INVESTIGATION #7: REFLECT, ABSORB, OR TRANSMIT
INVESTIGATING SELECTIVE ABSORPTION OF WAVES
Cell phones are all around us today and have
revolutionized how we communicate and “stay connected”.
Modern cell phones are more powerful and can do a variety
of complex things at the same time. They seem to be a
wonderful invention, but are they safe?
Scientists have begun to research the safety of cell phones. What are
they so concerned about? How dangerous can a cell phone be to us? They are
concerned about the exposure of the human brain
to the electromagnetic waves that are emitted by
cell phones. Newer models even have an
earpiece that keeps the cellular phone close to the
brain at all times. What will happen if you have
long term exposure to the waves created by our
cell phones?
In this section of the unit we will investigate
the ability of substances to absorb different forms of energy, in particular, the
energy carried by waves. This process is called selective absorption. You will
gain knowledge about the ability of a wave to transfer energy so that you can
read information about such things as cell phones and make informed decisions.
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Do cell phones emit mechanical or
electromagnetic waves?

If cell phones emitted x-ray waves
instead of radio waves, would cell
phones be safer to use?

How could a scientist design an
experiment to test whether cell
phones are dangerous to humans?
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GOALS:
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In this investigation, you will …
Describe the characteristics of the key sections of the electromagnetic
spectrum and their practical uses.
Describe how incident waves interact with objects and how these
interactions affect energy transfer and energy transformation processes.
Describe the difference between selective absorption and selective
reflection.
Describe natural frequency and how selective absorption of waves can
affect the natural frequency of a material.
Investigate how common materials selectively absorb or selectively reflect
EM energy transferred by EM waves.
INVESTIGATION OVERVIEW:
A synopsis of this lesson is as follows…
This investigation is broken into two inter-related parts. The first portion
of the investigation focuses on how waves deliver energy. The related topics of
selective absorption and selective reflection of waves will be discussed in this
part. In the second portion of the investigation, the focus turns to investigating
how specific waves are absorbed and/or reflected. Consequences of this
absorption and reflection are also discussed in this part.
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CONNECTIONS
Scientific Content  Electromagnetic waves are disturbances in the electric fields and
magnetic fields that are created by moving charged particles.
 Electromagnetic waves carry electrical energy and magnetic energy. This
energy is transported without the need for vibrating particles, so
electromagnetic waves can be transported through the vacuum of space.
 The amplitude and frequency of a wave together determine how much
energy is carried by the wave.
 Electromagnetic waves are divided into seven groups of waves. People
have used their scientific understanding of these waves to use all seven
groups of waves to improve our lives.
 Very often, when waves strike a surface, they divide. Some of the waves
reflect off of the surface and the rest enter the material. The waves that
reflect off of the surface carry their energy away from the material. The
waves that pass through the surface enter the substance and carry their
energy into the material. Once inside the material, the waves can
continue to travel, and ultimately leave the material, and/or they can be
absorbed by the material. If waves are absorbed, their energy is
transformed into a different form within the material. If waves pass
through the material, they carry their energy along with them.
 The different groups of waves behave differently when they strike
substances. The behavior depends on the type of electromagnetic wave
and the properties of the substance. Some materials absorb only a
narrow range of energy transferred by waves. This selection process,
called selective absorption, is dependent upon the properties of the
material.
 All objects vibrate at a characteristic frequency, determined by the
molecular structure of the material, called a natural frequency.
 The heat energy of an object can be transformed by the particles of the
object into a form of electric and magnetic energy that can be carried
away by electromagnetic waves. This process is referred to as cooling by
radiation (heat energy transfer by radiation).
Scientific Process  There will be opportunities for the students to design devices and design
their own investigations. Students will be asked to draw logical
conclusions from the results of their investigations, and extend their
understanding of the important science concepts to real-life situations.
Math/Graphing  There are a number of opportunities for displaying data and results using
a variety of graphical methods.
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Section I: Investigating How Waves Deliver Energy
MAKING SENSE OF ENERGY … Reviewing Wave Concepts
In the last investigation, we focused on what was creating the wave. We
discovered that when a particle oscillates in an organized manner, a wave is
created. If that particle is a molecule, a mechanical wave will be created. If that
particle is a charged particle, an electromagnetic wave will be created. Both
waves have similar characteristics as well as some noticeable differences. We
also created an extensive vocabulary that was used to discuss waves. A few of
the key terms are listed below.
Oscillation – any back-and-forth motion that repeats itself
Cycle – one completion of the ‘back and forth’ motion.
Crest – the highest point of a wave.
Trough – the lowest point of a wave.
Equilibrium line – the point midway between the crest and trough of a
wave.
Amplitude – the distance from the equilibrium line to the crest or to the
trough of the wave.
Period – the amount of time needed to complete one full cycle.
Frequency – the number of cycles completed every second.
In this investigation, we will turn our focus to how the waves deliver their energy
and the consequences of this delivery of energy. We will find that the ability of
waves to transfer energy to materials depends on the frequency of the waves
and the nature of the materials.
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MAKING SENSE OF ENERGY … Waves and the EM Spectrum
WHAT DO ELECTROMAGNETIC (EM) WAVES LOOK LIKE?
If you could see an electromagnetic wave traveling in front of you, it would
be shaped like the water waves that pass beneath boats in open water. The
distance between crests is called the wavelength. In water waves, this distance
is ordinarily around 10 meters. The wavelengths of electromagnetic waves range
from very large distances to extremely tiny distances. Some electromagnetic
waves have wavelengths longer than the State of Delaware. Other waves have
wavelengths that are so small that one of these waves would need to be millions
of wavelengths long to stretch across this little circle → ◦ (that’s a pretty short
wavelength!). Pick any distance between 1/10,000,000 of a centimeter and
hundreds of kilometers, and there are electromagnetic waves that have
wavelengths equal in length to that distance. Electromagnetic waves have a very
broad range of wavelengths! We will need a way to sort them into smaller
groups.
SORTING EM WAVES …
When discussing mechanical waves, people usually organize the waves
based on their frequency. For no particular reason, most electromagnetic
waves are organized based on their wavelength. The wavelength and
frequency of a wave are related mathematically, so it does not really matter
which characteristic is used to group the waves. In fact, some electromagnetic
waves, especially radio waves, are described by their frequency.
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As was described above, wavelengths of electromagnetic waves range
from hundreds of miles long to unimaginably tiny distances. This range is so
broad that electromagnetic waves are divided into seven (7) smaller groups;
radio waves, microwaves, infrared waves, visible light, ultraviolet light, X-rays,
and gamma rays. Collectively, these seven groups of waves make up the
electromagnetic spectrum.
WHAT IS ELECTROMAGNETIC RADIATION?
Electromagnetic radiation is the formal name for electromagnetic waves.
It is a form of energy that is produced from the disturbances of atoms or
disturbances within atoms. When an atom is disturbed and undergoes a change,
it can release energy in the form of an EM wave. The nature of these
disturbances determines the frequency of the EM waves (EM radiation)
produced. There are many different types of atoms, and many different ways
that each atom can be disturbed. So it makes sense that there are many
different types of EM waves produced by atoms.
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One important property of electromagnetic radiation is that it can travel in
the form of transverse waves through the vacuum of empty space. The
mechanical waves we have looked at all require matter to travel.
Electromagnetic waves do not. This is fortunate, because without this ability, we
would receive no energy from the sun. Another important property of these
waves is that they all travel at the same speed through space. The speed is
nearly 300 million m/s (186 000 miles/second). That means that radiation from
the sun, which is 150 billion meters (93 million miles) away, takes almost 8
minutes to get to Earth.
Electromagnetic radiation is often described as light. However, the light
we see is only a small part of the range of frequencies of electromagnetic
radiation. These frequencies range from below 60 cycles per second up to
trillions of cycles per second. The variation is a continuum we call the
electromagnetic spectrum, but we have divided it into groups based on the uses
we make of the various frequencies. In general, as the frequency of the wave
increases, so does the energy it carries.
Have you ever seen a rainbow in the sky? Each color seems to merge
into the neighboring one, and there are no firm lines between the colors. The
electromagnetic spectrum is actually a
continuation of the color spectrum we
see in a rainbow. Think of it spreading
out invisibly from either side of the
colors you see, with fuzzy boundaries
between layers. Spreading away from
the red side of our rainbow will be
radiation at a lower frequency (and
longer wavelength) than the red light.
Spreading away from the violet side will
be radiation at a higher frequency (and
a shorter wavelength) than the violet
light. Our eyes are not able to see this radiation, but it is there. We have other
types of detectors that we can use to find and measure it. We have names for
various sections of this spectrum, but the sections have fuzzy boundaries just as
the colors of the rainbow do.
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INFRARED (IR) WAVES
If we move away from the red side of the rainbow, in the direction of
longer wavelengths, the next grouping of waves is called infrared (IR). The
name infrared means “below red” because the wavelengths are longer than the
longest visible light waves. This means they vibrate at a lower frequency than
visible light. The infrared frequencies are most often associated with heat
energy since warm objects emit IR radiation as a way to cool.
Any time we hear of “heat” being radiated, what we generally mean is that
infrared radiation is being emitted by the object as it cools. Your body gives off
infrared radiation as a way to cool. The Earth does the same thing. The emitted
IR waves by warm objects can not be detected
by our human eyes, but it can be sensed by
specialized equipment. The image on the left
was created by an infrared camera and shows a
person holding a lit match. The colors in the
image are false colors to make the temperature
contrasts more visible. The colors distinguish
warmer and cooler areas. Notice that the tie is
the coolest part of the image and the areas of
exposed skin are much
warmer. Night vision goggles are sensitive to the
energy carried by infrared waves. The goggles are used
by the military and police to detect hiding and
approaching people at night. Infrared detectors are also
used to find survivors trapped under rubble after
disasters like earthquakes.
Energy companies use infrared cameras (sometimes called thermal
imaging cameras) to find where buildings leak heat energy. The red areas
around the windows show where the temperature of the outside surface of the
building is higher than the rest of the building. In these areas thermal energy is
being transferred to the air around the building.
Infrared photography is also used to analyze
vegetation patterns. Images from satellites are used to
show changes in vegetation all over the world.
Infrared detectors on satellites can be used to find the
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marijuana plants, carefully concealed and growing in the center of a cornfield,
which are invisible from the edge. Firefighters have also begun to use this
technology to see through the smoke in a burning building. Since the burning
building and hot smoke have higher temperatures than the people trapped inside,
the camera can distinguish the people from their surroundings by the IR waves
they emit. Infrared radiation is also used to carry information by some
communication devices.
MICROWAVES
The group of waves with a wavelength longer than infrared is known as
microwave. We are most familiar with microwaves as a cooking method, but
their applications are much more diverse. In a microwave oven,
water molecules readily absorb microwaves. The absorbed energy
of the microwaves is transformed into random kinetic energy of the
molecules. The temperature of the food then increases and the
food cooks as though it had been on a standard stove. How can we
be sure the microwaves will stay inside the oven and not escape
into the kitchen? There are holes in the screen that are big enough to let the tiny
wavelength visible waves through, but block the much longer wavelength
microwaves. Microwaves are 12.4 cm long, about half the width of this page.
These waves are reflected by the screen back into the oven to be absorbed by
the food.
The microwave group includes many frequencies. Some of these waves
are also used for radar imaging and measurement.
Meteorologists use microwaves in Doppler radar
systems to track the movement of storms. Some
of the microwaves emitted by the radar station
reflect off of the clouds, rain or snow and carry
information about the storm back to the station.
Microwaves are also used for communication.
Cell phones use microwaves to carry the
information in a phone conversation or a text
message to communication towers. From here,
the information is forwarded to the phone of the person being called.
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RADIO WAVES
The group of waves having the longest wavelengths is called radio. This
range is used mostly for communication. Radio waves have wavelengths that
may be as long as thousands of meters. The radio
range includes radio, short wave radio, and
television. Remember that wave frequency is
measured in Hertz. The numbers on your radio’s
AM dial are kilohertz (thousands of Hertz) and the
numbers on the FM range are in megahertz (millions
of Hertz). Each radio station in the AM range and
FM range broadcasts at a different and specific
frequency. Television stations broadcast at higher frequencies than radio
stations. The Federal Communications Commission reserves specific
frequencies for various organizations like police and fire companies so they will
not conflict with commercial broadcasters.
ULTRAVIOLET (UV) WAVES
Going back to our rainbow of colors, remember that the wave frequency
increases as we move from red to violet. Past violet we have EM waves with
frequencies too great to be visible. This group of waves is called ultraviolet
(UV) radiation. Ultraviolet means “beyond violet”. We cannot see this frequency
because our eyes are not sensitive to these waves. Many other organisms, such
as bees and deer, can see in this range. This is the reason that deer can detect
the UV brighteners found on clothing that was
washed with modern detergents.
Like the radio waves, UV is divided into sub
categories based on their wavelength. UV-A
waves are the longest wavelength UV waves,
followed by the UV-B waves. Both UV-A and UVB waves deliver energy to your skin that can produce sunburn damage or worse.
UV-C waves have the shortest wavelength (highest frequency) of all the UV
waves. Most UV-C waves coming from the Sun do not reach the earth because
they are absorbed by ozone molecules in the upper atmosphere. On the surface
of the Earth, we create and use UV-C waves to kill bacteria and sterilize things
such as the goggles that you wear in science class.
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As we have seen with microwaves, the energy transferred by certain
frequency waves can be readily absorbed by common materials. This absorbed
energy can have a damaging effect. When ultraviolet radiation is absorbed by
skin cells, it causes changes in the cells. The ultraviolet radiation can actually
damage the cells and cause sunburn. Frequent and long exposure to ultraviolet
radiation can cause skin cancer. This is why sun screens should be used from
infancy. UV radiation can also cause changes to the lenses of our eyes.
Prolonged exposure to UV waves caused the lens to turn cloudy making it
difficult to see, a condition called cataracts. The retina of the eye can also be
damaged. These changes can cause blindness. Glasses and sunglasses have
UV protective coatings to reduce this effect. It is important to remember that
clouds do not block UV-A and UV-B radiation,
even though they do block a lot of visible light.
That is why you can get sunburned on a cloudy
day.
Some materials will fluoresce (give off
light) when they are exposed to UV radiation.
The colors that certain minerals fluoresce are
very different from the colors of the minerals
themselves and can be used to help identify the minerals. This sample comes
from Franklin, New Jersey and shows calcite fluorescing in red and Willemite
fluorescing in green.
X-RAY WAVES
At a higher frequency (and shorter wavelength) than ultraviolet radiation is
X-ray radiation. X-rays got their name because William Roentgen, their
discoverer, didn’t know exactly what they were. X-rays have many applications
because these high energy waves can penetrate materials that lower frequency
waves cannot. The medical field has benefited greatly from the use of X-rays to
look inside of the human body. Special machines are
used to generate these high energy waves and
receptors that are sensitive to these waves are used to
detect them. Most X-ray waves simply pass through
the muscles and organs of human body, but they can
not pass through bones. On an X-ray image, the bones
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show up as white images giving the doctor the ability to see inside the human
body without making any incisions.
In addition to their medical
applications, X-rays have many industrial
and scientific applications. X-rays are
used to examine the welds on a pipe.
They can find weak spots that would leak
or break under pressure, causing major
problems.
These
weak
spots can
not be recognized without using X-rays. At the
airport, security has been enhanced through the
use of X-ray machines that scan passenger bags,
detecting items that are prohibited on a plane for
safety reasons.
GAMMA WAVES
The highest frequency electromagnetic radiation is known as gamma
radiation. This is extremely high energy radiation and is highly toxic to
organisms. In carefully controlled
doses the energy of gamma waves
can be put to use killing cancer cells
without killing the cancer patient.
This type of radiation is also used to
sterilize medical instruments and
bandages. Over exposure to these
waves can cause immediate death,
but even low doses of gamma
radiation is also associated with cell
damage that causes cancer. People who work with substances that emit gamma
radiation monitor their exposure very carefully.
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Let’s Investigate … Investigating How Waves Transfer Energy
FOCUS QUESTION: How do waves transfer energy?
To answer the focus question, how do waves transfer energy, we will need
to do some experimentation. For this activity, our source of waves will be a laser.
The laser will give us a very specific wave, that of visible red light having a
wavelength of 633 nanometers (that’s 633 billionths of a meter long). The light
coming from a laser is very special. All of the light waves leaving the laser are
aligned. This alignment enables the laser light to transfer energy very effectively.
So effectively, that laser light is very dangerous, especially to the receptors in
your eyes. The laser light emitted from inexpensive laser pointers can
permanently damage your eyes, so never look directly into a laser beam.
Along with the laser, two dishes of gelatin will be used in this activity, one that is
red (strawberry) and the other will be blue (blueberry).
PROCEDURE:
1. Shine the red (Helium-Neon) laser into both dishes of gelatin. Make a
drawing in your journal of this mini-investigation and record your
observations.
2. In your journal explain the results. Why did they produce two distinctively
different outcomes?
3. If you are having difficulty explaining why the light behaves differently in
the two dishes of different colored gelatin, think energy! It may be easier
to explain what you observe by thinking about the laser beam as a
‘stream’ of EM energy. What happens to the energy coming from the
laser? Where does it go? Does it change forms? What evidence
supports this conclusion?
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MAKING SENSE OF ENERGY … How Waves Deliver Their Energy
HOW DO WAVES DELIVER THE ENERGY THEY CARRY?
Waves transfer energy to substances by passing into the substance. This
transfer process is complicated because waves very seldom deliver all of their
energy to the substance. When an incoming wave (called an incident wave)
strikes an object or substance, usually the wave divides. Part of the wave
reflects off of the surface and the rest passes into the substance.
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The part of the wave that reflects off of the surface of the substance
(called the reflected wave) carries energy with it. This part of the incident
wave’s energy never really enters the object or substance. Instead, it is
carried elsewhere by the reflected wave.
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The part of the wave that passes into the material changes speed and is
called the refracted wave. This wave can keep going and pass right
through the substance, or it can be absorbed by the substance. Often, the
refracted wave does both.
 The part of the refracted wave that passes right through a
substance and out the other side is called the transmitted wave.
The energy carried by this part of the original wave also passes
right through the substance, and does not affect the substance.
 Usually, part of the wave passing through the substance seems to
disappear. What really happens is that part of the energy carried
by the refracted wave is transformed into a different form of energy
by the particles in the substance. We have studied many examples
of objects transferring energy to substances. When a wave
transfers energy to a substance, the process is called wave
absorption. When a part of a wave is absorbed, that part of the
wave disappears, but the energy it carried does not. We know
energy never disappears.
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DEMONSTRATING HOW LIGHT DIVIDES
This is a demonstration that will be conducted by your teacher. It involves the
use of a common laser pointer, but just like in the laser gelatin activity we will
need to be very careful with the laser beam. The laser light emitted from even
inexpensive laser pointers can permanently damage your eyes, so never look
directly into a laser beam.
Students: After looking at the laser demonstration, make a sketch of the water
container. Draw and label the following beams of laser light: the incident
beam, the reflected beam(s), the refracted beam, and the transmitted
beam(s). Is there an absorbed beam?
Question #1: Why couldn’t you see the laser beam traveling through the
air? Why could you see this beam if it passed through fog or chalk
dust?
Question #2: What happened to the energy carried by the laser beam that
was absorbed by the water?
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WHEN AN ELECTROMAGNETIC WAVE STRIKES A SUBSTANCE, HOW WILL THE
WAVE REACT?
It is very difficult to predict whether an electromagnetic wave will be
reflected off of a substance, be absorbed by it, or simply pass right through it.
There are two important factors that determine the behavior of the waves when
they strike a substance:
 The properties of the substance and its surface.
 The type of electromagnetic waves involved.
Very few substances absorb all electromagnetic waves. For example,
light cannot travel through the concrete block walls of your classroom, but radio
waves can pass right through them. Exactly the opposite happens with water.
Most radio waves cannot travel through water, but visible light passes right
through water. Next we are going to conduct a series of investigations to test
what effect different materials have on different waves in the electromagnetic
spectrum.
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MAKING SENSE OF ENERGY … The Selective Absorption and the Selective
Reflection of Waves
Why are some materials good reflectors of waves, while others are good
absorbers? Why do materials allow some waves to pass through, relatively
unchanged, but readily absorb different waves? The properties of a substance
that determine which waves it will absorb and which waves it will transmit are
much too complex to be discussed at this point. But we have learned enough
about waves to see relationships that are important when answering these
questions.
Let’s review the important ideas we have learned about how waves are created
and how they travel through matter and space.
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The molecules that make up a solid or liquid are bound together by
electric forces that behave like the elastic forces exerted by springs.
These connecting forces allow the molecules to vibrate and pass energy
back and forth to each other.
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Waves are created by the vibrations of molecules and charged particles
that are in the molecules.
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Mechanical waves are created and travel through materials by the
coordinated vibrations of molecules in the matter.

Electromagnetic (EM) waves are created when charged particles vibrate.
These waves can move through the emptiness of space and through
matter.
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The molecules in a substance vibrate with natural frequencies. These
natural frequencies are different for different substances, and are
determined by the masses of the molecules and the size of the elastic
binding forces that connect the molecules.

The charged particles within a molecule also have natural frequencies that
are different for different types of materials.
A key to the selective ways which substances reflect, transmit and absorb waves
is the existence of the natural frequencies of the molecules and charged particles
that make up the substance.
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Have you ever pushed a small child, perhaps a younger brother or sister,
on a swing? If so, you most likely found out that the only way to get him or her to
swing higher is to push when the child is at the top of the swing cycle. If you
push while the child is on the way up, the action is disruptive to the swinging
motion. Selective absorption of waves is very similar to this example. A wave
can increase the amplitude of a molecule’s motion if the frequency of the wave
matches a natural frequency of the molecule’s motion. When the frequencies
match, the molecule can easily absorb the energy of the wave. If the frequency
of the wave is a little too high or a little too low, the molecule will not readily
absorb the wave’s energy, and the amplitude of the molecule’s motion will not be
affected. It is this sensitivity to a narrow range of wave frequencies that allows
molecules in matter to absorbed incoming waves selectively.
A good example of this phenomenon is the collapse of the Tacoma
Narrows Bridge in the state of Washington. Soon after it opened, the bridge
began to sway. The wind coming
through the canyon struck the bridge
in gusts that arrived at intervals
matching the natural frequency of
the bridge. This caused an increase
in the amplitude of the motion of the
bridge, eventually leading to its
collapse on November 7, 1940. This
is an example of an engineering
disaster that could have been
avoided by better understanding
natural frequencies.
Summarizing what we just discussed …
 If a wave has a frequency that matches a natural frequency of the
molecules in a substance, when the waves strike the substance the
energy the wave carries will be readily transferred to the molecules (or
charged particles) in the substance.
 If the wave has a frequency that does not match a natural frequency of the
molecules in a substance, the molecules (or charged particles) will not
readily accept the energy carried by the wave.
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This is an important difference but it is not always easy to predict what will
happen to the energy carried by these two different groups of waves.
Nevertheless, it is easy to conclude that the substance will react differently to a
wave, depending upon whether or not the frequency of the wave matches a
natural frequency of the substance.
Using the Example of a Colored Object
We know that white light is a combination of EM waves traveling together.
The waves include all of the colors of the rainbow, but when combined together,
they give the illusion of the ‘color’ white. So why is a red shirt red? Does it emit
red light? Would the shirt appear to be red in a dark room?
Unless the shirt plugs into an energy source, it will not emit red light by
itself. When white light strikes a red shirt, only the red light reflects back. What
happens to all the other colors in the white light?
It is obvious that the other colors did not reflect off the shirt, so there are
only two possibilities. They either passed through the shirt or were absorbed by
it. It would not be difficult to demonstrate that orange, yellow, green and blue
light do not pass right through a red shirt. All of the frequencies that represent
these colors must have been absorbed by the pigment that dyes the shirt red.
There are two ways to look at this phenomenon:
 the orange, yellow, green blue and violet
frequencies are all selectively absorbed
 the red frequencies are selectively reflected
Using either description, it is clear that the pigment
in the shirt reacts differently to the different
wavelengths of visible light.
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Section II: Investigating Selective Absorption & Reflection
MAKING SENSE OF ENERGY … Selective Absorption and Microwaves
Joe and his younger brother Bob are attempting to make hot chocolate in
the microwave. Both brothers have made chocolate in the microwave many
times before, but this time Joe decides a different
approach. While Bob goes to fetch the container of
milk from the refrigerator, Joe decides to preheat his
powdered hot chocolate mix. He pours the dried
powder into his mug, places in the microwave, closes
the door to the microwave oven, sets the timer to one
minute and pushes the start button. After a minute, Joe is expecting a hot syrupy
chocolate mix, all ready for the cool milk. To his surprise, the dry powdered mix
is very much the same as it was when it was first placed in the microwave. It did
not melt into hot syrup. In fact it did not get very hot at all. It was as if the
microwaves in the oven had somehow missed the powder!
Journal Entry: Think about the problem described in the passage. Have you
ever experienced a similar problem? Have you noticed that some objects
do not warm up much, but at the same time other materials become very
hot in the same microwave oven? In your journal, use your knowledge of
waves to provide an explanation for this unusual event.
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Let’s Investigate … Selective Absorption of Microwaves
FOCUS QUESTION: How do water molecules respond to microwaves?
PROCEDURE:
1. Take two paper towels and fold them together. Then fold them in half
twice.
2. Place the folded towels into a microwave oven and turn the oven on for 15
seconds. After the 15 seconds of exposure to the microwaves, carefully
remove the paper towels from the oven. If you have a digital
thermometer, record the average temperature of the dry paper towels
before and after placing them in the microwave oven.
3. Next dampen the paper towels with room temperature water and repeat
the experiment. Be very careful when removing the wet paper towels from
the microwave oven; they will probably be hot. If you have a digital
thermometer, record the temperature of the wet paper towels before and
after placing them in a microwave oven.
Investigation Reflection:
Question #3: Do the wet paper towels respond differently to the 15 second
exposure to the microwaves? Why / why not?
Question #4: Do the microwaves in a microwave oven carry energy in the
form of heat to the food?
Question #5: Why do you think a popcorn kernel ‘pops’ when placed in a
microwave oven? What might be ‘wrong’ with those kernels that do not
pop?
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Let’s Investigate … Selective Absorption of Infrared Light Waves
The IR waves that leave a TV remote have wavelengths that are much
shorter than microwaves but still too long to stimulate your eyes. These
wavelengths are only slightly longer than the longest visible waves (the deep
reds). You should always be very careful when working with invisible EM waves.
All EM waves carry energy and some invisible waves can damage your eyes. We
do not have any evidence that the radiation from these devices is harmful, but it
is never a good idea to stare directly into any source of radiation.
In this activity, a TV remote control will be used as our source of IR waves. The
IR waves will be detected with an IR probe.
FOCUS QUESTION: What materials selectively absorb IR waves?
PROCEDURE:
1. Place the TV remote on the lab table and place the IR probe roughly 10
cm from the remote. Be sure to face the probe sensor towards the
remote.
2. Open the program Investigating IR Waves. Start collecting data (click on
the green collect button on the Logger Pro screen) while one of the TV
remote keys is depressed. A wave pattern should be visible on your
screen.
3. Now it is time to test materials to see if they absorb or transmit the IR
waves leaving the remote. Make sure to position the sample material
between the remote and the sensor so that the IR waves must pass
through the material to reach the sensor. Then start collecting data while
one of the TV remote keys is depressed. A wave pattern should be visible
on your screen if the material does not absorb IR waves. If the material
absorbs or completely reflects IR waves, the wave pattern may be altered
or non-existent.
4. Make a list of materials that you test and the outcome of your investigation
in your journal. Include numeric data if possible. Later, your teacher may
ask you to discuss your findings.
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Investigation Reflection:
After completing your investigations write an answer to the following
questions about the selective absorption of IR waves. You may have to do some
more investigating to find an answer. Write your responses in your journal.
Question #6: Does the remote source emit a narrow focused beam of IR
waves, or does it emit a broad spreading beam?
Question #7: What materials allowed both visible light and IR waves to pass?
Question #8: What materials allowed visible light waves to pass, but blocked
IR waves?
Question #9: What materials blocked visible light waves but allowed IR waves
to pass?
Question #10: Can IR waves be reflected by a standard mirror?
Question #11: You have just moved into a new house and the dish/satellite
Serviceman installs the receiver unit inside your TV cabinet behind a
glass door. Will the remote control signal be affected by the glass door?
Will the thickness of the glass matter? Will it work if the door is wooden
and not glass?
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© 2009 DE Science Coalition / Delaware Department of Education
Let’s Investigate … Selective Absorption of UV Waves
The UV probe
We will use a UV probe, which is sensitive to only one of the three regions
of the UV spectrum, UVA radiation. When UVA waves strike the sensor in the
end of the UV probe, the sensor absorbs the waves and transforms the energy
they carry into an electrical signal that is relayed to the computer. This signal is
recorded and plotted by the software as the intensity of the UV radiation.
Different materials react differently when exposed to UV radiation. In this
investigation, we will use the UV probe to investigate how materials react to UV
radiation. We will use the sun as a source of UV radiation (a UV lamp may be
substituted as the source).
Remember, UV waves can permanently damage your eyes.
Never look directly into a source of UV light, including the Sun.
FOCUS QUESTION: What materials selectively absorb UV waves?
In this activity, your challenge is to design an experiment to find materials that
selectively absorb or reflect UV waves. Write your plan in your journal. Listed
below are some questions that may help to guide your investigation.

What kinds of materials absorb UV waves? Will common materials (glass,
paper, wood, etc.) absorb UV waves?

What kinds of materials allow UV waves to pass through them?

Will UV waves pass through materials that are transparent in
visible light (like water and glass)?

Do your sunglasses block UV waves?

Can you find evidence that higher SPF number sun-block products absorb
UV waves better than lower SPF sun-block?

Do UV waves reflect off smooth surfaces like glass, mirrors, or aluminum
foil?
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Consider following the steps below for testing how well different materials
absorb UV light.
1. You can create a testing “card” by cutting four holes in an index card.
Three holes will be for testing
materials and the fourth will be
the control. For example, you
could cover all four holes with
light plastic and then place
sunscreen lotion of 5 SPF, 15
SPF, and 45 SPF over three of
the holes to test the
effectiveness of each
sunscreen.
2. Open the file Investigating UV Waves. This program will only record the
intensity of the UV light wave.
3. Whatever approach you use to test how well materials absorb or reflect
UV waves, be sure to construct a data table and record the results of your
investigation. Repeat your measurements if necessary or expand your
investigation by making a second card to test more materials.
4. Write a summary of your results in your journal.
Investigation Reflection:
Question #12: Why go to the trouble and expense of wearing sun-block?
What would happen to the energy carried by the UV waves if the waves
struck bare skin?
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© 2009 DE Science Coalition / Delaware Department of Education
Let’s Investigate … Selective Absorption of Radio Waves
Radio waves include the group electromagnetic waves having
wavelengths between roughly 30 cm long (about one foot) to waves having
wavelengths hundreds of meters long. FM radio stations use electromagnetic
waves that have wavelengths about 3 m long to carry the energy to your radios.
Most TV stations use electromagnetic waves having wavelengths close to half a
meter (50 cm) long. Cell phones make use of the shortest radio waves. Their
wavelengths range from 34 cm to 36 cm long. To test the properties of these
waves, you will need a portable radio or a cell phone. In this case, our concern is
whether the radio waves will reach our radio or cell phone.
FOCUS QUESTION: What materials selectively absorb Radio waves?
In this activity, your challenge is to design an experiment to find materials that
selectively absorb radio waves. Write your plan in your journal. Listed below are
some questions that may help to guide your investigation.

What kinds of materials absorb radio waves?

What kinds of materials allow these waves to pass through them?

Will the radio or cell phone receive a signal when it is enclosed by plastic?

Will the radio or cell phone receive a signal when it is enclosed by a dark
colored fabric?

Will the radio or cell phone receive a signal when it is enclosed in
aluminum foil?

Can you find any common dry substances that will absorb the radio waves
before they reach the radio or cell phone?
You will want to protect the radio or cell phone by placing it in multi-layers
of plastic bags. Carefully cover the devices, don’t just dump the
substances onto the devices.
Interpreting Your Data
Use the conclusions of your investigations with radio waves and make
predictions about the kinds of places where radios and cell phones would
have the most difficulty receiving a clear signal.
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© 2009 DE Science Coalition / Delaware Department of Education
Applying what you have learned … The Greenhouse Effect
WHAT IS THE GREENHOUSE EFFECT?
Have you ever opened the car door on a hot summer day and felt the rush
of hot air come from inside the car? How can the car’s interior heat up so much?
When you leave a closed car parked in the sun, light enters through the windows
and is absorbed by the interior of the car. The molecules that make up the
interior then begin to release their own radiation. They release energy in the
infrared range, but the type of infrared waves they emit cannot get back through
the windows, and the car
becomes very hot. We
call this the green house
effect because growers
use this principle to heat
their green houses.
Scientists have
learned that certain types
of gases in the
atmosphere trap some of
the IR waves the Earth
emits. This trapping of
Earth’s waves produces a
warming effect, called the Greenhouse Effect. There has always been a
Greenhouse Effect in our atmosphere that helps keep our planet warmer. In fact,
without in, Earth’s average temperature would drop well below the freezing
temperature of water, making Earth an ice covered planet. Scientists have
learned from geological evidence that the average temperature of Earth is at
least partially determined by the concentration of greenhouse gases in the
atmosphere. When the concentrations of greenhouse gases in the atmosphere
have been unusually low, parts of our planet at Delaware’s latitude were frigidly
cold year round, if not completely covered by glaciers. When the concentration
of these gases was unusually high, tropical plants grew in Delaware year round.
It is clear that our local climate and climates elsewhere on Earth are strongly
influenced by the Greenhouse Effect.
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Journal Entry: Is the Greenhouse Effect a good thing or a bad thing for the
Earth?
HOW DOES THE EARTH ‘COOL OFF’?
Every day the Earth receives huge quantities of energy from the Sun. We
now know that that energy mainly arrives in the form of IR waves, Visible Light
waves and UV waves. The atmosphere, oceans and dry land absorb about 70%
of the solar energy reaching Earth. The remaining energy is reflected back into
space, mostly by the Earth’s atmosphere, the oceans and the polar icecaps.
Most of the absorbed energy eventually becomes thermal energy.
If the Earth did not have some way to transfer some of this energy away,
the temperature of our planet would continue to rise every day, and long ago
Earth would have become too hot to support life of any form. Fortunately, like
everything else, the Earth does emit electromagnetic waves. Our planet emits IR
waves into space to cool. These are long wavelength IR waves, similar to those
emitted by humans. Remember that infrared waves are a broad family of waves
having wavelengths longer than visible waves but shorter than microwaves.
Roughly 45% of the energy in sunlight is in the form of IR waves. Most of these
are short wavelength IR waves. Earth’s long wavelength IR waves are different
from most of the IR waves in sunlight.
At any instant of any day, EM waves from the Sun bathe Earth with
energy. To ‘cool down’, Earth emits its own EM waves, different from most of the
waves in sunlight.
Journal Entry: Why doesn’t the Earth transfer heat energy away from its
surface and into space by conduction or convection?
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GLOBAL WARMING
There are some gases that are good absorbers of the long wavelength IR
waves emitted by Earth. We call these greenhouse gasses. These gases
include carbon dioxide and methane. These same gases are not good absorbers
of the electromagnetic waves arriving from the Sun, even the IR waves.
Remember, the IR waves emitted by Earth are long wavelength IR waves. The
Sun emits the whole range of IR waves, but most are shorter wavelength IR
waves. These shorter wavelength IR waves pass right through the carbon
dioxide and methane in the atmosphere. In effect, these greenhouse gases
allow most solar energy to enter, but capture some of the IR waves that the Earth
emits. As the concentration of greenhouse gases increases in our atmosphere,
more escaping energy is trapped, and the temperature of the Earth gradually
increases. Some of the sources of carbon dioxide are natural other sources are
man-made. Water vapor and carbon dioxide are also greenhouse gases, yet the
most troublesome greenhouse gases are the chlorofluorocarbons (CFC). A
single CFC molecule has the same effect as 10,000 carbon dioxide molecules.
Are we humans changing climates across the planet by increasing the
concentration of greenhouse gasses into the atmosphere? This is a question
being hotly
debated in the
scientific
community
and in politics.
The answer is
extraordinarily
important to
everyone, but
especially
anyone living
close to a
coastline.
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Explain how changing the amount of these “greenhouse gases” in the
atmosphere can influence the Earth’s ability to ‘cool’ itself. Use an energy chain
to trace the flow of energy into and away from the Earth in your answer.
Summary of Investigation …
In your journal, write a concise summary of this investigation. Be sure to
address the following questions and use your data to support your responses.
 What can happen to a wave when it strikes the surface of an object?
 What happens to the energy carried by a wave when the wave is
absorbed by a material?
 What is selective absorption?
 What is selective reflection?
 Do all materials absorb and/or reflect EM waves the same way?
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© 2009 DE Science Coalition / Delaware Department of Education
Investigating Further …
THE EM SPECTRUM AND ASTRONOMY
Each year thousands of people gaze into the nighttime sky and stare at
the awesome sights above. It is easy to be amazed by the vast numbers of stars
in the sky. How do astronomers know so much about these objects and places?
The answer is that they study the light given off by these objects. The “light” may
simply be visible light, but it may also mean any electromagnetic (EM) wave.
Aside from our Sun, most things we come in contact with emit a very
narrow range of EM radiation, mostly on the IR scale. In space, though stars and
star clusters can emit radiation in a much greater range of wavelengths.
Astronomers use special telescopes that collect information on a specific type of
wave being emitted
and then a computer
converts it into a
colored picture so that
we can interpret the
“image” obtained by
the telescope. For
instance, the Crab
Nebula can be seen
with an ordinary
optical telescope, but
if you were to use a special telescope that was sensitive to other regions of the
EM spectrum the nebula would look different. These astronomers view the star
clusters by investigating the amount and the intensity of visible, IR, UV, and radio
waves that these objects emit.
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THE CRAB NEBULA AS SEEN THROUGH
DIFFERENT EM WAVE TELESCOPES
Image source: www.fas.org
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