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Although they seem different, radio
waves, microwaves, x-rays, and even visible light are
all waves of energy called electromagnetic waves. They
are part of the electromagnetic spectrum, and each has a
different range of wavelengths, which cause they waves
to affect matter differently. The electromagnetic waves
have amplitude, wavelength, velocity and frequency. The
creation and detection of the wave depend much on the
range of wavelengths.
Questions you may have include:
-
What is the electromagnetic
spectrum?
-
What are the characteristics of
electromagnetic waves?
-
How are these waves created and
detected?
Electromagnetic spectrum
The range of wavelengths for
electromagnetic waves--from the very long to the very
short--is called the Electromagnetic Spectrum:
-
Radio and TV waves are
the longest usable waves, having a wavelength of 1
mile (1.5 kilometer) or more.
-
Microwaves are used in
telecommunication as well as for cooking food.
-
Infrared waves are barely
visible. They are the deep red rays you get from a
heat lamp.
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Visible light waves are the
radiation you can see with your eyes. Their
wavelengths are in the range of 1/1000 centimeter.
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Ultraviolet rays are what
give you sunburn and are used in "black lights" that
make object glow.
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X-rays go through the body
and are used for medical purposes.
-
Gamma rays are dangerous rays
coming from nuclear reactors and atomic bombs. They
have the shortest wavelength in the electromagnetic
spectrum of about 1/10,000,000 centimeter.
Characteristics of electromagnetic waves
Electromagnetic waves are transverse
waves, similar to water waves in the ocean or the waves
seen on a guitar string. This is as opposed to the
compression waves of sound. As you learned in Wave
Motion, all waves have amplitude, wavelength, velocity
and frequency.
The amplitude of electromagnetic waves
relates to its intensity or brightness (as in the case
of visible light).
With visible light, the brightness is
usually measured in lumens. With other wavelengths the
intensity of the radiation, which is power per unit area
or watts per square meter is used. The square of the
amplitude of a wave is the intensity.
The wavelengths of electromagnetic waves
go from extremely long to extremely short and everything
in between. The wavelengths determine how matter
responds to the electromagnetic wave, and those
characteristics determine the name we give that
particular group of wavelengths.
The velocity of electromagnetic waves in
a vacuum is approximately 186,000 miles per second or
300,000 kilometers per second, the same as the speed of
light. When these waves pass through matter, they slow
down slightly, according to their wavelength.
The frequency of any waveform equals the
velocity divided by the wavelength. The units of
measurement are in cycles per second or Hertz.
Creation and detection
When electrons move, they create a
magnetic field. When electrons move back and forth or
oscillate, their electric and magnetic fields change
together, forming an electromagnetic wave. This
oscillation can come from atoms being heated and thus
moving about rapidly or from alternating current (AC)
electricity.
The opposite effect occurs when an
electromagnetic wave hits matter. In such a case, it
could cause atoms to vibrate, creating heat, or it can
cause electrons to oscillate, depending on the
wavelength of the radiation.
Electromagnetic radiation is emitted
from all matter with a temperature above absolute zero.
Temperature is the measure of the average energy of
vibrating atoms and that vibration causes them to give
off electromagnetic radiation. As the temperature
increases, more radiation and shorter wavelengths of
electromagnetic radiation are emitted.
Microwaves, radio, and television waves
are emitted from electronic devices. Sparks and
alternating current cause vibrations at the appropriate
frequencies.
Visible light is emitted from matter
hotter than about 700 degrees Celsius. This matter is
said to be incandescent. The sun, a fire, and the
ordinary light bulb are incandescent sources of light.
As the element in an electric stove gets
warms, it gives off infrared radiation, and then when it
gets hotter than 700 degrees, it starts to glow. Visible
light is being emitted from the hot element.
By smashing high-energy electrons into
other particles, such as atoms in a metal, X-rays are
created.
Gamma rays are emitted from nuclear
reactions, atomic bombs, and explosions on the Sun and
other stars.
There are a number of different types of
detectors of electromagnetic radiation. We know the
common ones for detecting visible light: the eye, camera
film, and the detectors on some calculators. Your skin
can also detect both visible light and infrared heat
rays.
Electronic devices are necessary to
detect most of the longer waves, such as radio waves.
Special film can detect shorter wavelengths such as
X-rays
Electromagnetic waves are transverse
waves that travel at the speed of light. The spectrum of
their wavelengths results in waves that are used in many
of our useful devices. The waves are formed by heat,
electronics, and nuclear forces. |
