Physics 5.4Physics 5.4

WavesWaves

A vibratory disturbance that moves A vibratory disturbance that moves through a medium.through a medium.

Waves can be one pulse or a periodic Waves can be one pulse or a periodic wave (which is just a pulse repeated wave (which is just a pulse repeated many times!)many times!)

Which diagram does Which diagram does not not represent a periodic represent a periodic wave?wave?

1. 2.1. 2.

3. 4.3. 4.

Answer: Answer: 44

4

More wave stuff…More wave stuff…

Only energy is transferred by a wave, no Only energy is transferred by a wave, no mass!mass!

Mechanical waves require a medium to Mechanical waves require a medium to pass through, where as an pass through, where as an electromagnetic wave can pass through electromagnetic wave can pass through a vacuum. a vacuum.

Longitudinal waves:Longitudinal waves:

Ex/ sound waves.Ex/ sound waves. The particles vibrate parallel to the The particles vibrate parallel to the

motion of the wave.motion of the wave.

Transverse waves:Transverse waves:

Ex/ light waves.Ex/ light waves. The particles vibrate perpendicular to the The particles vibrate perpendicular to the

motion of the wave. motion of the wave.

AmplitudeAmplitude

Amplitude (Amplitude (AA)): the maximum height of : the maximum height of the wave measured from the average the wave measured from the average height of the wave (the wave’s center). height of the wave (the wave’s center). Unit: meters (m) Unit: meters (m)

WavelengthWavelength

Wavelength (Wavelength (λλ)): The length of one wave, : The length of one wave, or the distance from a point on one wave or the distance from a point on one wave to the same point on the next wave. to the same point on the next wave. Units: meters (m). In light, Units: meters (m). In light, λλ tells us the tells us the color.color.

FrequencyFrequency

Frequency of oscillation (Frequency of oscillation (ff)) (or just (or just frequencyfrequency): the number of times the ): the number of times the wave pattern repeats itself in one wave pattern repeats itself in one second. Units: seconds-1 = (1/s) = hertz second. Units: seconds-1 = (1/s) = hertz (Hz) In sound, (Hz) In sound, ff tells us the pitch. The tells us the pitch. The inverse of frequency is the period of inverse of frequency is the period of oscillation.oscillation.

PeriodPeriod

Period of oscillation (Period of oscillation (TT)) (or just (or just periodperiod): ): duration of time between one wave and duration of time between one wave and the next one passing the same spot. the next one passing the same spot. Units: seconds (s). The inverse of the Units: seconds (s). The inverse of the period is frequency. Use a capital, italic period is frequency. Use a capital, italic TT and not a lowercase one, which is used and not a lowercase one, which is used for time.for time.

Lets take a look:Lets take a look:

Wave SpeedWave Speed

The speed of the wave is denoted by The speed of the wave is denoted by velocity velocity (v)(v) equals the wavelength equals the wavelength ((λλ)) times the frequency times the frequency (f).(f).

v = λfv = λf

Compared to the speed of microwaves in a Compared to the speed of microwaves in a vacuum, the speed of x-rays in a vacuum is:vacuum, the speed of x-rays in a vacuum is:

1.1. less less

2.2. greater greater

3.3. the same the same Answer:Answer:3 3

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A student plucks a guitar string and A student plucks a guitar string and the vibrations produce a sound wave the vibrations produce a sound wave with a frequency of 650 hertz. with a frequency of 650 hertz. Calculate the wavelength of the sound Calculate the wavelength of the sound wave in air at STP. Round your answer wave in air at STP. Round your answer to the nearest hundredth of a meter.to the nearest hundredth of a meter.

λ = 0.51 mλ = 0.51 m

Wave Fronts:Wave Fronts: Flat wave fronts become spherical after Flat wave fronts become spherical after

passing through a lens.passing through a lens.

The Doppler effectThe Doppler effect

The Doppler effect is the perceived The Doppler effect is the perceived change in frequency of sound emitted by change in frequency of sound emitted by a source moving relative to the observer: a source moving relative to the observer: as a plane flies overhead, the note of the as a plane flies overhead, the note of the engine becomes noticeably lower, as engine becomes noticeably lower, as does the siren noise from a fast-moving does the siren noise from a fast-moving emergency vehicle as it passes. emergency vehicle as it passes. 

more Doppler effect…more Doppler effect…

An observer detects an apparent change in the An observer detects an apparent change in the frequency of sound waves produced by an frequency of sound waves produced by an airplane passing overhead. This phenomenon airplane passing overhead. This phenomenon illustrates:illustrates:

1.1. the Doppler effect the Doppler effect

2.2. the refraction of sound waves the refraction of sound waves

3.3. wave amplitude increase wave amplitude increase

4.4. wave intensity increasewave intensity increase AnswerAnswer11

Interference from Interference from superposition:superposition:

Constructive interference is when two waves Constructive interference is when two waves are in phase and the resultant wave is the sum are in phase and the resultant wave is the sum of the two waves. Two waves 0of the two waves. Two waves 0˚ in phase ˚ in phase create max constructive interference.create max constructive interference.

Destructive interference is when two waves are Destructive interference is when two waves are out of phase and resultant wave is smaller than out of phase and resultant wave is smaller than both. Two waves 180both. Two waves 180˚ out of phase create max ˚ out of phase create max destructive interference.destructive interference.

The diagram shows two pulses of equal amplitude, The diagram shows two pulses of equal amplitude, AA, , approaching point approaching point P P along a uniform string.along a uniform string.

When the two pulses meet at When the two pulses meet at PP, the vertical , the vertical displacement of the string at displacement of the string at P P will bewill be

AA 22AA 0 0 A/2A/2 Answer:Answer:3 3

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Radio waves diffract around buildings Radio waves diffract around buildings more than light waves do because, more than light waves do because, compared to light waves, radio wavescompared to light waves, radio waves

1.1. move faster move faster 2.2. move slower move slower 3.3. have a higher frequency have a higher frequency 4.4. have a longer wavelengthhave a longer wavelength AnswerAnswer4 4

Standing WaveStanding Wave

Standing waves are produced whenever Standing waves are produced whenever two waves of identical frequency interfere two waves of identical frequency interfere with one another while traveling opposite with one another while traveling opposite directions along the same medium. directions along the same medium.

Node and AntinodesNode and Antinodes

Standing wave patterns are characterized by Standing wave patterns are characterized by certain fixed points along the medium which certain fixed points along the medium which undergo no displacement. These points of no undergo no displacement. These points of no displacement are called displacement are called nodesnodes (nodes can be (nodes can be remembered as points of remembered as points of no displacementno displacement). ).

Midway between every consecutive nodal point Midway between every consecutive nodal point are points which undergo maximum are points which undergo maximum displacement. These points are called displacement. These points are called antinodes.antinodes.

A Standing Wave:A Standing Wave:

One end of a rope is attached to a variable speed drill One end of a rope is attached to a variable speed drill and the other end is attached to a 5.0-kilogram mass. and the other end is attached to a 5.0-kilogram mass. The rope is draped over a hook on a wall opposite the The rope is draped over a hook on a wall opposite the drill. When the drill rotates at a frequency of 20.0 Hz, drill. When the drill rotates at a frequency of 20.0 Hz, standing waves of the same frequency are set up in the standing waves of the same frequency are set up in the rope. The diagram below shows such a wave pattern.rope. The diagram below shows such a wave pattern.

Calculate the speed of the wave in the rope.Calculate the speed of the wave in the rope. Answer: Answer: 60.60. m/s m/s

ResonanceResonance

In sound applications, a resonant In sound applications, a resonant frequency is a natural frequency of frequency is a natural frequency of vibration determined by the physical vibration determined by the physical parameters of the vibrating object. parameters of the vibrating object.

A tuning fork is struck and vibrates at a A tuning fork is struck and vibrates at a frequency of 512 Hz, and causes a frequency of 512 Hz, and causes a second tuning fork (untouched) to vibrate second tuning fork (untouched) to vibrate at a frequency of 512 Hz.at a frequency of 512 Hz.

DiffractionDiffraction

Diffraction manifests itself in the apparent Diffraction manifests itself in the apparent bending of waves around small obstacles bending of waves around small obstacles and the spreading out of waves past and the spreading out of waves past small openings. small openings.

If the opening is larger than the If the opening is larger than the wavelength of light, than there will be no wavelength of light, than there will be no diffraction. Why?diffraction. Why?

A beam of monochromatic light approaches a barrier A beam of monochromatic light approaches a barrier having four openings, having four openings, AA, , BB, , CC, and , and DD, of different , of different sizes as shown below.sizes as shown below.

Which opening will cause the greatest diffraction?Which opening will cause the greatest diffraction? A A B B CC DD

Answer:Answer:11

RefractionRefraction

The bending of light when it goes from The bending of light when it goes from one medium (example: air) to a different one medium (example: air) to a different medium (example: water) is called medium (example: water) is called refraction.refraction.

The diagram shows a ray of light,The diagram shows a ray of light, R R , entering glass , entering glass from air.from air.

Which path is the ray most likely to follow in the glass?Which path is the ray most likely to follow in the glass?1.1. AA 2.2. BB 3.3. CC 4.4. DD AnswerAnswer3 3