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Waves

Waves. A “wiggle” or “oscillation” or “vibration” produces a Wave. Types of Waves. Mechanical Waves Require a material through which to travel- a “material medium” Examples: water, rope, sound, slinky. Electromagnetic Waves

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Waves

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  1. Waves A “wiggle” or “oscillation” or “vibration” produces a Wave

  2. Types of Waves Mechanical Waves Require a material through which to travel- a “material medium” Examples: water, rope, sound, slinky

  3. Electromagnetic Waves They can travel through empty space- a vacuum- they don’t require a material medium. Examples: x-rays, UV, visible light, infrared, … In a vacuum, they all travel at the same speed— The “speed of light” This speed is constant and is called “c”. c = 3 x 108 m/s

  4. Wave Motion Transverse Waves The wave disturbance is PERPENDICULAR to the direction of the wave’s velocity. “Crest”, the peak of the wave “Trough”, the valley of the wave “Equilibrium” line

  5. Longitudinal Wave (compression wave) The wave disturbance is PARALLEL to the direction of the wave’s velocity.

  6. How does this motion make a “wave”??

  7. Sound is a longitudinal wave. * • Molecules move parallel to the direction of the waves velocity. • Areas of high pressure and low pressure • “compression” and “rarefaction”

  8. Wave pulse- one disturbance

  9. Polarized waves If there are many waves and ALL the waves are vibrating in the same plane, they are said to be “polarized”

  10. Measurements Wavelength, l Distance between points where the wave pattern repeats- Measured in meters

  11. Amplitude, A Maximum distance above or below equilibrium- Measured in meters As the amplitude increases, the energy the wave transmits also increases.

  12. Period, T Shortest time interval during which the pattern repeats--- measured in seconds Frequency, f The number of waves per second-- Measured in Hz f = 1 / T and T = 1 / f Example: While watching waves go by a pier, you count 6 waves every 12 seconds. What is the frequency and period of the waves? f = 6 waves / 12 s = 0.5 Hz T = 1 / f = 2 s

  13. Velocity, v The velocity of a wave depends on what kind of material through which it is traveling. For example, ALL sound waves, regardless of their pitch, travel at the same speed through air and at the same speed through water. But the speed in water is faster than the speed in air!

  14. The velocity of a wave depends on the medium through which is travels. If you know some things about the medium, you can find the velocity by “Modulus”- a characteristic of different substances Bulk modulus- fluids Elastic modulus- solids

  15. Velocity, v You can find that speed if you know both the wave’s period and its wavelength: Velocity = Distance / time = l/T, so v = l/T but since frequency, f = 1/T, v = lf

  16. Water Wave “Surface” water waves are combinations of transverse and longitudinal waves.

  17. Waves transmit energy without transmitting matter. Most waves move through a substancebut only move it backwards and forwards (longitudinal)or up and down (transverse) while the wave passes.After the wave has gone, the substance is back where it started but energyhas been carried by the wave from its origin (where it begins) to its destination (where it finishes).

  18. Get a calculator!!!

  19. Period = time per wave Frequency = waves per time If you observe 3 waves go by every 9 seconds, what is the period of the wave? The frequency? T = 3 s f = 1/3 Hz

  20. A sound wave moves at about 343 m/s through air. The musical note “C” has a frequency of 128 Hz. What is its wavelength in air? v = lf l= v/f = 343 m/s / 128 Hz l = 2.7 m

  21. What is the frequency of red light, whose wavelength is 620 nm? nm = 10-9 m Velocity = c = 3 x 108 m/s v = lf f = c / l f = 3E8 / 620E-9 f = 4.8E14 Hz

  22. v = lf Two different sound waves are produced by different band instruments- 512 Hz and 106 Hz. Which has a longer wavelength? Since the velocity is the same for both, the smaller frequency has the larger wavelength!

  23. Behavior of Waves

  24. Behavior of All Waves Reflect: To bounce back from a surface Law of Reflection: The angle of reflection is equal to the angle of incidence.

  25. Refraction: The change in direction as a wave passes from one medium into another. Particles ALSO refract when they enter a new medium!

  26. Diffraction: The curving of a wave around boundaries or barriers or through small openings. This behavior is unique to waves. Particles do NOT diffract (spread out, curve)

  27. Simulations More simulations

  28. What happens to a wave when…. …the medium through which it travels changes? If the medium changes, the velocity changes! (as well as the wavelength) … and the wave REFRACTS!

  29. What happens to a wave when…. …it runs into another wave? The two waves will pass right through each other During the time of intersection, the size of the resulting wave is determined by SUPERPOSITION- Adding the displacements from equilibrium together.

  30. Constructive Interference: Waves are on same side of equilibrium

  31. Destructive Interference: waves are on opposite side of equilibrium

  32. “in Phase”

  33. “out of Phase” The peaks and troughs do NOT line up with each other

  34. What happens when…. … a wave reflects back upon itself? It MAY result in a standing wave. Node: the locations along a standing wave where the medium is undisturbed. Antinode: the locations where there is maximum displacement.

  35. Sound

  36. Sound is a longitudinal, mechanical wave. * • Molecules move parallel to the direction of the waves velocity. • Areas of high pressure and low pressure • “compression” and “rarefaction”- molecules are compressed and than move apart

  37. Requires a vibrating object • Guitar string • Stereo speaker • Voice: vocal cords *

  38. Speed of sound • As sound travels through air, at 20˚C (68˚F) and sea level pressure, v is about 343 m/s v = lf • As the temp goes up, the velocity increases- Why?? • As the density of the medium goes up, the velocity increases- Why?? • Travels much slower than light Count time from when you see the flash of lightning to when you hear the thunder- divide by 5 = miles to lightning

  39. The velocity of a wave depends on the medium through which is travels. If you know the medium, you can find the velocity by Bulk modulus- fluids Elastic modulus- solids

  40. Sound Wave Behavior Reflect: an echo Refract: changes direction when the medium changes Diffract: curves around barriers and through openings

  41. You stand at the edge of a canyon and yell, “Hello!”. If you hear the echo 3 seconds later, how wide was the canyon? vsound = 343 m/s d = vt The time for the sound wave to strike the opposite canyon wall is ½ the total time. d = (343 m/s) (1.5 s) d = 514.5 m

  42. If you drop a rock at the top of a 40 meter high cliff, how long will it be until you hear the sound when it hits the ground? Total time = time for the rock to fall + time for the sound to travel back to you. d = vot + ½ at2 Time for the rock to fall: vo = 0, a = g d = ½ at2 Time for the sound: a = 0, v = 343 m/s d = vt

  43. What kind of sound wave is produced when the source of the sound is moving?

  44. A “shock wave” is produced from these overlapping waves. It produces a loud “sonic boom”. Sonic booms occur when the source of sound exceeds the speed of sound * Sonic Booms captured on video

  45. Reflection • Echo • Sonar: invented in 1915- a reflected sound wave is used underwater instead of light because light is more easily absorbed by water, so sound will travel much farther. • Ultrasound • Autofocus cameras

  46. Pitch • Determined by the frequency • Hi frequency = high pitch • Musical notes- if you double the frequency you go up by one OCTAVE Example: 400 Hz, 200 Hz, 800 Hz • Range of hearing humans 20 Hz up to about 20,000 Hz dogs up to about 50,000 Hz cats up to about 70,000 Hz

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