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 interference- occurs 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