Diffraction
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Diffraction. 17.3. Diffraction. What happens when a wave encounters a barrier The wave curves around it. Diffraction. As the wave moves through the air, each molecule vibrates Thus each molecule creates it own little wave The wave radiates away from the molecule forming circular ripples

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Diffraction

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Diffraction

Diffraction

17.3


Diffraction1

Diffraction

  • What happens when a wave encounters a barrier

  • The wave curves around it


Diffraction2

Diffraction

  • As the wave moves through the air, each molecule vibrates

  • Thus each molecule creates it own little wave

  • The wave radiates away from the molecule forming circular ripples

  • The circular ripples create the bending effect


Single slit diffraction

Single Slit Diffraction

  • Where

    •  = angle of first minimum

    •  = wavelength of wave

    • D = width of slit


Circular opening diffraction

Circular Opening Diffraction

  • Where

    •  = angle of first minimum

    •  = wavelength of wave

    • D = width of slit


Think about this

Think About This

  • According to the formula

    • The angle depends only on the ratio of the wavelength to the size of the opening

    • Bigger wavelength  bigger angle

    • Which has a bigger wavelength; AM (kHz) or FM (MHz)?

      • AM (that is why big cities have AM stations, they bend around the tall buildings easier


Example

Example

  • Why are high pitched speakers smaller than low pitched subwoofers?

  • Lets say that the same sized (25 cm diameter) speakers produce two sounds, 1715 Hz and 10290 Hz. What are the angles of the first minimum?

    • Low sound 1715 Hz  9.36°

    • High sound 10290 Hz  77.42°

    • Person standing to side of speakers only hears low pitches

    • To make high pitches bend more, decrease size of opening


Practice problems

Practice Problems

  • Try bending these problems

  • 506 CQ 4 – 5, P 10 – 13

  • Total of 6 Problems


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