One of the most important diagrams in both science and engineering is the
chart of the EM spectrum which is illustrated in Figure
. The typical EM
spectrum diagram summarizes the ranges of frequencies, or bands that are
important to understanding many things in nature and technology. EM waves can
be classified according to their frequency in Hz or their wavelength in meters.
The EM spectrum has eight major sections, which are presented in order of
increasing frequency and energy, and decreasing wavelength:
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Power waves – These are the slowest of all EM radiation and
therefore also have the lowest energy and the longest wavelength.
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Radio waves – This is the same kind of energy that radio stations
emit into the air for a radio to capture and play. However, other things such
as stars and gases in space also emit radio waves. Many communication functions
use radio waves.
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Microwaves – Microwaves will cook popcorn in just a few minutes. In
space, astronomers use microwaves to learn about the structure of nearby
galaxies.
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Infrared (IR) light – Infrared is often thought of as being the same
thing as heat, because it makes our skin feel warm. In space, IR light maps the
dust between stars.
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Visible light – This is the range that is visible to the human eye.
Visible radiation is emitted by everything from fireflies to light bulbs to
stars. It is also emitted by fast-moving particles hitting other particles.
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Ultra-violet (UV) light – It is well known that the sun is a source
of ultraviolet (UV) radiation. It is the UV rays that cause our skin to burn.
Stars and other hot objects in space emit UV radiation.
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X-rays – A doctor uses X-rays to look at bones and a dentist uses
them to look at teeth. Hot gases in the universe also emit X-rays.
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Gamma rays – Natural and man-made radioactive materials can emit
gamma rays. Big particle accelerators that scientists use to help them
understand what matter is made of can sometimes generate gamma rays. However,
the biggest gamma-ray generator of all is the universe, which makes gamma
radiation in many ways.
The most important range for this course is the RF spectrum. The RF
spectrum includes several frequency bands including microwave and the Ultra
High Frequencies (UHF) and Very High Frequencies (VHF) bands of terrestrial
radio and television broadcasting. This is also where WLANs operate. The RF
spectrum ranges from nine kHz to thousands of GHz. It actually consists of two
major sections of the EM spectrum, radio waves and microwaves. For historical
reasons, many people refer to both sections together as the RF spectrum. The RF
frequencies, which cover a significant portion of the EM radiation spectrum,
are used heavily for communications.
Most of the RF ranges are licensed,
though a few key ranges are unlicensed.