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

## Presentation on theme: "AP Physics B Summer Course 2012 2012 年 AP 物理 B 暑假班 M Sittig Ch 23: Waves."— Presentation transcript:

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

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

Transverse and Longitudinal

Wave Terminology

Wave Speed

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

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?

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

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.

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

Constructive Interference

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

Destructive Interference

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

Interference AB DESTRUCTIVE CONSTRUCTIVE

Interference AB DESTRUCTIVE CONSTRUCTIVE

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.

Standing Waves

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

Standing Waves

 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.

Standing Waves

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

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.

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

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

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.

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.

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

Electromagnetic Waves  See EM Waves PPT.EM Waves PPT

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

Double Slit

Single Slit

Single Vs Double

Double in Real Life

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

Review  See PPT: PPA6_ConcepTests_Ch_24.pptPPA6_ConcepTests_Ch_24.ppt

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.

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

Thin Films

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.

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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