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AP Physics B Summer Course 2012 2012 年 AP 物理 B 暑假班 M Sittig Ch 23: Waves

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Waves Waves are all around us. They are a form of simple harmonic motion.

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Transverse and Longitudinal

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Wave Terminology

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Wave Speed

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Velocity of a wave Wave speed (m/s) Wavelength (m) Frequency (1/s)

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Practice Problem A harmonic wave is traveling along a rope. It is observed that the oscillator that generates the wave completes 40.0 vibrations in 30.0 s. Also, a given maximum travels 425 cm along a rope in 10.0 s. What is the wavelength?

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Practice Problem The displacement-time graph and displacement- distance graph of a particular wave are shown below. What is the speed of the wave?

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Interference When two waves overlap, interference occurs. Superposition principle: When two or more waves occupy the same space at the same time, the displacements (+ and -) add at every point on the waves. The resultant displacement at any point is the sum of the separate displacements of the waves.

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Constructive Interference It occurs whenever two waves combine to make one wave with a larger amplitude.

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Constructive Interference

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Destructive Interference When the amplitudes of two waves combine producing a smaller amplitude. (The two waves need not cancel each other totally.)

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Destructive Interference

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Interference http://physics.ham.muohio.edu/waveapplet s/pulseadd.htm http://physics.ham.muohio.edu/waveapplet s/pulseadd.htm

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Interference AB DESTRUCTIVE CONSTRUCTIVE

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Interference AB DESTRUCTIVE CONSTRUCTIVE

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Standing Waves A standing wave is a wave that appears to be stationary, even though it is really two separate waves interfering as they pass through each other. It is due to superposition of incident and reflected waves of same frequency, amplitude and wavelength traveling in opposite directions.

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Standing Waves

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http://www.phys.unsw.edu.au/jw/module1 _nodes.swf http://www.phys.unsw.edu.au/jw/module1 _nodes.swf

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Standing Waves

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There are points where the displacement is always zero. These points are called nodes of the standing wave. Points which have the greatest amplitude are called the antinodes.

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Standing Waves

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Number of loops in standing wave depends on the vibration frequency. f 2f 3f 4f

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Standing Waves in Pipes Number of loops in standing wave depends on the vibration frequency. This FIRST SOUND is called the FUNDAMENTAL FREQUENCY or the FIRST HARMONIC.

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Standing Waves in Pipes Harmonics are MULTIPLES of the fundamental frequency In a closed pipe, you have a NODE at the 2nd harmonic position, therefore NO SOUND is produced

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Standing Waves in Pipes You will get your FIRST sound when the length of the pipe equals one-half of a wavelength.

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Example Problem The speed of sound waves in air is found to be 340 m/s. Determine the fundamental frequency (1st harmonic) of an open-end air column which has a length of 67.5 cm.

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Practice Problem The windpipe of a typical whooping crane is about 1.525-m long. What is the lowest resonant frequency of this pipe assuming it is a pipe closed at one end? Assume a speed of sound of 340 m/s.

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Doppler Effect Imagine a motorcycle or racecar, how the frequency changes (not volume).

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Electromagnetic Waves See EM Waves PPT.EM Waves PPT

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Double Slit See PPT: AP_Physics_B_-_Diffraction.pptAP_Physics_B_-_Diffraction.ppt

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Double Slit

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Single Slit

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Single Vs Double

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Double in Real Life

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Diffraction Gratings http://hyperphysics.phy- astr.gsu.edu/hbase/phyopt/mulslidi.html http://hyperphysics.phy- astr.gsu.edu/hbase/phyopt/mulslidi.html

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Review See PPT: PPA6_ConcepTests_Ch_24.pptPPA6_ConcepTests_Ch_24.ppt

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Example Problem A viewing screen is separated from a double slit source by 1.2 m. The distance between the two slits is 0.030 mm. The second -order bright fringe (m=2) is 4.5 cm from the central maximum. Determine the wavelength of light.

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Index of Refraction n, a number that describes how much light slows down (and bends) when it passes through a material.

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Thin Films

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Thin films 1. Count the phase changes. 2. The extra distance traveled in the material is a round trip (2t). 3. The wavelength in the material is λ n =λ/n. 4. Apply 2t = mλ n. If phase changes is even (0, 2…) then integers for m means constructive interference, half-integers means destructive. If phase changes is odd (1, 3…) then it is the reverse.

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Example/Practice Problem A thin film of material is to be tested on the glass sheet for use in making reflective coatings. The film has an index of refraction n f = 1.38. White light is incident normal to the surface of the film as shown below. It is observed that at a point where the light is incident on the film, light reflected from the surface appears green ( = 525 nm). a. What is the frequency of the green light in air? b. What is the frequency of the green light in the film? c. What is the wavelength of the green light in the film? d. Calculate the minimum thickness of film that would produce this green reflection.

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