique sound of the violin.

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