18.2 The Electromagnetic Spectrum
William Herschel
measured the
temperature of
different colors of light.
The temperature was
lowest at the blue end
and highest at the red
end. Curiosity led
Herschel to discover
evidence of radiation
past the red end of the
band of visible light.
18.2 The Electromagnetic Spectrum
II. The Electromagnetic Spectrum- The full range of frequencies of
electromagnetic radiation.
A. The Wave of the Spectrum
1. The electromagnetic spectrum includes radio waves,
infrared rays, visible light, ultraviolet rays, X-rays, and
gamma rays
2. Visible light is the only part of the electromagnetic
spectrum that you can see, but it is just a small part.
3. Each kind of wave is characterized by a range of
wavelengths and frequencies. All of these waves have
many useful applications.
18.2 The Electromagnetic Spectrum
The Waves of the Spectrum
The electromagnetic spectrum consists of
radio waves, infrared rays, visible light,
ultraviolet rays, X-rays, and gamma rays.
18.2 The Electromagnetic Spectrum
B. Radio waves are used in radio and television technologies, as
well as in microwave ovens and radar.
1. Radio waves have the longest wavelengths in the
electromagnetic spectrum.
2. Radio waves also have the lowest frequencies in the
spectrum—300,000 megahertz (MHz) or less.
18.2 The Electromagnetic Spectrum
3. Radio
a. In a radio studio, sound is changed into electronic
signals that are coded onto radio waves and then
broadcast.
b. Your radio receives the radio signal, decodes it, and
changes it back into sound waves you can hear.
18.2 The Electromagnetic Spectrum
c. There are two ways that signals are encoded for radio.
1) amplitude modulation-the amplitude of the wave is
varied. The frequency remains the same.
a) AM radio stations broadcast by amplitude
modulation.
2) frequency modulation- the frequency of the wave is
varied. The amplitude remains the same.
a) FM stations broadcast by frequency modulation.
18.2 The Electromagnetic Spectrum
Radio Waves
The announcer’s voice and the music on CD
leave the radio studio as electronic signals.
Those signals are used to produce a wave
with either a varying amplitude or a varying
frequency.
A. AM waves have a varying amplitude.
B. FM waves have a varying frequency.
Amplitude modulation
Frequency modulation
18.2 The Electromagnetic Spectrum
d. FM radio signals do not travel as far as AM signals
along Earth’s curved surface.
1) Particles in Earth’s upper atmosphere reflect the
lower-frequency AM radio waves much better than
the FM radio waves. The reflection helps transmit
AM signals farther.
18.2 The Electromagnetic Spectrum
4. Television
a. Radio waves also carry signals for television
programming.
1) The radio waves carry information for pictures as
well as for sound.
b. Location and weather can affect the reception of
television signals by an antenna.
18.2 The Electromagnetic Spectrum
c. With satellite transmission, TV broadcasts are sent to
satellites, which then retransmit the signals back to
Earth.
1) Signals can go to home satellite receivers or to cable
services that receive the signals and send them
through cables.
18.2 The Electromagnetic Spectrum
5. Microwaves
a. The shortest-wavelength radio waves are called
microwaves. Microwave wavelengths are from about 1
m to about 1 mm.
b. Microwaves cook and reheat food.
c. Microwaves also carry cell phone conversations.
1) The process works much like a radio broadcast.
18.2 The Electromagnetic Spectrum
6. Radar
a. Radar technology uses a radio transmitter to send out
short bursts of radio waves.
1) The waves reflect off the objects they encounter
and bounce back toward where they came from.
2) The returning waves are then picked up by a radio
receiver.
18.2 The Electromagnetic Spectrum
Radio Waves
A speed-monitoring trailer uses radar to
measure the speed of an approaching car.
18.2 The Electromagnetic Spectrum
C. Infrared rays are used as a source of heat and to discover
areas of heat differences.
1. Infrared rays have higher frequencies than radio waves
and lower frequencies than red light.
a. Infrared wavelengths vary from about 1 millimeter to
about 750 nanometers (10–9 meter).
b. Your skin senses infrared radiation as warmth.
c. Restaurants use infrared lamps to keep foods warm.
18.2 The Electromagnetic Spectrum
d. Warmer objects give off more infrared radiation than
cooler objects.
e. A device called a thermograph uses infrared sensors to
create thermograms- color-coded pictures that show
variations in temperature.
1) Search-and-rescue teams use infrared cameras to
locate people who are trapped during disasters.
18.2 The Electromagnetic Spectrum
Infrared Rays
A thermogram can be used to diagnose problems in a utility line.
A. When viewed in visible light, the wires all look the same.
B. The colors in the thermogram image show that the
electric current in the center wire is not flowing as it
should.
18.2 The Electromagnetic Spectrum
D. People use visible light to see, to help keep them safe, and to
communicate with one another.
1. The visible part of the electromagnetic spectrum is light
that the human eye can see.
2. Each wavelength in the visible spectrum corresponds to a
specific frequency and has a particular color.
18.2 The Electromagnetic Spectrum
Visible Light
Each color of light
corresponds to a
different range of
wavelengths. The
wavelengths of visible
light are quite small.
Wavelengths of red
light, for example, are
about one hundredth
the thickness of a
human hair.
18.2 The Electromagnetic Spectrum
E. Ultraviolet rays have applications in health and medicine, and
in agriculture.
1. Ultraviolet rays vary from about 400 nm to about 4 nm.
2. Some exposure to ultraviolet rays helps your skin produce
vitamin D, which helps the body absorb calcium from
foods.
3. Excessive exposure can cause sunburn, wrinkles, skin
cancer, and eye damage.
4. Ultraviolet rays are used to kill microorganisms. In winter,
plant nurseries use ultraviolet lights to help plants grow.
18.2 The Electromagnetic Spectrum
F. X-rays are used in medicine, industry, and transportation to
make pictures of the inside of solid objects.
1. X-rays have very short wavelengths, from about 12 nm to
about 0.005 nm.
2. X-rays have high energy and can penetrate matter that
light cannot.
3. Too much exposure to X-rays can kill or damage living
tissue.
18.2 The Electromagnetic Spectrum
4. Your teeth and bones absorb X-rays. X-ray photographs
show softer tissue as dark, highly exposed areas. Bones and
teeth appear white.
5. The lids on aluminum cans are sometimes inspected with
X-rays to make sure they are sealed properly.
6. X-rays can be used to identify the contents of entire truck
trailers.
18.2 The Electromagnetic Spectrum
X-Rays
Airport security
screeners use Xrays to search
baggage for
potentially
dangerous
objects.
18.2 The Electromagnetic Spectrum
Gamma Rays
Gamma rays emitted
by radioactive tracers
in the brain are used
to produce colorcoded images. Areas
of high activity show
up in red.
18.2 The Electromagnetic Spectrum
G. Gamma rays are used in the medical field to kill cancer cells
and make pictures of the brain, and in industrial situations as
an inspection tool.
1. Gamma rays have the shortest wavelengths in the
electromagnetic spectrum, about 0.005 nm or less.
2. They have the highest frequencies, the most energy, and
the greatest penetrating ability of all the electromagnetic
waves.
3. Exposure to tiny amounts of gamma rays is tolerable, but
overexposure can be deadly.
18.2 The Electromagnetic Spectrum
4. Gamma rays are used in radiation therapy to kill cancer
cells without harming nearby healthy cells.
5. Gamma rays are also used to make pictures of the human
brain, with different levels of brain activity represented by
different colors.
6. Pipelines are checked with machines that travel on the
inside of a pipe, taking gamma ray pictures along the entire
length.
18.2 The Electromagnetic Spectrum
Assessment Questions
1. Which waves have the longest wavelength?
a.
b.
c.
d.
radio waves
infrared rays
visible light
ultraviolet rays
18.2 The Electromagnetic Spectrum
Assessment Questions
1. Which waves have the longest wavelength?
a.
b.
c.
d.
radio waves
infrared rays
visible light
ultraviolet rays
ANS: A
18.2 The Electromagnetic Spectrum
Assessment Questions
2. What type of electromagnetic radiation is used to
keep prepared foods warm in a serving area?
a.
b.
c.
d.
ultraviolet rays
infrared rays
X-rays
gamma rays
18.2 The Electromagnetic Spectrum
Assessment Questions
2. What type of electromagnetic radiation is used to
keep prepared foods warm in a serving area?
a.
b.
c.
d.
ultraviolet rays
infrared rays
X-rays
gamma rays
ANS: B