WavesWaves

WavesWaves• Disturbance that carries energy through matter or space

• The matter through which a wave travels is called a medium

• Waves spread out in circles (wave front) that get bigger as the waves move farther from the center  

Wave front

energy most concentrated

energy most spread out

Energy in the form of Energy in the form of WavesWaves

Essentially energy is being transferred through waves

Two main types of waves: 1. Electromagnetic

– “no stuff needed”-Particles– Ex: Light

2. Mechanical– “stuff needed”-Particles– Ex: Sound

Room ExampleRoom Example

Pretend we take all of the air out of this room...

Besides exploding, what would happen if we tried to conduct class?

The speed of waves depends on the medium through which they travel.

• Sound waves travel faster in solids than in gas or in liquid.

• Light waves travel slower when they have to pass through solids, gases or liquids.

Waves that require a medium are called mechanical waves.

Most waves are mechanical with the exception of light waves – which make up the electromagnetic

spectrum.

http://aspire.cosmic-ray.org//labs/seismic/

Wave MovementWave Movement

Types of WavesTypes of Waves

1. Transverse • motion of particles is perpendicular to motion of wave

Ex. Rope

2. Longitudinal • motion of particles is parallel to motion of wave

Ex. Slinky

UShttp://aspire.cosmic-ray.org//labs/seismic/

Types of WavesTypes of Waves3. Surface •motion of particles move both parallel &

perpendicular producing a circular motion

Ex. Water waves

Wave CharacteristicsWave Characteristics1. Crest highest point

2. Trough lowest point

3. Amplitude half the distance between crest & trough

4. Wavelength (lambda) – distance from one crest/trough to next crest/trough; measured in meters (m)

5. Frequency (f) - # of wavelengths that pass a point in a given time interval; measured in Hertz (Hz)

http://aspire.cosmic-ray.org//labs/seismic/long_waves.swf

Period vs. FrequencyPeriod vs. Frequency

Period– # of seconds it takes

for one wave to pass

Frequency– # of waves that pass

by in one second

Sound WavesSound Waves

Properties of Sound Waves: Are longitudinal waves, like that of a stretched spring

Caused by vibrations

Spread in all directions from the source

Like the coils in a slinky, some areas are compressed, while others are stretched

These areas are called

CompressionsCompressions and

RarefactionsRarefactions

Speed of SoundSpeed of Sound Depends on the material of the vibrating

medium Sound can vibrate water, wood (speaker

enclosures, pianos), metal, plastic, etc. Sound speed in dry air is 330 meters/second

at 0o C Faster in warm air, slower in cold Water 4 times faster, steel 15 times faster

EVERYONE STAND UP AND LINE THE BACK OF THE ROOM

Sound WavesSound Waves

Sound WavesSound WavesRelative intensity of sound is measured in decibels, dB

Loudness is perceived on a logarithmic scale

Molecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of sound the pitch.

Sound WavesSound WavesPitchPitch is determined by frequencyfrequency : High-pitched sounds have high frequencies

Low-pitched sounds have low frequencies

InfrasoundInfrasound - Sound waves with frequencies lower than 20 Hz

UltrasoundUltrasound - Sound waves with frequencies higher than 20,000 Hz

Go to EMS, Wave Basics 2 and Wave Behaviors

Sound WavesSound WavesResonance occurs when vibrations of one object cause the vibration of

another object at a natural frequency

Tacoma Narrows Bridge Disaster aka “Galloping Gertie”

Tacoma Narrow Bridge Disaster Video

Sound WavesSound WavesHow do humans hear?

Our eardrums act somewhat like normal drums, only in a reversed fashion

How do bats hear?

Bats use ultrasonic sonar location

Bats use reflected sound waves to determine the distance to, and location of, objects

Sound WavesSound WavesUltrasound is also used in medicine

This method is known as Ultrasonography and we see a picture known as a SonogramSonogram

Doppler EffectDoppler Effectan observed change in the frequency of a wave when an observed change in the frequency of a wave when the source or observer is movingthe source or observer is moving

http://www.explorelearning.com/index.cfm?method=cResource.dspView&ResourceID=19

Wave SpeedWave SpeedWave speed is equal to the frequency

multiplied by the wavelength; v = f (lambda)

Sample Problems:

1. The average wavelength of ocean waves is 15.0 m and the frequency is 0.10 Hz. What is the average speed of the waves?

2. A wave along a guitar string has a frequency of 440 Hz and travels at a speed of 660 m/s. What is the wavelength?

3. The speed of sound in air is about 340 m/s.What is the frequency of a sound wave thatHas a wavelength of 1.50 m?

significant figures practice site

Electromagnetic Spectrum

Wave InteractionsWave Interactions Occur when waves meet objects or other waves in a medium

Reflection the bouncing back of a wave as it meets a surface or boundary

Ex. Light waves striking a shiny surface

Reflection at a Free Boundary the reflected wave is exactly like the original wave

Ex. A

US

Wave InteractionsWave Interactions Reflection at a Fixed Boundary

the reflected wave is like the original wave, but is turned upside down and travels in the opposite direction

follows Newton’s Third Law…Equal but opposite force is exerted

Ex. B

DiffractionDiffraction the bending of a wave as it passes an edge or an opening

Ex. Hearing people speak when standing outside the doorway of a room (picture B/A)

Picture A/B demonstrates waves bending and spreading out into the space behind the block

Picture B/A demonstrates waves bending around two edges and spreading out as they pass through the opening

RefractionRefraction the bending of waves as they pass from one medium to another

Ex. a spoon in a glass of water

the top of the spoon handle has light waves passing directly from the spoon through the air to your eyes

the rest of the spoon has light waves passing through the water, then the glass, and finally to your eyes (each time bending slightly)

InterferenceInterference the combination of two or more waves that exist in the same place at the

same time

these waves combine to produce a single, new wave that is different than the original

Ex. water waves passing through each other

Constructive InterferenceConstructive Interference

Destructive InterferenceDestructive Interference

any interference in which waves combine so that the resulting wave is bigger than the original waves

any interference in which waves combine so that the resulting wave is smaller than the largest of the original waves

Interference of Light wavesInterference of Light wavesmay produce colorful displays

Ex. soap bubbles

two sets of light waves reach our eyes

1. those that bounce off the outside of the bubble

2. those that bounce off the inside of the bubble

Interference of Sound WavesInterference of Sound Waves   produce beats

• if sound wave frequencies are different, their compressions arrive at your ear at different rates

Standing WavesStanding Wavesa wave form caused by interference that appears not to move along the medium and that shows some regions of no vibration (nodesnodes) and other

regions of maximum vibration (antinodesantinodes)

• cause the medium to vibrate in a loop or in a series of loops

Nodes – lie @ points where the crests of the original waves meet the troughs of the reflected waves

Antinodes – form where the crests of the original waves line up with the

troughs of the reflected waves

Standing Waves (continued)Standing Waves (continued)

can only have certain wavelengths

can exist whenever a multiple of half-wavelengths will fit exactly in the length

of the string

Musical instruments rely on standing waves

The End