Light and Sound in the Ocean

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Light and Sound in the Ocean

• Both light and sound are transmitted by energy in
  their waves.
• The term refraction refers to the bending of
  waves.
• When a light wave or sound wave leaves a medium
  of one density, such as air, and enters a medium
  of a different density, such as water, at an
  angle other than 90-degrees, it is bent from its
  original path.

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Light and Sound in the Ocean

• The refraction (bending) causes the waves to slow
  down.
• The speed of light in water is only about
  three-quarters its speed in air. Glass bends
  light even more.
• The degree to which light is refracted from one
  medium to another is expressed as a ratio called
  the refractive index.

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Light and Sound in the Ocean

• The higher the refractive index the greater the
  degree of bending.
• The refractive index of seawater increases as the
  salinity increases.
• The wavelength of light determines its color.

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Light and Sound in the Ocean

• Shorter wavelengths are bluer and longer
  wavelengths are redder.
• Except for very long radio waves, water rapidly
  absorbs nearly all electromagnetic radiation.
• Only blue and green wavelengths of light pass
  through water to any appreciable depth.

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Light and Sound in the Ocean

• When light is absorbed, molecules vibrate and the
  lights electromagnetic energy is converted to
  heat.
• In the clearest of tropical waters, light only
  penetrates to a depth of about 200 meters (660
  ft.).
• The zone of light penetration from the surface
  down is called the photic zone.

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Light and Sound in the Ocean

• In open ocean waters which are not as clear as in
  the tropics the typical depth of light
  penetration is about 100 meters (330 ft.).
• In the coastal waters in which we typically swim
  light may only penetrate to about 40 meters (130
  Ft..).

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Light and Sound in the Ocean

• Only about 45 of the light striking the surface
  reaches 1 meter. About 16 of the light striking
  the surface reaches 10 meters. Only about 1 of
  the energy striking the surface remains at 100
  meters.

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Light and Sound in the Ocean

• Sound
• Sound is a form of energy transmitted by rapid
  pressure changes. The intensity of sound
  decreases as it travels through seawater because
  of spreading, scattering and absorption.
• The intensity loss due to spreading is
  proportional to the square of the distance from
  the source.

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Light and Wound in the Ocean

• Eventually sound is absorbed and converted by
  molecules into very small amounts of heat.
• Sound waves can travel for a much greater
  distance than light.
• Because sound travels efficiently in water many
  organisms use sound rather than light to see in
  the ocean.

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Light and Sound in the Ocean

• The speed of sound in seawater is 1500 meters per
  second (3,345 miles per hour).
• The speed of sound in seawater increases as
  temperature and pressure increase.
• The speed of sound in seawater decreases with
  depth.

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Light and Sound in the Ocean

• The Sofar Layer
• The depth at which the speed of sound reaches its
  minimum varies with conditions, but it is usually
  located near 1,200 meters (3,900 ft.) in the
  North Atlantic. It is only slightly different
  the other major oceans.

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Light and Sound in the Ocean

• The speed of sound in this layer may be slow but
  the transmission is considered efficient.
• Sound energy tends to remain within the layer.
  Therefore loud noises at this depth can be heard
  for thousands of kilometers.

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Light and Sound in the Ocean

• In recent test sound generated by a U.S. Navy
  ship in the Indian Ocean was heard as far as the
  coast of Oregon.
• Because of test carried out in the 1960s to see
  if this layer could be used to locate survivors
  in life rafts this layer was called sofar (sound
  fixing and ranging).

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Light and Sound in the Ocean

• Sonar
• Crews aboard surface ships and submarines employ
  active sonar(sound navigation and ranging).
• In modern systems, electrical current is passed
  through crystals, which respond by producing
  powerful sound pulses above the limit of human
  hearing.

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Light and Sound in the Ocean

• Some of the sound bounces off any object larger
  than the wavelength of sound and returns to a
  microphone sensor.
• An experienced sonar operator can tell the
  direction of the contact, its size and heading,
  and even something about its composition.

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Light and Sound in the Ocean

• Side Scan Sonar
• Side scan sonar is a type of active sonar.
• In this type of sonar there may be as many as 60
  transceivers tuned to high sound frequencies.
• They are towed in the quiet water beneath a ship
  and are sometimes capable of near-photographic
  resolution.

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Light and Sound in the Ocean

• Side-scan systems are used for geological
  investigations, archaeological studies and
  locating downed ships and airplanes.
• Echo sounders transmit a pulse of sound toward
  the ocean floor, measures the time of the round
  trip from transducer to seabed and back, computes
  the depth from the time delay and displays the
  depth on a screen.

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• The End