Text Book Flashcards
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a Figure 14-15 Circular waves
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spread outward from their source
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(a). The wave can be represented
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by circles drawn at their crests
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(b). Notice that the rays are
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perpendicular to the wave fronts.
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Ray
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• Figure 14-16 A wave pulse in a
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ripple tank is reflected by a barrier
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(a). The ray diagram models the
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wave in time sequence as it
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approaches the barrier and is
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then reflected to the right (b).
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Wave
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front
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Waves in Two Dimensions
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You have studied waves on a rope and on a spring reflecting rom rigid
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supports
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where the amplitude of the waves is forced to be zero by destruc-
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tive interference. These mechanical waves move in only one dimension.
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However
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waves on the surface of water move in two dimensions
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sound waves and electromagnetic waves will later be shown to move in
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three dimensions. How can two-dimensional waves be demonstrated?
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Picturing waves in two dimensions When you throw a small stone into
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a calm pool of water
you see the circular crests and troughs of the result-
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ing waves spreading out in all directions. You can sketch those waves by
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drawing circles to represent the wave crests. If you dip your finger into
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water with a constant frequency
the resulting sketch would be a series of
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concentric circles
called wave fronts
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is a line that represents the crest of a wave in two dimensions
and it can
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be used to show waves of any shape
including circular waves and straight
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waves. Figure 14-15a shows circular waves in water
and Figure 14-15b
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shows the wave fronts that represent those water waves. Wave fronts drawn
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to scale show the wavelengths of the waves
but not their amplitudes.
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Whatever their shape
two-dimensional waves always travel in a
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direction that is perpendicular to their wave fronts. That direction can be
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represented by a ray
which is a line drawn at a right angle to the crest of
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the wave. When all you want to show is the direction in which a wave is
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traveling
it is convenient to draw rays instead of wave fronts.
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Reflection of waves in two dimensions A ripple tank can be used to
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show the properties of two-dimensional waves. A ripple tank contains a
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thin layer of water. Vibrating boards produce wave pulses
or
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Barrier
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Normal
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Reflected
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Incident ray
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Chapter 14 Vibrations and Waves
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U1
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Figure 14-lba
traveling waves of water with constant frequency.
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A lamp above the tank prouaces shadows below the tank that
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snow the locations of tremo sa of the waves. The wave pulse trav
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els toward a rigid barrier that reflects the wave: the incident wave
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moves upward
and the reflected wave moves to the right.
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The direction of wave motion can be modeled by a ray diagram.
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Figure 14-16b shows the ray diagram for the waves in the ripple
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tank. The ray representing the incident wave is the arrow pointing
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upward. The ray representing the reflected wave points to the right.
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The direction of the barrier also is shown by a line
which is
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drawn at a right angle
or perpendicular
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normal. The angle between the incident ray and the normal is called
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the angle of incidence. The angle between the normal and the
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reflected ray is called the angle of reflection. The law of reflection
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states that the ungle of incidence is equal to the angle of reflection.
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Refraction i waves is to dimensions A ripple tank also can be
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used to model the behavior of waves as they move from one
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medium into another. Figure 14-17a shows a glass plate placed in
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a ripple tank. The water above the plate is shallower than the water
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in the rest of the tank and acts like a different medium. As the
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waves move from deep to shallow water
their speed decreases
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and the direction of the waves changes. Because the waves in the
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shallow water are generated by the waves in the deep water
their
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frequency is not changed. Based on the equation 2 = v/f
the decrease in
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the speed of the waves means that the wavelength is shorter in the shallower
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water. The change in the direction of waves at the boundary between two
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different media is known as refraction. Figure 14-17b shows a wave front
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and ray model of refraction. When you study the reflection and refraction
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of light in Chapter 17
you will learn the law of refraction
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You may not be aware that echoes are caused by the reflection of sound
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off hard surfaces
such as the walls of a large warehouse or a distant
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cliff face. Refraction is partly responsible for rainbows. When white
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light passes through a raindrop
refraction separates the light into its
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• Figure 14-17 As the water
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waves move over a shallower
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region of the ripple tank where
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a glass plate is placed
they slow
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down and their wavelength
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decreases (a). Refraction can be
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represented by a diagram of wave
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fronts and rays (b).
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14.3 Section Review
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27. Waves at Boundaries Which of the following
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wave characteristics remain unchanged when a
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wave crosses a boundary into a different medium:
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frequency
amplitude
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direction?
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28. Refraction of Waves Notice in Figure 14-17a how
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the wave changes direction as it passes from one
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medium to another. Can two-dimensional waves
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cross a boundary between two media without
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changing direction? Explain.
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Physics
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nline physicspp.com/self_check quiz
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29. Standing Waves In a standing wave on a string
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fixed at both ends
how is the number of nodes
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related to the number of antinodes?
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30. Critical Thinking As another way to understand
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wave reflection
cover the right-hand side of each
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drawing in Figure 14-13a with a piece of paper. The
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edge of the paper should be at point N
the node.
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Now
concentrate on the resultant wave
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darker blue. Note that it acts like a wave reflected
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from a boundary. Is the boundary a rigid wall
or is it
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open-ended? Repeat this exercise for Figure 14-13b.
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Section 14.3 Wave Behavior
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