chapter 20. waves – are oscillations that move from one place to another. waves are a traveling...

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 workThey 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