What is a wave? Most of us think of the waves we see at the beach. The waves come in from out in the ocean and crash against the shore. If we consider how the water is actually moving in a wave, the situation becomes quite interesting. Although the water is in motion, the wave is travelling across the water, and if we could examine a small drop of water, we would find the water is actually moving mostly up and down.
Water waves are quite complicated, so let’s begin by examining waves in a simpler system – a slinky.
Transverse and longitudinal waves
There are two ways to make waves on a slinky. The first is called a transverse wave. In a transverse wave, the material of the slinky moves in one direction (up and down) while the wave travels along the slinky in another direction. The second type of wave is called a longitudinal wave. In a longitudinal wave, the slinky moves back and forth, and the wave moves in the same direction as the slinky is moving.
This video shows examples of both waves. Watch a particular point on the slinky as each type of wave passes.
This video shows a series of waves moving along the slinky.
Illustrating transverse and longitudinal waves
Most illustrations of waves show transverse waves, primarily because they are easier to visualise.
Since longitudinal waves are difficult to draw, most waves are drawn as transverse waves. The high points of the waves are the peaks and represent more dense areas, and the low points are troughs, representing less dense areas.
Wavelength, frequency and amplitude
The three terms used when describing a wave are wavelength, amplitude and frequency.
Wavelength is the length of one wave. Waves come in all different lengths. Sound waves vary in length from 2 centimetres (a very high note) to nearly 20 metres (a very low note). Light waves vary in length from 0.0004 millimetres (violet) to 0.0007 millimetres (red).
Amplitude describes the height of waves from the starting position to the top of the wave. In the case of a transverse wave, it describes how much the material of the wave is compressed or expanded. The amplitude of a sound wave is its volume or loudness. The amplitude of a light wave is its brightness.
Frequency is how many waves pass a certain point in one second, measured in hertz (Hz). In music, middle C is about 262 Hz. This means that, if we could see the waves passing when someone was playing middle C, we would see about 262 waves pass our eyes in each second. The wavelength of middle C is about 132 centimetres. Human hearing ranges from approximately 20 Hz to 20,000 Hz, depending on age.
The frequency of a sound is commonly referred to as its pitch. High-frequency notes are high notes, and low frequency notes are low notes. Frequency in light describes the colour of the light. Red light has a relatively low frequency, and violet light has a high frequency.
The article Waves and energy – energy transfer discusses how waves transmit energy.
Nature of science
There are many words – like wave – that have an everyday meaning and a scientific meaning. Discussion about the different meanings helps to avoid confusion and alternative conceptions. Developing the knowledge of scientific vocabulary is part of the ‘Communicating in science’ achievement aim.
In the activity Modelling waves with slinkies, students model how sound travels by sending waves along two stretched plastic slinkies tied together.
In the activity Sound on an oscilloscope, students play different kinds of sounds near a computer microphone and watch the resulting visual display created by oscilloscope software.