Properties of Waves
What are Waves
A wave is a transfer of energy in the form of a disturbance, usually
through a material substance or medium. Most energies are transfered by waves. The transference of both light eneergy and heat energy is by an electromagnetic wave requiring no medium of transference. Sound, on the other hand needs a medium to travel.
We live surrounded by waves. Some waves are
visible and some are not. By observing waves that are visible we can discover
some characteristics that all waves, including invisible ones, have in common.
Vibrations
When we describe the motion of a vibrating object, we call one complete
oscillation a cycle. The number of cycles per second is called the
frequency.
If the unit of time is a second, then we call one cycle per second a hertz,
Hz: 1 cps = 1 Hz. The unit used to measure frequency is named after the German
scientist, Heinrich Hertz (1857-1894), the discoverer of electromagnetic
waves. The period of vibration is the time taken for one full cycle. Period is
represented by T. Note: Frequency and period are reciprocals, i.e.
The distance from rest to maximum position in either direction is called the amplitude.
Objects are said to be in phase if the displacement is equal distance from rest, and moving in the same direction. Objects that are not in phase are said to be out of phase.
Wave Motion
Periodic waves are the motions of a wave that are repeated at regular time intervals. But a wave can also consist of a single disturbance called a pulse, or shock wave.
To the left is an oscilloscope tracing of the sound.
In a transverse wave the particles of the medium vibrate at right angles to the direction in which the wave travels. The high section of a wave is called a crest and the low section is called a trough. The crest is sometimes referred to as a positive pulse since it lies above the rest point. The trough is referred to as the negative pulse since it lies below the rest point.
A wavelength is the distance between two successive particles vibrating in phase. For example a wavelength is the distance from the midpoint of one crest to the midpoint of the next crest or from the midpoint of one trough to the midpoint of the next trough. The wavelength is represented by the Greek letter lambda, l.
The following is an example of a sine wave.
In a longitudinal wave the particles of the medium vibrate parallel to the
direction in which the wave travels. In a longitudinal wave, the regions where
the particles are closer together are called compressions. The regions
where they are farther apart are called rarefactions. In longitudinal waves,
one wavelength is the distance between the midpoints of successive compressions
or rarefactions. The maximum displacement of the particles from rest position
is the amplitude of the longitudinal wave.
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Notes to be Made
Note: Mathematically both types of waves can be represented by the same function: a sine or cosine function.
What is the wavelenth of this train?
If it takes 20 s for the entire wave train to pass, what is the period of these waves.