Light is a form of electromagnetic radiation.
Other forms of electromagnetic radiation include radio waves,
microwaves, infrared radiation, ultraviolet rays, X-rays, and
gamma rays. All of these, known collectively as the
electromagnetic spectrum, are fundamentally similar in that they
move at 186,000 miles per second, the speed of light. The only
difference between them is their wavelength,
which is directly related to the amount of energy the waves
carry. The shorter the wavelength of the radiation, the higher
the energy.
The rainbow of colors that we see in visible light represents only a very small portion of the electromagnetic spectrum. On one end of the spectrum are radio waves with wavelengths billions of times longer than those of visible light. On the other end of the spectrum are gamma rays. These have wavelengths millions of times smaller than those of visible light. The following are the basic categories of the electromagnetic spectrum, from longest to shortest wavelength:
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Radio waves are used
to transmit radio and television signals. Radio waves
have wavelengths that range from less than a centimeter
to tens or even hundreds of meters. FM radio waves are
shorter than AM radio waves. For example, an FM radio
station at 100 on the radio dial (100 megahertz) would
have a wavelength of about three meters. An AM station at
750 on the dial (750 kilohertz) uses a wavelength of
about 400 meters. Radio waves can also be used to create
images. Radio waves with wavelengths of a few centimeters
can be transmitted from a satellite or airplane antenna.
The reflected waves can be used to form an image of the
ground in complete darkness or through clouds. Microwave
wavelengths range from approximately one millimeter (the
thickness of a pencil lead) to thirty centimeters (about
twelve inches). In a microwave oven, the radio waves
generated are tuned to frequencies that can be absorbed
by the food. The food absorbs the energy and gets warmer.
The dish holding the food doesn't absorb a significant
amount of energy and stays much cooler. Microwaves are
emitted from the Earth, from objects such as cars and
planes, and from the atmosphere. These microwaves can be
detected to give information, such as the temperature of
the object that emitted the microwaves. Infrared is the region
of the electromagnetic spectrum that extends from the
visible region to about one millimeter (in wavelength).
Infrared waves include thermal radiation. For example,
burning charcoal may not give off light, but it does emit
infrared radiation which is felt as heat. Infrared
radiation can be measured using electronic detectors and
has applications in medicine and in finding heat leaks
from houses. Infrared images obtained by sensors in
satellites and airplanes can yield important information
on the health of crops and can help us see forest fires
even when they are enveloped in an opaque curtain of
smoke. The rainbow of colors we know
as visible light
is the portion of the electromagnetic spectrum with
wavelengths between 400 and 700 billionths of a meter
(400 to 700 nanometers). It is the part of the
electromagnetic spectrum that we see, and coincides with
the wavelength of greatest intensity of sunlight. Visible
waves have great utility for the remote sensing of
vegetation and for the identification of different
objects by their visible colors. Ultraviolet radiation
has a range of wavelengths from 400 billionths of a meter
to about 10 billionths of a meter. Sunlight contains
ultraviolet waves which can burn your skin. Most of these
are blocked by ozone in the Earth's upper atmosphere. A
small dose of ultraviolet radiation is beneficial to
humans, but larger doses cause skin cancer and cataracts.
Ultraviolet wavelengths are used extensively in
astronomical observatories. Some remote sensing
observations of the Earth are also concerned with the
measurement of ozone. X-rays are high energy
waves which have great penetrating power and are used
extensively in medical applications and in inspecting
welds. X-ray images of our Sun can yield important clues
to solar flares and other changes on our Sun that can
affect space weather. The wavelength range is from about
ten billionths of a meter to about 10 trillionths of a
meter. Gamma rays have
wavelengths of less than about ten trillionths of a
meter. They are more penetrating than X-rays. Gamma rays
are generated by radioactive atoms and in nuclear
explosions, and are used in many medical applications.
Images of our universe taken in gamma rays have yielded
important information on the life and death of stars, and
other violent processes in the universe.
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