properties of waves

What is a wave?

We use the word wave in everyday conversation to refer to ocean, light, sound, or earthquake waves. But what do all of these seemingly different phenomena have in common, and why is it important to understand the nature of waves? Let's explore these topics.

Waves transmit the energy that topples buildings during an earthquake, energy that allows us to communicate in the modern world, and energy that allows for life on earth at all. Our observations of the earth from space are also dependent on waves, those that are received by satellites. Thus, waves are a basic feature of the natural world and our ability to understand waves has resulted in many useful devices, cell phones, garage door openers, and microwave ovens, to name a few. With such a variety, what do all waves have in common? Ocean, light, sound, and earthquake waves share the characteristics contained in the scientific definition of wave.

Definition The Random House dictionary tells us that a wave is: Physics. a progressive disturbance propagated from point to point in a medium or space without progress or advance by the points themselves, as in the transmission of sound or light.It's a wave if: 1) energy moves from one place to another and 2) matter doesn't move from one place to another, for the most part.

For example, ocean waves ceaselessly arrive at the shore without piling up infinite amounts of water. The wave arrives, but the water doesn't.

We know that ocean waves carry energy because they are able to beat up and move objects at the shore. It takes a wave the same amount of energy to move a large boulder as it would for us to do the same, manually or with a bulldozer.

In understanding the earth, it's useful to concentrate on two general classes of waves, mechanical and electromagnetic waves.

Mechanical waves
Common types of mechanical waves include sound or acoustic waves, ocean waves, and earthquake or seismic waves. In order for compressional waves to propagate, there must be a medium, i.e. matter must exist in the intervening space. For our purposes, we use the term matter to mean that atoms must exist in the intervening space. To learn more about different types of mechanical waves such as earthquake waves, link to our module on Mechanical Waves.

Electromagnetic waves
Common types of electromagnetic waves include visible light, infrared, and ultraviolet radiation, among others. The transmission of electromagnetic waves does not require a medium and electromagnetic waves are able to travel through vacuums. Unlike mechanical waves such as sound, electromagnetic waves can travel successfully across the near emptiness of outer space. Thus humanity has been entertained for eons by the stars that light night skies. To learn more about different types of electromagnetic waves such as ultraviolet radiation, link to our module on Electromagnetic Waves.

Wavelength
Wavelength is the distance between two consecutive and equivalent points on a wave. Wavelength can be quantified by measuring the distance between two equivalent and consecutive points, such as the distance between two peaks or two troughs. The scientific symbol for wavelength is a Greek letter called lambda.
Watch the animation to see examples of wavelength.

Natural waves come in many different wavelengths, covering a vast range relative to human senses of scale. Gamma rays which are a form of electromagnetic radiation have wavelengths as short as one trillionth of a meter. Tsunami waves can have wavelengths greater than 100 miles (161 km).

Amplitude
Amplitude is a measurement of the vertical distance of the wave from the average. The wave axis is the average height of the wave over one cycle, and is usually considered to be zero. Heights above and below the average are given positive and negative values, respectively.
The wave shape shown below is called a sine wave. The maximum and minimum amplitudes of this sine wave are the heights measured from the wave axis (white line) to the top of the wave peak and bottom of the wave trough. Everywhere else along the wave, amplitudes are less than the maximum.
Watch the animation to see examples of amplitude.

Bigger ocean waves have larger wave heights or amplitude. The amplitude of typical ocean waves is between 2 and 8 feet.

Frequency
Frequency is a measurement of how often a recurring event such as a wave occurs in a measured amount of time. One completion of the repeating pattern is called a cycle. Only moving waves which vary their positions with respect to time possess frequency. Frequency is one way to define how fast a wave moves.
Waves can move in two ways. The frequencies of progressive waves or those that move forward indicate how fast a wave moves forward in units of cycles per unit time. The frequencies of standing waves or those that oscillate in place are the rate of oscillation in units of cycles per unit time.

Definition The dictionary definition of frequency is:Physics. a) the number of periods or regularly occurring events of any given kind in a unit of time, usually one second. b) the number of cycles or completed alternations per unit time of a wave or oscillation. Symbol: f; Abbr.: freq.

Units
Frequency is expressed in units of cycles per unit time.

Although frequency is a measurement of rate of movement, it is not identical to velocity. For example, if we think of a car that moves at 60 miles per hour, we mean just that. However, if we say that a wave has a frequency of 60 cycles per hour, points on the wave may be traveling faster or slower depending on wavelength. Comparing two waves of the same wavelength, a higher frequency is associated with faster movement. Comparing two waves of different wavelengths, a higher frequency doesn't always indicate faster movement, although it can. Waves of different wavelengths can have the same frequency. For some purposes, the measurement frequency is more useful than absolute velocity.
The unit, Hertz
The unit Hertz (Hz) is used to describe frequency in cycles per second. In a sentence the proper format for writing this relationship is:

One cycle represents the movement of one wavelength.
Radio call numbers
Often you might hear radio frequencies given in Megahertz (MHz) ... (under construction).
Wave period
Wave frequency is also related to another measurement called the wave period (T). The wave period is just how much time it takes for one cycle to pass and the units are always in terms of time. The faster a wave moves, its wave period becomes smaller.
Instead of measuring in terms of a fixed time unit, the second, the wave period uses a fixed number of cycles, one cycle ...

How do you measure wave period?
Wave period can be determined by measuring how much time it takes two peaks to pass a certain point. You can do this for ocean waves by standing on a pier and using a stop watch.