MI BIG
Waves and Vibrations IV.4
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(Inspiration)
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All students will describe sounds and sound waves.
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All students will explain shadows, color, and other light phenomena.
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All students will measure and describe vibrations and waves.
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All students will explain how waves and vibrations transfer energy.
Overview
What do sound, water, and light have in common? They can all be associated
with waves. When we drop a stone in water, waves are produced. Sound is a
wave, manifesting itself in the form of pressure vibrations. Light also
has wave properties. Simply put, waves are the mechanism of vibrations
(and energy) being transported from one point to another. An example of
waves carrying energy can be observed in the erosive effect of wave action
on beaches or shorelines. The pendulum of a grandfather clock repeats its
swing on a regular schedule that allows us to tell time: this is a useful
model for the study of waves and vibrations. Humans gather information
from their environment through waves: mechanical waves like sound and
electromagnetic waves like light. Humans have the ability to translate the
energy of sound and light waves into information that the brain can
interpret.
Essential Background Narratives
All students will describe sounds and sound waves.
All students will explain shadows, color, and other light phenomena.
All students will measure and describe vibrations and waves.
All students will explain how waves and vibrations transfer energy.
During the elementary grades, children should begin to describe and
analyze their rich experiences with sounds. In particular, they should be
able to distinguish sounds from the objects that make them. They need to
understand that sounds exist in the air, and that they are separate from
the objects that make them. Elementary students should learn that all
sounds originate with some kind of vibrating object or substance.
As middle school students experience sound traveling through different
media; they should identify other ways that matter can affect sound. Many
students think of echoes only as sounds that repeat themselves in open
spaces. They do not relate them to the movement of sound waves, nor do
they understand that echoes are produced whenever sound waves bounce back
(reflect) off large surfaces.
Light and vision are complex phenomena, and it will take many years of
study for students to clearly explain what they see in terms of light and
its properties. Although young children experience light every day, they
need to acquire the concept of light as a form of energy that moves
through space. Light is something that is constantly streaming out from
light sources in all directions, traveling rapidly through space and
bouncing around in "lighted" rooms. Shadows are another concept
that is difficult for elementary students to explain without an
understanding of light moving through space. Most children notice the
similarity between the shape of the object and its shadow. Some students
may think of shadows as dark "images" or "reflections"
of an object rather than as areas where an object has blocked the light
from the light source. As students begin to use the idea of light moving
in space to explain shadows they should say, "The shadow was formed
because light could not pass through the object."
Some middle school students have naïve conceptions. Although most
students recognize that light is necessary for vision, they believe its
only role is to "light up" the object. Since they can't feel
light reaching their eyes, they think their eyes detect objects without
anything linking the object to the eye. Once students understand that
light travels through space in straight paths, they can use this knowledge
to help understand how humans see. They must learn that, in order for them
to see non-luminous objects, light must strike the object and then be
reflected from the object to their eyes. If the light is completely
absorbed by the object, then it cannot be seen. Students should be able to
explain how reflected light from an object enters the eye.
Students at the middle school level have ideas about reflection that are
limited by their perceptions of the effects of light. Most students have
daily experiences with mirrors in their homes and rear view mirrors on
cars. Their limited understanding of reflection becomes clear when
students are asked to compare light shined on a mirror with light shined
on a white piece of paper. Students might say, "The light bounces off
the mirror, but when the light falls on the paper it stays there."
This explanation is directly related to the students' ability to detect
the effects of reflection. As students light up something else with the
light reflecting off a mirror, they can see that the light has moved from
the mirror to another place. With the paper, the only effect they can see
is on the paper itself. Students need to understand that light may reflect
off ordinary objects. They also need to know that some objects, like
mirrors reflect the light in a regular pattern. Other objects, like the
white paper, scatter the reflected light, destroying the regular pattern
in the light that reaches our eyes.
Students encounter colors everywhere they go. They can see colors in a
rainbow or in patterns reflected by a soap bubble, yet few children relate
these colors to white light. Many students think of white light as being
colorless and that colored light is darker than white light. They believe
that prisms or bubbles "make" the colored patterns they see. As
high school students learn about waves, they discover that white light is
actually made of all wavelengths of light mixed together. Students must
learn that as white light passes through objects, such as prisms or drops
of water, the light is bent. Each wavelength of light is bent differently,
separating the light into its different wavelengths or colors. Once
students understand that white light is made of a mixture of all colors of
light, they can see that color is a property that light already possesses.
Nothing needs to be added to white light to give it color. To get colored
light from white light, wavelength of other colors must be taken out. Even
in high school, students may think color resides in the object, and the
only role light plays is to "show" the color. As scientifically
literate students move toward the concept that color is a property of
light, they can then explain how we see the color of objects. Students
should know that the color we see is the color that reaches our eye,
because objects reflect some wavelengths of light and absorb others. If an
object appears black, it is not reflecting any light; if an object appears
white, it is reflecting all wavelengths of light.
Students at the middle school level have the naïve conception that sounds
cannot travel through liquids and solids, and sounds can travel through
empty space. They should come to understand that sound travels only
through some kind of substance or medium and sound does not travel in a
vacuum-it requires the vibration of molecules. Sounds can travel not only
through air but also through solids such as glass windows or the strings
in string telephones. Many students believe that the medium itself moves
in the direction of the waves going through it. This is not really the
case. Students may also have experienced "the wave" performed by
crowds at concerts and ball games. This is when people in the audience
stand up and sit down in a regular wave-like motion that moves around the
stadium. This model gives a much more accurate picture of the nature of a
wave. Scientifically literate students should describe the motion of a
wave in terms of period, frequency and amplitude.
The ability of scientific theories to reveal the unity underlying
seemingly diverse phenomena is especially apparent with regard to waves.
Mechanical waves include sound, ultrasound, water waves, and seismic
waves. All are alike in that they involve moving vibrations in some kind
of medium. Scientific literate high school students also need to know that
waves travel at different velocities and that sound travels faster in
liquids and solids than in gases. Wave velocities are measured in meters
per second. Students are able to experience these different speeds every
time there is a thunderstorm. If students are asked which comes first,
thunder or lightning, they often say that lightning comes first during a
storm. However, if students are asked why this happens, most cannot
answer. Students should be able to explain that light waves travel at a
faster velocity than sound waves.
Middle school students should be able to explain how waves transfer energy
from sources to objects that absorb them. Sources of waves, such as light
bulbs and musical instruments, are constantly emitting energy via the
waves that are propagated from them. The energy is "stored" in
the waves and travels with them wherever they go. Usually, energy in
electromagnetic or mechanical waves is changed into heat (thermal energy),
but there are important exceptions. Solar cells transform some of the
energy from the light that strikes them into electrical energy. When waves
reach an object they may be absorbed. When a wave is absorbed it ceases to
exist. The energy, however, is never destroyed; it is changed into some
other form.
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