Waves

If you drop a pebble into a still pond, a wave ripples out in all directions across the water. The surface of the water rises and falls in a regular pattern as the wave moves forward. This rising and falling is called a disturbance. The falling pebble has energy that is transferred to the water molecules. These molecules transfer energy to the molecules near them. The energy from the pebble moves outward as the water rises and falls. Therefore, a wave is a disturbance that transfers energy from one place to another. Types of waves include water waves, sound waves, and light waves.

For a water wave, energy is transferred by the water molecules. For sound waves, energy is transferred by the air molecules. A wave that needs a substance to travel or transfer its energy is called a mechanical wave. The substance that the wave travels through is called the medium. A medium can be a solid, liquid, or gas. Not all waves need a medium to transfer energy. Light is a wave that can travel through a vacuum. A wave can cause the particles of the medium to move in two ways, up and down or side to side. These movements produce two different kinds of waves, transverse and longitudinal.

In a transverse wave, the particles of the medium move up and down while the energy moves forward. The wave has a high point called the crest and a low point called the trough. Examples of transverse waves are light and water waves. You can see the crest and trough of the water wave as it approaches the shore.

In a longitudinal wave, the particles of the medium move back and forth horizontally, while the energy moves forward. When the particles are pushed together, it is called compression. When the particles are spread apart, it is called rarefaction. You can observe the motion of a longitudinal wave in a spring. Stretch out the spring and then pinch some of the coils together. When you let go, the energy will move through the spring with the compressions and rarefactions. An example of a longitudinal wave is sound.

Properties of waves include amplitude, wavelength and frequency. Amplitude is a measure of how big the wave is. In a transverse wave, the amplitude is the height of the wave from the rest position to the point of greatest displacement. In a longitudinal wave, amplitude is related to how compressed the particles are in the medium. The more energy a wave has, the higher the amplitude. You can observe amplitude changes in a length of rope. With two people holding the rope, one person can move the rope up and down to produce waves. If the rope is moved faster, the amplitude will increase.

Another property of waves is wavelength. Wavelength is the distance between two corresponding parts of a wave, such as crest to crest distance. Wavelength is measured in units of distance such as mm, cm, and m, among others. The shorter the wavelength of a wave the more energy it has. With two people holding a rope, one person can move the rope up and down. Observe the distance between crests. If the rope is moved faster, the distance between the crests will decrease.

Frequency is the number of complete waves that pass a point in a given time. Frequency is measured in Hertz (Hz). For waves, one Hertz equals one wave per second (1 Hz = 1/s). For example, if 10 waves pass a point in 5 seconds the frequency is 2 Hz. The more energy a wave has the higher its frequency.

The speed of a wave is determined by the wavelength and frequency of the wave. Wave speed equals wavelength times frequency. What will happen when the frequency increases? If a wave has a frequency of 10 Hz and a wavelength of 2 m the speed of the wave will be 20 m/s. If the frequency is increased to 20 Hz with the wavelength of 2 m than the speed will increase to 40 m/s. If the wavelength increases the speed will also increase.