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