Waves in 2D and 3D

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Waves in 2D and 3D

• Chapter 8.4

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1D Waves

• We have really only talked about 1D waves like
  springs and ropes

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Some Common 2D/3D Waves

• Water
• A stone dropped in water creates circular waves
• An ocean wave creates waves with straight
  wavefronts
• Sound
• Light

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Reflection of Waves

• Waves reflecting from a rigid support are in 1D
• Waves reflecting on the surface of water are in
  2D
• Electromagnetic waves (light, TV, radio) are in
  3D.
• Sound waves are in 3D
• We show the movement of these waves with ray
  diagrams.

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Key Terms

• Incident ray - the ray that falls or strikes a
  barrier.
• Reflected ray - the ray that reflects or bounces
  off the barrier.
• Normal Line a perpendicular line to the barrier
  surface
• Angle of incidence (?i) - the angle between the
  incident ray
• and the normal line.
• Angle of reflection (?r) - the angle between the
  normal line and
• the reflected ray.

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Reflection and Refraction

• If a straight wave meets a straight barrier, the
  wave reflects directly backward.
• If a straight wave meets a barrier at an angle,
  it reflects at an angle.
• http//www.physicsclassroom.com/mmedia/optics/lr.c
  fm

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Law of Reflection

• The angle of incidence (?i) is equal to the angle
  of reflection (?r).

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Refraction

• The change in direction of waves at the boundary
  between two different media (material).
• NOTE the speed changes!!!
• Water is also affected by its depth as the
  depth increases, the speed increases
• http//www.absorblearning.com/media/attachment.act
  ion?quick15hatt2975

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Undertow (just for Cody!)

• To understand how an undertow works, think about
  what happens when waves break on shore. The water
  obviously has to go somewhere, and this is what
  causes an undertow as waves break, water from
  previous waves runs underneath them, creating a
  gentle current which runs back out to sea. When
  there is heavy wave action, the undertow may not
  be able to get out, and as a result the water
  builds up and looks for a weak point in the
  breaking waves. When the water finds a weak
  point, it pushes out to sea, creating a rip
  current.

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Refraction

• Just like with boundaries, waves behave
  differently when going from a less dense material
  to a more dense material (or vice versa).
• Light twists inward

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Refraction

• More dense to less dense
• Light twists outward

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Diffraction

• The spreading of waves around the edge of a
  barrier or hole.

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Diffraction

• If waves encounter a barrier with a hole in it,
  they do not pass straight through. They bend
  around the edges of the barrier, forming circular
  waves that spread out.

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Diffraction

• If waves spread around 2 closely-spaced holes,
  diffraction occurs

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Diffraction
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Diffraction of Sound Waves

• We notice sound diffracting around corners or
  through door openings, allowing us to hear others
  who are speaking to us from adjacent rooms.
• Owls are able to communicate across long
  distances due to the fact that their
  long-wavelength hoots are able to diffract around
  forest trees and carry farther than the
  short-wavelength tweets of songbirds.
• Low-pitched (long wavelength) sounds always carry
  further than high-pitched (short wavelength)
  sounds.

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Echolocation

• Bats use high frequency (low wavelength)
  ultrasonic waves in order to enhance their
  ability to hunt.
• The typical prey of a bat is the moth - an object
  not much larger than a couple of centimeters.
  Bats use ultrasonic echolocation methods to
  detect the presence of bats in the air.
• As the wavelength of a wave becomes smaller than
  the obstacle that it encounters, the wave is no
  longer able to diffract around the obstacle,
  instead the wave reflects off the obstacle.
• Bats use ultrasonic waves with wavelengths
  smaller than the dimensions of their prey. These
  sound waves will encounter the prey, and instead
  of diffracting around the prey, will reflect off
  the prey and allow the bat to hunt by means of
  echolocation.

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What is the wavelength?

• The wavelength of a 50 000 Hz sound wave in air
  (speed of approximately 340 m/s) can be
  calculated as follows

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Questions for You

• Page 370
• 1, 3
• http//phet.colorado.edu/en/contributions/view/304
  3

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Questions

• 1. Light in air is incident upon a piece of
  crown glass at an angle of 45.0. What is the
  angle of refraction?
• 2. A ray of light passes from air into water at
  an angle of 30.0. Find the angle of refraction.
• 3. A ray of light is incident upon a diamond at
  45.0.
• A) What is the angle of refraction?
• Compare you answer for A to your answer for 1.

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• 1. 45.0
• 2. 30.0