Waves
A
wave is an oscillation that travels from one
place to another.
 If
you poke a floating ball, it oscillates up and
down.
 The
oscillation spreads outward from where it
started.
Why learn about waves?
 Waves carry
 Waves are
all around us:
light from the stoplight
ripples in a puddle of
electricity flowing in wires
radio and television and cell
phone transmissions
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Recognizing waves around you
 Waves are
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present:
when you see a vibration that moves.
when something makes or responds
to sound.
when something makes or responds
to light.
when technology allows us to “see
through” objects.
when information travels through the
air (or space) without wires.
Wave pulses
useful information
and energy.
Waves
 Waves are
a traveling form
of energy because they can
change motion.
 Waves also
carry
information, such as sound,
pictures, or even numbers.
Transverse waves
A
A
 It
wave pulse is a short ‘burst’ of a traveling wave.
is sometimes easier to see the motion of wave
pulses than it is to see long waves with many
oscillations.
transverse wave has its oscillations
perpendicular to the direction the wave moves. A
A
wave pulse along a rope attached to a wall moves
left to right, while the boys hand moves up and
down.
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Longitudinal waves
 The
oscillations of a longitudinal wave are in the
same direction that the wave moves.
Longitudinal waves
A
sharp push-pull on the
end of the spring results in a
traveling wave pulse as
portions of the spring
compress, then relax.
 Sound
waves are
longitudinal waves.
 Like
a wave pulse on a
spring, air molecules
oscillate back and forth as
sound travels.
Frequency, amplitude, and wavelength
 You
can think of a wave as a moving series of
high points and low points.

A crest is the high point of the wave.

A trough is the low point.
Frequency
 The
frequency of a wave is the rate at which
every point on the wave moves up and down.
 Frequency
means “how often”.
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Amplitude
 The
amplitude of a water wave is the maximum
height the wave rises above the level surface.
Wavelength
 Wavelength
is the distance from any point on a
wave to the same point on the next cycle of the
wave.
 The
distance between one crest and the next
crest is a wavelength.
The speed of waves
The speed of waves
 The
speed of a water wave is how fast the wave
spreads, NOT how fast the water surface moves
up and down or how fast the dropped ball moves
in the water.
A
wave moves one
wavelength in each cycle.
 Since
a cycle takes one
period, the speed of the
wave is the wavelength
divided by the period.
How do we measure the wave speed?
The speed of waves
 The
speed is the distance traveled (one
wavelength) divided by the time it takes (one
period).
 We
usually calculate the speed of a wave by
multiplying wavelength by frequency.
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Calculating wave speed
Cooking with waves
A wave has a wavelength of 0.5 meters, and its frequency is
40 hertz. What is the speed of the wave?
A
1.
Looking for: …speed of the wave.
 The
2.
Given: …wavelength (0.5 m) and frequency (40 Hz).
3.
Relationships: Use formula: speed = ƒ x 
4.
Solution: …speed = 40 Hz × 0.5 m = 40 1/s × 0.5 m
microwave heats food by
transferring wave energy to the
food.
magnetron is a device in a
microwave oven that creates a
wave with electricity.
 The
wave vibrates inside the
cooking space at 2.5
gigahertz- the frequency best
absorbed by water.
speed = 20 m/s
Standing waves on a string
Standing Waves and Harmonics
A
wave that is confined between boundaries is
called a standing wave.
 With all waves, resonance and natural frequency
are dependent on reflections from boundaries of
the system containing the wave.
Standing waves
 Standing
waves have nodes
and antinodes.
A
node is a point where the
string stays at its equilibrium
position.
 The
standing wave with the
longest wavelength is called the
fundamental.
 The fundamental has the lowest
frequency in a series of standing
waves called harmonics.
 The first five standing wave
patterns of a vibrating string
shows that patterns occur at
multiples of the fundamental
frequency.
Standing waves

It is easy to measure the
wavelength of a standing
wave on a string.
 Two
harmonics equals one
wave!
 An
antinode is a point where
the wave is as far as it gets
from equilibrium.
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Waves propagation
 Waves propagate,
which means they spread out
from where they begin.
 When you
drop a ball into water, some of the water
is pushed aside and raised by the ball.
Wave motion
A
wave front is the leading
edge of a moving wave which
is considered to be the crest
for purposes of modeling.
 The
crests of a plane wave
look like parallel lines.
 The
crests of a circular wave
are circles.
Four wave interactions

When a wave encounters a surface, four
interactions can occur:
1.
reflection,
2.
refraction,
3.
diffraction, or
4.
absorption.
Diffraction
Superposition principle
 Interference
 Diffraction usually
changes the direction
and shape of the wave.
 When a
plane wave
passes through a small
hole diffraction turns it
into a circular wave.
happens when two or more waves
mix together.
 When more
than one wave is present, the total
oscillation of any point is the sum of the
oscillations from each individual wave.
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Noise canceling headphones
 Specialized
headphones can
create “anti-noise.”
A
microphone in the headphone
samples the noise and generates
anti-noise, or sound that is 180
degrees out of phase with the
noise.
 The
anti-noise uses superposition
to reduce or muffle noise.
Constructive interference
 Constructive
interference happens when waves
add up to make a larger amplitude.
 Suppose
you make two wave pulses on a
stretched string.
 One
comes from the left and the other comes from
the right.
 When the
waves meet, they combine to make a
single large pulse.
Destructive interference
 What happens
when one pulse is on top of the
string and the other is on the bottom?
 When the
pulses meet in the middle, they cancel
each other out.
 During
destructive interference, waves add up
to make a wave with smaller or zero amplitude.
Resonance and light
A
wave has to be caught in a system with boundaries
to show resonance.
 Catch
light between two perfect mirrors and we can
get resonance of light waves.
 This
is exactly how a laser works!
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Resonance and elastic string
Waves and energy
 Resonance
 The
elastic strings is created by adding new
pulses so that each adds to the reflected pulse in
constructive interference.
energy of a wave is
proportional to its
frequency.
 Higher
frequency means
higher energy.
Waves that Shake the Ground
Waves and energy
 The
energy of a wave is
also proportional to its
amplitude.
 Given
two standing waves
of the same frequency,
the wave with the larger
amplitude has more
energy.

On January 12, 2010, a 7.0
magnitude earthquake struck the
Caribbean nation of Haiti.

Its capital, Port-au-Prince, was
nearly destroyed.

Many government buildings,
schools, hospitals, and
businesses collapsed.

The powerful earthquake was
caused by waves traveling
through Earth.
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