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Chapter 20
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• 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.
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• Properties of waves are:
• Frequency
• Period of time
• Amplitude
• Wavelength
• Wave speed
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• 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
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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)
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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.
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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
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Comparison of P & S Waves
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
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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.
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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.
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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?
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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.
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How tsunamis workThey start at a fault line in the ocean.
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Wave Interactions- waves are affected by boundaries (an edge or
surface) or different mediums (matter)
• Absorption
• Diffraction
• Reflection
• Refraction
• Constructive Interference
• Destructive Interference.
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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.
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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.
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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
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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
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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
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Constructive Interference
• Constructive interference- occurs when waves add up to make a larger wave or amplitude.
• Ex. when 2 sound waves interfere, loudness increases
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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
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