Waves

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Waves

  1. 1. Waves
  2. 2. Slinky Waves A wave is caused by a vibrating source, and travels outwards from the source. The particles of the medium through which a wave travels move about their equilibrium positions; they do not move along with the wave. Energy is transferred from place to place by a wave.
  3. 3. To think about <ul><li>Does the size of a pulse change as it travels along the spring?  </li></ul><ul><li>What is there was no friction? </li></ul><ul><li>What happens when a pulse reflects at the fixed end? (we need to clamp the slinky at one end) </li></ul><ul><li>How does the speed change if you change the tension? (does not work well, should be the same) </li></ul><ul><li>What happens if pulses set off from opposite ends at the same time? </li></ul>
  4. 4. What you are going to achieve today <ul><li>By the end of the lesson you will know </li></ul><ul><li>What are the differences between transverse and longitudinal waves? </li></ul><ul><li>What is a plane-polarised wave? </li></ul><ul><li>What physical test can distinguish transverse waves from longitudinal waves? </li></ul><ul><li>What is meant by the amplitude of a wave? </li></ul><ul><li>Between which two points can the wavelength be measured? </li></ul><ul><li>How is the frequency of a wave calculated from its period? </li></ul><ul><li>What is phase? </li></ul>
  5. 5. Important wave diagram
  6. 6. Radians and waves
  7. 7. Phase Dual pendulums to demo phase
  8. 8. Phase Demo
  9. 9. Phase on the oscilloscope
  10. 10. V = f λ <ul><li>V = velocity (m/s) </li></ul><ul><li>f = frequency (how many waves in a second) </li></ul><ul><li>λ = wavelength (m) </li></ul>λ c = f λ is used for electromagnetic waves as the velocity of light is always 3 x 10 8 m/s
  11. 11. All go at the speed of light
  12. 12. Speed, wavelength and frequency <ul><li>1)Light travels 300,000 km in one second. </li></ul><ul><li>a) How long does it take light to travel 300 m? </li></ul><ul><li>  b) How long does it take light to travel 300 mm? </li></ul><ul><li>2)The speed of sound in a metal is 1000 m s–1. </li></ul><ul><li>a.) What is the wavelength if the frequency is 2 kHz?   </li></ul><ul><li>  b.) What is the wavelength if the frequency is 2 MHz? </li></ul><ul><li>3)What is the frequency of sound waves of wavelength 100 mm? </li></ul><ul><li>4)What is the frequency of radio waves of wavelength 100 mm? </li></ul><ul><li>5)If you could 'freeze' ultrasound of wavelength 5 mm as it travels through tissue, how many waves would there be in a one metre length? This quantity is called the wave number . How is it related to wavelength? </li></ul><ul><li>6)If a thunder clap comes 1.5 s after the lightning strike, how far away is the strike? Explain your calculation. </li></ul>The speed of sound in air 330 m/s speed of light 3 x 10 8 m/s. s = v ∆ t
  13. 13. Answers <ul><li>1 microsecond b) 1 nanosecond </li></ul><ul><li>200 waves per metre; the wave number is the reciprocal of the wavelength. </li></ul>6) s = v ∆ t = 330 x 1.5 = 495 m, light transit time is negligible
  14. 14. Polarisation Polarisation can only happen with transverse waves http://www.youtube.com/watch?v=qlRriWu1zOI&feature=related http://www.youtube.com/watch?v=ozTH2zIKwC0&feature=player_embedded
  15. 15. 3D glasses
  16. 16. The water tubs waves in another medium <ul><li>The speed of a water wave </li></ul><ul><li>Create a water wave at one end of the tub and </li></ul><ul><li>Time it to the other end </li></ul><ul><li>Change the depth of water </li></ul><ul><li>There should be a difference but it is very fast </li></ul><ul><li>Speed = √g x depth (metres) </li></ul><ul><li>Wave height   </li></ul><ul><li>(from trough to crest), </li></ul><ul><li>Wavelength (from crest to crest), </li></ul><ul><li>Period (time interval between arrival of consecutive crests at a stationary point), </li></ul><ul><li>The direction of wave propagation. </li></ul>
  17. 17. CHALLENGE! Demonstrate longitudinal and transverse waves In a group of 5 demo both types of wave

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