What is a Wave?

We are all familiar with vibrations or oscillations in various forms: a pendulum, a tree swaying in the wind, the string of a guitar -these are all examples of oscillations.

What they have in common is that something, some medium or object, is swinging in a periodic manner, with a certain number of cycles per unit of time. This kind of wave is sometimes called a mechanical wave, since it is defined by the motion of an object or its propagating medium.

When such oscillations travel (that is, when the swinging does not stay bound to one place) then we speak of waves propagating in space. For example, a singer singing creates periodic oscillations in his or her vocal cords. These oscillations periodically compress and decompress the air, and this periodic change of air pressure then leaves the singers mouth and travels, at the speed of sound. A stone plunging into a lake causes a disturbance, which then travels across the lake as a wave.

A wave has a certain speed, frequency, and wavelength. These are connected by a simple relation:

Speed = Frequency * Wavelength

The wavelength (sometimes referred to as lambda, λ) is the distance measured from a point on one wave to the equivalent part of the next, for example from the top of one peak to the next. The frequency is the number of whole waves that pass a fixed point in a period of time. Speed is measured in meters/second, frequency is measured in cycles per second (or Hertz, abbreviated Hz), and wavelength is measured in meters.

For example, if a wave on water travels at one meter per second, and it oscillates five times per second, then each wave will be twenty centimeters long:

1 meter/second = 5 cycles/second * W
W = 1 / 5 meters
W = 0.2 meters = 20 cm

Waves also have a property called amplitude. This is the distance from the center of the wave to the extreme of one of its peaks, and can be thought of as the “height” of a water wave.. The relationship between frequency, wavelength, and amplitude are shown in Figure 2.1.

Waves in water are easy to visualize. Simply drop a stone into the lake and you can see the waves as they move across the water over time. In the case of electromagnetic waves, the part that might be hardest to understand is: “What is it that is oscillating?”

In order to understand that, we need to understand electromagnetic forces.

Figure 2.1: Wavelength, amplitude, and frequency. For this wave, the frequency is 2 cycles per second, or 2 Hz.