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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 • • • • 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