The Nature of Waves

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The Nature of Waves

• Physics
• Montwood High School
• R. Casao

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Energy Transfer

• A wave is a disturbance that propagates through a
  medium or space. A wave cannot exist in one
  place but must extend from one place to another.
• Waves provide a mechanism by which energy is
  transmitted from one location to another without
  the physical transfer of matter between these
  locations.
• The source of all waves is something that is
  vibrating.

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Mechanical Waves

• Mechanical waves those that require a medium to
  pass through.
• Medium any region through which a wave
  disturbance propagates.
• To generate mechanical waves, a source of energy
  is required to cause a disturbance and an elastic
  medium is required to transmit the disturbance.
• Sound waves and ocean waves are examples of
  mechanical waves.
• Without the medium - air, water, etc. - there can
  be no wave propagation.

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Mechanical Waves

• An elastic medium behaves as if it were an array
  of particles connected to springs with each
  particle occupying an equilibrium position.
• A displacement produced by an energy source
  travels from particle to particle.

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Longitudinal Waves

• Longitudinal wave a wave in which the
  displacement of particles of the medium is
  parallel to the direction of propagation of the
  wave.
• Compression the region of the longitudinal wave
  in which the distances separating the vibrating
  particles is less than their equilibrium
  position.
• When a compression is released, it attempts to
  return to its equilibrium position. Thus they
  compress the region adjacent to the compressions.
  
• Rarefaction the region of a longitudinal wave
  in which the vibrating particles are farther
  apart than their equilibrium position.
• Rarefactions travel along a medium just as
  compressions do.

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Longitudinal Waves

• Longitudinal wave motion continuous wave
  disturbance parallel to the direction the pulses
  are traveling.
• Energy is transferred from particle to particle
  along the medium without motion of the medium as
  a whole.

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Longitudinal Waves

• Longitudinal and Transverse Wave Motion
  Illustrations
• Waves

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Transverse Waves

• Transverse wave a wave in which the
  displacement of particles of the medium is
  perpendicular to the direction of propagation of
  the wave.
• Pulse a single non-repeated disturbance.
• Crest upward displacement (positive pulse).
• Trough downward displacement (negative pulse).
• The displacement of the particles of the medium
  caused by the pulse are perpendicular to the
  direction in which the pulse travels.
• If a pulse is produced in the middle of a medium,
  the disturbance will move in both directions.

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Transverse Waves

• Longitudinal and Transverse Wave Motion
  Illustrations
• Waves

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Electromagnetic waves

• Electromagnetic waves can travel through a
  vacuum they do not require a material medium for
  their propagation.
• Light, radio waves, television waves, microwaves,
  and x-rays are examples of electromagnetic waves.
  
• Speed 3 x 108 m/s

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Characteristics of Waves

• Frequency (f ) the number of crests (or
  troughs) passing a given point in a unit time,
  usually a second number of vibrating
  oscillations, or cycles, per unit time.
• Unit hertz, Hz.
• One hertz is equivalent to one cycle per second
  dimensionally it is s-1.

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Characteristics of Waves

• Period (T) amount of time required for one
  complete oscillation.
• Period is the reciprocal of frequency
• Period is measured in seconds.

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Characteristics of Waves

• Wavelength (?) is the distance between two
  points that occupy the same relative
  position on the wave.
• Wavelength is the distance advanced by the wave
  motion in one period and is often measured from
  crest to crest along a wave or from trough to
  trough along a wave.

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Characteristics of Waves

• Amplitude maximum displacement of vibrating
  particles of the medium from their equilibrium
  positions.

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Characteristics of Waves

• The amplitude of a wave is related to the energy
  which it transports.

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Characteristics of Waves
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Wave Speed

• Waves move with a speed that is related to
  frequency and wavelength.
• In general, waves travel with a speed
• Speed of light (electromagnetic radiation) 3 x
  108 m/s
• Speed of sound in air 334 m/s (although it
  varies with air temperature)

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Wave Interactions

• Reflection
• A wave is turned back, or reflected, when it
  encounters a barrier that is the boundary of the
  medium in which the wave is traveling.
• Angle of incidence (i) the angle between the
  incident wave and the normal (perpendicular)
  drawn to the point of incidence.
• Angle of reflection (r) the angle between the
  reflected wave and the normal drawn to the point
  of incidence.
• The angle of incidence equal to the angle of
  reflection i r.

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Wave Interactions

• Law of Reflection when a wave disturbance is
  reflected at the boundary of a transmitting
  medium, the angle of incidence is equal to the
  angle of reflection.

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Wave Interactions

• Refraction
• Refraction is the bending of a path of a wave
  disturbance as it passes obliquely from one
  medium into another of different propagation
  speed.
• As a wave passes into a medium where its speed is
  less and its wavelength shorter, it is refracted
  toward the normal drawn to the boundary.
• The angle of refraction is smaller than the angle
  of incidence.

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Wave Interactions

• As a wave passes into a medium where its speed is
  increased and its wavelength longer, it is
  refracted away from the normal.
• The angle of refraction is greater than the angle
  of incidence.
• The change in direction and the change in speed
  occur simultaneously.

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Wave Interactions

• Remember as light travels from one medium to
  another its wavelength changes, NOT its
  frequency.

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Wave Interactions

• The index of refraction for any substance is the
  ratio of the velocity of light in a vacuum to its
  velocity in the substance.

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Homework Conversions

• Megahertz and hertz 1 MHz 1 x 106 Hz
• Meters and nanometers 1 m 1 x 10-9 nm