Chapter 16
Types of Waves
ALABAMA 8TH GRADE SCIENCE STANDARDS COVERED IN THIS CHAPTER INCLUDE:
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Classify waves as mechanical or electromagnetic.
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Describing how earthquake waves, sound waves, water waves and
electromagnetic waves can be destructive or beneficial due to the
transfer of energy
Describing the electromagnetic spectrum in terms of frequencies
Example: electromagnetic spectrum in increasing
frequencies microwaves, infrared light,
visible light, ultraviolet light, X-rays
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SOUND WAVES
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A sound wave is a mechanical wave produced by a vibrating
object. When the object vibrates, it causes the molecules around it
to compress (come together) and expand (separate back out). You
learned in the previous chapter that this is called a longitudinal
wave. This compression and expansion pattern travels through the
medium (solid, liquid or gas) until it reaches your ear. Remember,
sound cannot travel through empty space or a vacuum. Sound
travels faster through solids than through liquids and gases. Can
you test this? You know the answer must be yes or we wouldn’t Figure 16.1 Speaker
have asked! Stand at
ends ofthe outside ofa brick building. Have a friend gently tap
a rhythm on a brick on his or her end. First listen and see if you can hear it in the air.
Then put your ear to a brick on your side. You should hear the sound loud and clear when
your ear is to the brick. .because sound really does travel faster through a solid! Both
temperature and humidity can affect the speed of sound waves through the air.
Temperature As the temperature increases, the speed of sound increases. The
motion in the atoms that make up air molecules is faster at hotter
temperatures, so it should make sense that sound would move faster.
Humidity
When the air becomes more humid, the speed of sound increases.
Why? Humidity is moisture being carried in the air, which makes the
air thicker. We already know that sound travels faster through a liquid
or solid, so it makes sense that the thicker (and more like a solid) air
becomes, the faster sound will travel through it.
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Types of Waves
VOLUME AND PITCH
What makes a sound louder or softer? Think about it this way; the amplitude is really the
wave’s intensity the amount of energy the wave is carrying. The higher the amplitude, the
more energy being carried and the louder, more intense the sound. The intensity, or volume of
a sound is measured in units called decibels. The higher amplitudes are louder and have a
higher number of decibels. Lower amplitudes produce softer sounds with a lower number of
decibels.
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Relationship Between Volume and Pitch
high pitch:
low pitch:
long wavelength
low frequency
short wavelength
high frequency
Figure 16.2 Volume Pitch Relationship
THE ELECTROMAGNETIC SPECTRUM
The entire range of electromagnetic waves is called the electromagnetic spectrum. Waves in
the electromagnetic spectrum include radio waves, microwaves and visible light, among others.
Figure 16.3 arranges the electromagnetic spectrum from long wavelengths to short wavelengths
(from right to left). Another way to look at this is that the waves are arranged from low
frequency to high frequency.
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Figure 16.3 The Electromagnetic Spectrum
Now let’s talk about each of these waves starting with microwaves.
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Microwaves: These invisible waves have a wavelength of only a
few millimeters. Your microwave oven uses these waves to heat
food. The microwaves generated by the oven’s instruments cause
the water molecules in the food to vibrate and rotate. It is the
movement ofthe water molecules that heats the food!
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Figure 16.4 Microwave
Infrared Waves: These wavelengths are
up to a few micrometers in length. These waves are invisible but you
can feel them as heat from the Sun. Your television’s remote control
uses a beam of infrared light to change the channel. The electronics in
the TV respond to the infrared beam. Your body also radiates infrared
light, but of a slightly different wavelength. That is how night vision
goggles can see living things moving in the dark.
Visible Light Waves: This is the only
Wavelength fnm)
part of the electromagnetic spectrum
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that our eyes can see. It’s the kind of
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very narrow band ofwavelengths, from
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different wavelengths in this band as
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order that the colors are arranged is
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Figure 16.6 Roy G Biv
name is the first letter of a color in the
visible light range. This is illustrated for you in Figure 16.6.
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Ultraviolet Waves: The ozone layer in our Earth’s atmosphere helps to
protect us from most of the harmful effects of these short wavelength
SPF 15
waves. Only some of the Sun’s ultraviolet light reaches the ground, and
those waves can cause sunburn or even worse, skin cancer. Ultraviolet
light penetrates the skin, interacts with molecules and can tear them
apart.
This is why it is important to use sunscreen!
Figure 16.7 Sunscreen
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BLOCKER
Blocks UV Light
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Types of Waves
X-rays: These waves have wavelengths in the nanometer range. They are
often used to image bones in a doctor’s office. X-rays can be dangerous,
so it is best to minimize your exposure to them. That is why the X-ray
technician or radiographer covers the parts of your body that are not
being X-rayed with a lead apron. This absorbs the radiation.
BENEFITS AND RISKS OF VARIOUS WAVES
Figure 16.8 X-ray
The energy transmitted by mechanical and electromagnetic waves effects objects as well as
people. Some effects are beneficial to humans, while others can pose risks to health and safety.
Waves that carry large amounts of energy can produce intense vibrations and cause extensive
damage. Let’s look at some examples of different types of waves and their potential risks and
benefits.
Sound waves that carry high energy can be harmful to humans. Loud sounds above 90 decibels
can cause permanent hearing loss. This sound level may be reached with live rock concerts,
power saws or by ajet plane taking off Hearing loss can occur at decibels lower than 90 when
exposure occurs over a prolonged or repeated period of time.
Other types ofmechanical waves can carry large amounts of energy and
be highly destructive. Two examples are earthquake waves and water
waves. Earthquake (seismic) waves are waves that move through the
Earth, usually as a result of tectonic plate movements or possibly
explosions. Earthquake waves are surface waves and cause much of the
damage associated with earthquakes. They can travel many miles from
where the earthquake originated. As they travel through the Earth, the
energy in the wave interacts with matter (in this case a solid) and loses
some of its energy. The farther the waves travel from their origin, the
more energy they lose and the less damage they cause.
Figure 16.9 Earthquake
Damage
Earthquakes that originate near an
ocean or at the bottom of an ocean can produce a tsunami.
Tsunamis are water waves with immense energy and power.
They form in the deep ocean with large wavelengths. As the
wave of a tsunami moves closer to shore into shallower
water, its wavelength decreases but its amplitude increases
significantly. This produces a really tall wave that causes
widespread damage along the coast and even several miles
inland. Ocean waves can also be beneficial. Ocean waves
transfer nutrients throughout the oceans and remove wastes
Figure 16.10 Tsunami
in coastal areas. In addition, ocean waves help many living
things travel to new locations. Ocean waves are helpful to
humans generating electricity by transferring mechanical energy into electrical current.
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Chapter 16
You’ve already seen some examples of how
electromagnetic waves are used (microwaves, radios,
TVs, etc.) and how they can be harmful (exposure to
X-rays). Humans can harness energy from
electromagnetic energy, especially solar radiation.
Solar radiation is radiation in the visible part of the
spectrum. It can be converted to electricity by the use
of solar cells and photoelectric cells. Solar cells use
a semiconductor, usually made out of silicon. When
photons of energy from the Sun hit the silicon
crystals, it makes the electrons move. This produces
an electric current. Solar cells power satellites and
space capsules. Photoelectric cells also convert the Figure 16.11 Solar Panels used for Electricity
Sun’s electromagnetic radiation into usable
electricity.
Electromagnetic waves carry electromagnetic energy of two types: ionizing radiation and
non-ionizing radiation.
Ionizing radiation comes from wavelengths in the electromagnetic spectrum such as
ultraviolet waves, X-rays and gamma rays that are smaller than visible light. The hazards of
ionizing radiation are clear and should NOT be ignored. These waves have enough energy to
remove electrons from the atoms ofliving cells. These ionized atoms, also called free radicals,
are then capable of damaging living tissue. If the damage is done to a cell’s DNA, the damage
can then be replicated to make more damaged cells, and that can result in cancerous growth.
Damage to the DNA in reproductive cells can result in genetic mutations in offspring.
Excessive radiation exposure can cause sunburn, nausea, vomiting, skin bums, fatigue, and
bleeding to name just a few symptoms.
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Non-ionizing radiation is made up of visible light waves and the wavelengths such as radio
waves, microwaves and radar that are longer than visible light waves. Non-ionizing radiation
also includes the electrical energy carried by power lines and emitted from electrical tools and
appliances. There is a great deal of research and debate on possible hazardous effects of
incidental non-ionizing radiation on the body. Some researchers argue that non-ionizing
radiation is safe, because it does not remove electrons from the atoms ofliving cells. However,
other concerns are based on the fact that this form does alter charged particles needed for living
cells to perform necessary functions. The jury is still out on this one as scientists continue to
research these questions to provide accurate information to the public.
Activity
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!Use the text and other sources to describe ways
each type ofwave is beneficial and harmful.
Benefit
Harm
earthquake
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Types of Waves
CHAPTER
16 REVIEW
A raggae band plays in Birmingham. In which location will the sound oftheir music
travel most quickly to their audience?
1.
A
B
C
B
jamming on a hot day in the park
inside an air conditioned concert hail
at a hot and rainy concert performance in the park
practicing in the cool basement of one of the band members
How can visible light waves be described?
Visible light waves are mechanical waves that must move through a medium.
A
Visible light waves mechanical waves that require no medium.
B
Visible light waves electromagnetic waves that require no medium.
C
Visible light waves electromagnetic waves that must move through a medium.
D
2.
3. Which of the following is not an example of a mechanical wave?
ocean waves
C
sunlight
A
B
sound waves
vibrations of a guitar string
B
4. Which type of wave has the most energy?
A
B
C
D
5.
a low-frequency wave
a high-frequency wave
a low pitch wave
a long wavelength wave
Which of the following would result in the largest amplitude wave with the most
energy?
A
B
C
D
distortion in a wave
destructive interference
constructive interference
no interference
have never seen sound waves but you know that they exist because you can hear them.
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What ifthere was a way for you to “see” sound waves being created. Look back to the Figure
w 16.1 Have you ever watched a large speaker like this while it was producing sound? Ifyou
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have you may have noticed the inner parts ofthe speaker vibrating. Speakers generate sound
using a diaphragm and electromagnets to create sound waves. Large speakers generate low
pitched sounds while tiny speakers generate high pitched sounds. Go to the Web site
http://e1ectronics.howstuffivorks.com/speaker5.htm to see an animation of a speaker in
motion. Draw your own diagram of a speaker in motion.
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