Sound and Light

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Sound and Light

• CHAPTER 15

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All sound waves

• Are caused by vibrations

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All sound waves

• Are longitudinal waves

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All sound waves

• Require a medium -- solid, liquid, or gas.

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The speed of sound

• Depends on the medium and temperature
• Fastest in most solids
• (except ones like rubber that DAMPEN or
  weaken the sound waves -- these make good
  soundproofing materials)

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The speed of sound is

• Slowest in gases
• The colder the gas,
• the slower the speed
• Speed of sound in room temperature air 346
  m/s (760 mi/hour)

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Why does sound travel fastest in solids?

• The molecules are closer together than in a
  liquid or gas so they pass on the vibrations more
  quickly

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Loudness of sound depends on wave intensity

• Intensity in turn is determined by the amplitude
  and distance from the source of the sound.

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Intensity

• Increases when amplitude increases
• Decreases when amplitude decreases

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Intensity increases as distance from source
decreases

• Louder when closer because waves have spread out
  less

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Loudness is measured in decibels

• Logarithmic scale --
• 70 decibels is twice
• as loud as 60

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Frequency determines pitch.

• Higher frequency higher pitch
• Lower frequency lower pitch

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Beat Frequencies

• Interference of two or more frequencies to form a
  new wave
• Frequencies must be close to one another but not
  the same

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Doppler effect

• Frequency (and pitch) go up when source of sound
  is approaching because wavelength is shortened
• Go down when source is retreating

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Resonance

• Tendency of an object to vibrate with larger and
  larger amplitudes
• Examples
• Opera singer breaking a glass
• Pushing a kid on a swing
• Running fingers on rim of a glass

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Sonar

• Measures distance by measuring time for sound
  wave to reflect off a surface, calculated
  distance using d v x t

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Range of human hearing

• 20Hz to 20,000 Hz

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Ultrasound Frequency too high for us to hear

• Can be used in sonar systems
• Above 20,000 Hz

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Infrasound

• Frequencies that are too low to be heard by
  humans
• Below 20 Hz
• Examples
• Earthquakes
• Tornadoes
• Storms at sea

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Light

• Sometimes acts
• like a wave
• Sometimes acts
• like a particle

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Light acts like a wave when it

• Reflects
• Refracts
• Produces interference

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Light acts like a particle when it

• Travels without a medium
• Delivers packets of energy (photons) to solar
  collectors or chlorophyll

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The energy of light

• Is proportional to its frequency
• Gamma rays have the most energy because they have
  the highest frequency.

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The speed of light

• Depends on the medium
• In a vacuum, the speed 3 X 108 m/s.
• This speed is known as c
• (as in Emc2).

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Lights brightness depends on wave intensity.

• Wave intensity decreases when amplitude decreases
• Increases when amplitude increases

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Intensity increases as distance from source
decreases

• brighter when closer because waves have spread
  out less

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Electromagnetic spectrum

• Light at all possible energies, frequencies, and
  wavelengths
• Entire spectrum given off by sun and other stars

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Electromagnetic spectrum
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Radio waves

• Longest wavelength, least energy
• Used in communication and radar

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microwaves

• Next longest waves
• Used in cooking, communication

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Infrared waves

• Thermal energy -- heat

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Visible light

• Red is longest wavelength of visible light
• Violet is shortest

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Ultraviolet (UV) light

• 9 of suns energy
• Can cause sunburn, cancer
• More dangerous than visible light because it
  has a shorter wavelength

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X-rays

• Shorter wavelength than UV light, so more
  dangerous
• Used in medicine, but can disrupt DNA so
  must limit exposure

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Gamma rays

• Shortest wavelengths, so most energy (and most
  dangerous)
• Rays can be focused to kill cancer
• Given off by radioactive materials

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Reflection

• Light bouncing off a surface

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Rough surfaces scatter the light they reflect

• so they look dull, not shiny.

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Smooth surfaces reflect the light in the same
direction

• so they are shiny and can reflect an image.

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Polarization

• Filtering of light

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Curved mirrors distort images, because different
parts of the mirror reflect the light in
different directions.
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Objects have the color of the wavelength they
reflect.
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Fiber optics use internal reflection to transmit
light along the length of the fibers.

• This is more efficient than transmission through
  metal wires.

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Refraction of Light

• Light waves can bend (refract) when they pass
  from one medium into another.
• The waves bend because the new medium changes the
  waves speed.

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Refraction of light

• The bending of light waves changes the apparent
  position of objects.

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Because lenses diffract light, they can

• magnify images
  (microscope, magnifying glass)
• focus images (eye, telescope).
• Correct the poor focus of a near- or far-
• sighted eye (glasses).

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Prisms separate light into its component colors.

• This happens because the different colors travel
  at different speeds and therefore bend different
  amounts. (Red is fastest and bends the least, so
  it appears at the top of a rainbow.)

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In rainbows, water droplet act as prisms.