Waves

Waves occur in many settings. If a bird drops a stone into the middle of a still pond, waves ripple out in concentric circles. The wave propagates. As the wave propagates, its height diminishes, it attenuates. It loses energy.

Waves occur in many settings. For example, sound propagates in waves of oscillating compression of air or another medium. At the ball game, the crowd may do "the wave." Some fans stand up, then sit down. When they sit, a group next to them stands. When they sit, the fans next to them stand, and so forth. The wave propagates around the stadium until the fans get tired.

Note that when a wave propagates, the medium does not move. In a stadium wave, the fans stand up and sit down, but they are in same seats after the wave passes. When a ripple in a pond passes a floating leaf, the leaf rises then settles back down, but remains in the same spot.

We are interested in waves because they can be used to communicate information. If a bird drops a pebble in a pond, and the wave is strong enough to reach the shore, someone watching may detect it. The bird has sent a message.

Waves can be described in terms of their frequency and amplitude.

These two waves have the same amplitude, but different frequencies. The higher frequency wave has a shorter wavelength (distance between successive peaks).

These two wave have the same frequency, but different amplitudes. The higher amplitude wave is more powerful.

If this were a wave caused by a bird dropping a pebble, the amplitude would depend on the weight of the pebble and the height from which the bird dropped it. If it were a radio wave, it would depend upon the transmitter power.

The unit of measure for frequency is the hertz, named after Heinrich Hertz who discovered radio transmission. One hertz = 1 cycle per second, 100 hertz = 100 cycles per second, etc. Wireless communication occurs at higher frequencies, so we commonly speak of: 


   Kilohertz (1 thousand cycles per second), abbreviated Khz
   Megahertz (1 million cycles per second), abbreviated Mhz
   Gigahertz (1 billion cycles per second), abbreviated Ghz

Propagation speed, frequency and wavelength are related. For example, if a wave propagates at ten feet per second, and has a frequency of ten Hz, the wavelength must be one foot. Sketch the wave and convince your self that this is true.

If the frequency of the wave were increased to twenty Hz, but the propagation speed remained ten feet per second, the wavelength would have to be reduced to half a foot.

Play around with sketches and examples until you are comfortable with the relationships between propagation speed, frequency and wavelength.

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