Properties of Waves
Introduction

To describe a wave, you need to know a few of its properties.

One property is its wavelength. For a sound wave, this corresponds to how high or how low a note sounds. For a light wave, it corresponds to the colour of light.

Another property is the amplitude, which corresponds to loudness for a sound wave or brightness for a light wave.
The wavelength of a wave
One of the basic characteristics of a wave is its wavelength. This is the distance from one peak, or crest, to the next. This is the same as the distance from one dip, or trough, to the next.

Figure 1.   A diagram showing wavelength.
Display as fullscreen


All the arrows on Fig.1 indicate the
wavelength
The wavelength of a wave is the distance from one peak to the next, or from one trough to the next.
wavelength
of the wave.

Select the sentence that best describes the wavelength of each of the following waves.
  • Click here to mark the question


The Greek letter λ (lambda) is used for the wavelength of a wave.

This is how waves with the same wavelength would appear in a ripple tank:

Figure 2.   A ripple tank. The wavelength of the wave is indicated.
Display as fullscreen


The black lines shown in the diagram above are called wavefronts, and represent either all the crests or all the troughs in a wave.

The amplitude of a wave
The volume of sound, the
energy
Energy is the capacity to do work. The SI unit of energy is the joule (J).
energy
in a water wave, and the brightness of light all depend upon the amplitude of the wave. The arrows on the wave in Fig.3 below indicate the
amplitude
In general, the amplitude of a quantity is a measure of its size. When talking about waves, the amplitude of a wave is the maximum height of a wave measured from its rest position.
amplitude
of the water wave. It is equal to the height of the crest of the wave above the rest position. This is the same as the depth of a trough of a wave below the rest position.

Figure 3.   A diagram showing the amplitude of a wave.
Display as fullscreen


The amount of energy in a wave is proportional to its amplitude. The larger the amplitude, the more energy it carries, and the more dangerous it can be. For example, low-energy sound is perfectly safe. However, prolonged exposure to high-intensity sounds can cause loss of hearing and even deafness.

Select the sentence that best describes the energy of each of the following waves.
  • Click here to mark the question


You cannot really tell the amplitude of a wave in a ripple tank. If the amplitude is too large, the water may splash out of the tank!

Wave period and frequency
'Keep your radio tuned to 102.2 FM for all the latest and greatest hits.' Have you ever wondered what the DJ means by 102.2? It is the
frequency
In general, the frequency of an event describes how often it occurs. When talking about waves, the frequency is a measure of how many waves go past a fixed point in a given time.
frequency
of the radio waves. The frequency of a wave has the same meaning as the frequency of anything else. It means how often something happens in a given time. For example, three buses an hour might be the frequency of buses on one route.

With waves, frequency is how many peaks go past in a given time, usually one second.

Figure 4.   A moving wave.
Display as fullscreen


Look at the waves in Fig.4 above.

How many waves go past any one point in 10 seconds?
  • Click here to mark the question
Does it matter at which point you count the waves?
  • Click here to mark the question


Divide the number of waves by the time you counted for. This will give you the frequency of the waves.

The unit of frequency is the hertz (Hz). One hertz is equal to one particular event per second. In this example, it is one wave per second. If you were counting buses, one hertz would mean one bus per second.

What is the frequency of the waves you have just studied?
  •  Hz
  • Click here to mark the question


The period of a wave is the time between one crest and the next appearing (this is also true for troughs). Can you use the clock to time the period of the waves above?

It is easier to use the equation below, which relates the frequency f to the period


Click on the figure below to interact with the model.

 Figure 5.  A flashing light.



We can work out the period of the light from its frequency.

The light flashes on and off with a frequency of 4 Hz.
The period, T, of the flash is found by dividing 1 by the frequency.
     The period is equal to 1/4, or 0.25 seconds.

Summary


A wave has a wavelength, an amplitude, a frequency, and a period.

The wavelength is the distance from one peak to the next, or from one trough to the next.

The amplitude is the maximum height of a wave from its rest position.

The frequency is the number of waves per second.

The period and the frequency are related by the equation:

Exercises
1. On Fig.6 below, which letter corresponds to the wavelength?
  • Click here to mark the question
Figure 6.  
Display as fullscreen
2. On Fig.6 above, which letter corresponds to the amplitude?
  • Click here to mark the question
3. Select the correct terms to complete the sentences.
  • A wave with low energy has a...
    A wave with high energy has a...
    A wave with a high frequency has a...
    A wave with a low frequency has a...
  • Click here to mark the question
4. You are at the beach, counting the waves as they hit the shore. In 2 minutes you count 15 waves. What is the period and frequency of these waves?

  • Period: s
    Frequency: Hz

  • Click here to mark the question
Figure 7.  
Display as fullscreen
5. What are the amplitude and wavelength of the
transverse wave
In a transverse wave, the material moves at right angles to the direction of motion of the wave. Ripples on a pond are an example of transverse waves.
transverse wave
shown in Fig.7 above?

  • Amplitude: cm
    Wavelength: cm

  • Click here to mark the question
Figure 8.  
Display as fullscreen
6. What are the amplitude and wavelength of the transverse wave shown in Fig.8 above?

  • Amplitude: cm
    Wavelength: cm

  • Click here to mark the question
Well done!
Try again!