|
|
EM Basics Applet: Want to see and hear a sine wave? Click on the above image.
To view the applet you must have Java enabled in your browser and have a Java runtime environment installed on your computer. These are available for free by clicking here.
|
|
EM Wave Basics
Although we can't actually see them, we often represent EM waves as sine waves like the one shown below. Their wavelength is measured from peak to peak or from trough to trough (low point to low point). Their frequency is a measure of how many peaks would pass a fixed point in a given length of time. For example, with a frequency of 1.0 gigahertz (GHz), 1,000,000,000 peaks will pass a fixed point in a second.
Amplitude indicates a wave's intensity or brightness and is measured from a wave's center to its lowest or highest point.
Each wavelength has a specific frequency determined as follows:
|
 |
Human Wireless Communication
Much of the EM spectrum with wavelengths longer than visible light is used for human communications. For example, infrared is used for TV remote controls, microwaves for cell phones, radio waves for (you guessed it) radio, etc.
Waves in this section of the spectrum are not considered ionizing and at low levels of intensity they're not considered harmful. However, at high levels such as those found in microwave ovens, they can cause burns. The extremely high levels of infrared given off by nuclear bomb blasts can actually vaporize objects including people. |
Frequency = (Speed of Light) / (Wave Length)
To better understand this relationship click on the applet image at right and try adjusting the sine wave. You guessed it; the light blue shape in the applet is a wave length vs. frequency plot just like the light blue area at the left side of this page.
Sounds can also be modeled as a combination of different sine waves. The frequency of a sound wave is related to its wavelength just like EM waves except that the speed of sound is substituted for the speed of light. How does a sound wave compare to an EM wave? First, it's a mechanical wave and can only be transmitted through matter, Em waves need no matter. Second, sound waves are about a million times slower than EM waves. An easily heard sound of say 1000 Hz would have a wave length of 34 centimeters. An EM wave of the same frequency would have a wave length of 300 kilometers or 186 miles! By contrast, the longest wave length used in AM radio broadcast is about 0.56 kilometers.
You can listen to sine waves of various frequencies using the applet at right. A pure sine wave sounds irritating because you rarely hear one. Almost all sounds are a combination of many sine waves and together they usually sound much better. You'll notice that the shorter waves sound louder but that's because the human ear is more sensitive to them. For a given wave length, wave amplitude is a measure of loudness.
Typically broadcasts, such as those sent from TV stations, are carried on a specific EM wavelength, which acts like its voice. The electronic circuits in the transmitter cause electrons to move up and down in the antenna. This motion of a charged particle generates the electromagnetic wave. A receiver has to be tuned to the same wavelength in order to select the broadcast from all the others being transmitted. For more details on how this is done click here.
