1. Two ways to get in touch with a friend in adistant city are to write a letter and to useTelephone.2. The first choice (the letter) involves aconcept of particle.3. A material object moves from one point toanother, carrying with it information andenergy.4. The second choice (the Telephone)involves the concept of wave.Wave motion
5. In a wave, information and energy move fromone point to another but no material object makesthat journey.6. In your Telephone call, a sound carries yourmessage from your vocal cords to the Telephonealthough the message is passed nothing that youhave touched reaches your friend7. In Telephone an electromagnetic wave takesover, passing along a copper wire or an optical fiberare through the atmosphere, possibly by way of acommunications satellite.8. Particle and wave are two great concepts inclassical physics.
What is a wave?A wave is a disturbance in a medium that travelsoutward from its source.It travels from one place to another by means ofa medium, but the medium itself is nottransported.All material media- solids, liquids, and gases-can carry energy and information by means ofwaves.There are many types of wave.All types of waves use similar mathematicaldescriptions.
5Waves are everywhere innatureSound waves,visible lightwaves,radio waves,microwaves,water waves,sine waves,telephone chordwaves,stadium waves,earthquakewaves,waves on astring,slinky waves
6What is a wave?a wave is a disturbance that travelsthrough a medium from one location toanother.a wave is the motion of a disturbance
7Slinky WaveLet’s use a slinky wave as an example.When the slinky is stretched from end toend and is held at rest, it assumes anatural position known as theequilibrium or rest position.To introduce a wave here we must firstcreate a disturbance.We must move a particle away from itsrest position.
8Slinky WaveOne way to do this is to jerk the slinky forwardthe beginning of the slinky moves away from itsequilibrium position and then back.the disturbance continues down the slinky.this disturbance that moves down the slinky iscalled a pulse.if we keep “pulsing” the slinky back and forth,we could get a repeating disturbance.
9Slinky WaveThis disturbance would look something like thisThis type of wave is called a LONGITUDINAL wave.The pulse is transferred through the medium of theslinky, but the slinky itself does not actually move.It just displaces from its rest position and thenreturns to it.So what really is being transferred?
10Slinky WaveEnergy is being transferred.The metal of the slinky is the MEDIUM in thattransfers the energy pulse of the wave.The medium ends up in the same place as itstarted … it just gets disturbed and then returnsto it rest position.The same can be seen with a stadium wave.
11Longitudinal WaveThe wave we see here is a longitudinal wave.The medium particles vibrate parallel to themotion of the pulse.This is the same type of wave that we use totransfer sound.Can you figure out how??
12Transverse wavesA second type of wave is a transversewave.We said in a longitudinal wave the pulsetravels in a direction parallel to thedisturbance.In a transverse wave the pulse travelsperpendicular to the disturbance.
13Transverse WavesThe differences between the two can beseen
14Transverse WavesTransverse waves occur when we wigglethe slinky back and forth.They also occur when the sourcedisturbance follows a periodic motion.A spring or a pendulum can accomplishthis.The wave formed here is a SINE wave.
15Anatomy of a WaveNow we can begin to describe theanatomy of our waves.We will use a transverse wave to describethis since it is easier to see the pieces.
16Anatomy of a WaveIn our wave here the dashed line represents theequilibrium position.Once the medium is disturbed, it moves awayfrom this position and then returns to it
17Anatomy of a WaveThe points A and F are called the CRESTSof the wave.This is the point where the wave exhibitsthe maximum amount of positive orupwards displacementcrest
18Anatomy of a WaveThe points D and I are called theTROUGHS of the wave.These are the points where the waveexhibits its maximum negative ordownward displacement.trough
19Anatomy of a WaveThe distance between the dashed line andpoint A is called the Amplitude of the wave.This is the maximum displacement that thewave moves away from its equilibrium.Amplitude
20Anatomy of a WaveThe distance between two consecutive similarpoints (in this case two crests) is called thewavelength.This is the length of the wave pulse.Between what other points is can a wavelength bemeasured?wavelength
21Anatomy of a WaveWhat else can we determine?We know that things that repeat have afrequency and a period. How could wefind a frequency and a period of awave?
22Wave frequencyWe know that frequency measure howoften something happens over a certainamount of time.We can measure how many times a pulsepasses a fixed point over a given amountof time, and this will give us thefrequency.
23Wave frequencySuppose I wiggle a slinky back and forth,and count that 6 waves pass a point in 2seconds. What would the frequency be?3 cycles / second3 Hzwe use the term Hertz (Hz) to stand forcycles per second.
24Wave PeriodThe period describes the same thing as itdid with a pendulum.It is the time it takes for one cycle tocomplete.It also is the reciprocal of the frequency.T = 1 / ff = 1 / T
25Wave SpeedWe can use what we know to determinehow fast a wave is moving.What is the formula for velocity?velocity = distance / timeWhat distance do we know about a wavewavelengthand what time do we knowperiod
26Wave Speedso if we plug these in we getvelocity =length of pulse /time for pulse to move pass a fixed pointv = λ / Twe will use the symbol λ to representwavelength
27Wave Speedv = λ / Tbut what does T equalT = 1 / fso we can also writev = f λvelocity = frequency * wavelengthThis is known as the wave equation.
28Wave BehaviorNow we know all about waves.How to describe them, measure them andanalyze them.But how do they interact?
29Wave BehaviorWe know that waves travel throughmediums.But what happens when that medium runsout?
30Boundary BehaviorThe behavior of a wave when it reachesthe end of its medium is called the wave’sBOUNDARY BEHAVIOR.When one medium ends and anotherbegins, that is called a boundary.
31Fixed EndOne type of boundary that a wave mayencounter is that it may be attached to afixed end.In this case, the end of the medium willnot be able to move.What is going to happen if a wave pulsegoes down this string and encounters thefixed end?
32Fixed EndHere the incident pulse is an upwardpulse.The reflected pulse is upside-down. It isinverted.The reflected pulse has the same speed,wavelength, and amplitude as theincident pulse.
34Free EndAnother boundary type is when a wave’smedium is attached to a stationary objectas a free end.In this situation, the end of the medium isallowed to slide up and down.What would happen in this case?
35Free EndHere the reflected pulse is not inverted.It is identical to the incident pulse, exceptit is moving in the opposite direction.The speed, wavelength, and amplitudeare the same as the incident pulse.
37Change in MediumOur third boundary condition is when themedium of a wave changes.Think of a thin rope attached to a thinrope. The point where the two ropes areattached is the boundary.At this point, a wave pulse will transferfrom one medium to another.What will happen here?
38Change in MediumIn this situation part of the wave is reflected,and part of the wave is transmitted.Part of the wave energy is transferred to themore dense medium, and part is reflected.The transmitted pulse is upright, while thereflected pulse is inverted.
39Change in MediumThe speed and wavelength of thereflected wave remain the same, but theamplitude decreases.The speed, wavelength, and amplitude ofthe transmitted pulse are all smaller thanin the incident pulse.
41Wave InteractionAll we have left to discover is how wavesinteract with each other.When two waves meet while travelingalong the same medium it is calledINTERFERENCE.
42Constructive InterferenceLet’s consider two waves moving towardseach other, both having a positiveupward amplitude.What will happen when they meet?
43Constructive InterferenceThey will ADD together to produce agreater amplitude.This is known as CONSTRUCTIVEINTERFERENCE.
44Destructive InterferenceNow let’s consider the opposite, twowaves moving towards each other, onehaving a positive (upward) and one anegative (downward) amplitude.What will happen when they meet?
45Destructive InterferenceThis time when they add together theywill produce a smaller amplitude.This is know as DESTRUCTIVEINTERFERENCE.
46Check Your Understanding Which points will produce constructive interferenceand which will produce destructive interference? ConstructiveG, J, M, N DestructiveH, I, K, L, O