Transcript Waves - compcolts

Chapter 20
• Waves – are oscillations that
move from one place to another.
• Waves are a traveling form of
energy.
• Waves are a series of high and
low points.
• Waves move information, energy
or motion over a distance without
anything obviously moving.
• Properties of waves are:
• Frequency
• Period of time
• Amplitude
• Wavelength
• Wave speed
• Types of Waves and how they travel:
• Transverse Wave- has oscillations
perpendicular (transverse) to the
direction the waves moves.
• Longitudinal Wave- has oscillations in
the same direction as the wave moves.
• Plane waves- wave crests form a
pattern of parallel lines, like ocean
waves.
• Circular waves- wave crests form a
pattern of circles
How do waves get started in the
ground?
• Pressure builds up in the
underground rocks causing them
to expand and contract.
• The rocks store energy.
• When the rocks break or change
shape, stored energy is suddenly
converted to ground shaking
energy (called earthquakes)
What is the result?
• Seismic waves radiate from
the place where the rocks
released the energy.
• Ground shaking lasts from
seconds to minutes.
• The longest earthquake lasted
4 minutes in 1964, in Alaska.
What are characteristics of
seismic waves?
• Travel 20x faster than speed
of sound.
• P (primary waves) are
longitudinal waves.
• S (secondary waves) are
transverse waves.
• Surface waves
Comparison of P & S Waves
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•
•
•
P Waves
Travel fast (4-6
km/s) so reach
earth’s surface first
Pull and push on
rocks as move
through earth’s
Low frequency
(0.05 Hz)
Longitudinal wave
S Waves
Slow in travel (3-4
km/s)
Move sideways and
up and down.
Transverse wave
What are surface waves?
• Surface waves- are the P and S waves that
reach the earth’s crust.
• Speed of surface waves are reduced by 10%.
They travel at 4 km/s.
• They are Rayleigh and Love waves.
• Rayleigh waves- rolls along the surface in a
circular motion, like an ocean wave. This is
most of the ground shaking we feel.
• Love waves- move the ground from side to
side.
What scientific instrument records
earthquakes?
• Seismograph
• It records the
amplitudes of the
recorded waves.
• These are related to
the magnitude of the
earthquake.
• Placed worldwide on
land, in oceans.
Where do most earthquakes
occur?
• On the tectonic plates- as
edges of the plates move
against each other, pressure
builds up and an earthquake
occurs.
• Common earthquake
locations?
What is a tsunami?
• Tsunami- a water wave that
occurs from an ocean floor
earthquake.
• Speed can be up to 700 km/hr.
• Amplitude is small in the ocean
and grows as it reaches shallower
areas…..where the wave can get
as high as 25 meters.
How tsunamis work
They start at a fault line in the ocean.
Wave Interactions- waves are
affected by boundaries (an edge or
surface) or different mediums (matter)
•
•
•
•
•
Absorption
Diffraction
Reflection
Refraction
Constructive Interference
• Destructive Interference.
Absorption
• What happens when the amplitude of
the wave gets smaller as it passes
through a material.
• It is absorbed and disappears.
• Ex. A sponge absorbs a water wave.
• Ex. Tinted sunglasses absorb light
waves.
Diffraction
• If a wave reaches an obstacle the
wave bends around the object or
passes through holes in the object.
• The direction and shape of the
wave is changed.
• Ex. hearing through a door open
only a tiny crack.
Reflection
• If a wave collides with an
obstacle and bounces off, it is
reflected,
it goes in a new direction.
- The wavelength and frequency
are not changed.
Ex. mirror , echo
Refraction
• If the wave passes from one
medium to another, as it changes
velocity it deflects or bends, a
phenomenon known as refraction.
• The wave bends as it passes into
and through an object.
• Ex. light through eyeglasses
Interference
• You see or hear a mixture of waves of
many different frequencies or amplitudes.
• Interference happens when 2 or more
waves mix together. Some can be useful
and some dangerous.
• Ex. radio and TV use the interference of 2
waves to carry music and video.
• Ex. water waves can add up to make one
gigantic wave
Constructive Interference
• Constructive interferenceoccurs when waves add up to
make a larger wave or amplitude.
• Ex. when 2 sound waves
interfere, loudness increases
Destructive Interference
• Destructive interference- is
when waves add up to make a
smaller wave or cancel each
other out. The result could be
a zero amplitude.
• Ex. noise cancelling
headphones use destructive
interference