Chapter 17 EM Waves

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Chapter 17EM Waves
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17.1 Traits of EM Waves
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• An EM wave is a disturbance that transfers energy
  through a field.
• A field is an area around an object where the
  object can apply a force on another object
  without touching it.

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• The electric and magnetic fields vibrate at right
  angles to each other and to the direction in
  which the EM wave moves.

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• Sources of EM waves include the sun (largest
  source) and technology.
• EM waves are transverse waves
• The transfer of energy in the form of EM waves is
  called radiation.

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• In a vacuum, or empty space, EM waves spread
  outward in all directions from the source of the
  disturbance.
• The waves travel in a straight line until
  something interferes with them.
• In a vacuum, EM waves travel at a constant speed,
  about 186,000 mi/s.
• At this speed, EM waves from the Sun take about 8
  minutes to reach Earth.

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• Unlike a mechanical wave, an EM wave can travel
  without any medium at all and does not lose
  energy as it moves.
• When EM waves encounter a medium, they can
  transfer energy to the medium itself. That energy
  can be converted into many other forms, like
  thermal energy.

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17.1 Quiz
Write QA

1. Name two waves EM waves are different than
   mechanical waves.
2. What are two sources of EM waves ?
3. How can EM waves transfer energy differently in a
   medium than in a vacuum?
4. What would happen to an EM wave if it never came
   in contact with a medium?

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17.2 EM Spectrum

• The frequency of an EM wave determines its
  characteristics and uses. Higher-frequency EM
  waves, with short wavelengths, have more energy.
  Lower-frequency EM waves, with longer
  wavelengths, have less energy.

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• Frequency is measured in hertz (Hz).
• Radio waves and microwaves have long wavelengths
  and low frequencies.
• Radio waves are EM waves that have the longest
  wavelengths, the lowest frequencies, and the
  lowest energies. Radio waves travel easily
  through the atmosphere and many materials.
• Example A radio transmitter converts sound waves
  into radio waves and broadcasts them. Radio
  receivers pick up the radio waves and convert
  them back into sound waves.

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• Microwaves are EM waves with shorter wavelengths,
  higher frequencies, and higher energy than other
  radio waves.
• Examples Radar - It works by transmitting,
  reflecting, and receiving microwaves from objects
  the waves strike. A cell phone is a radio
  transmitter and receiver that uses microwaves.

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• Infrared light is the part between microwaves and
  visible light. Infrared radiation is the type of
  EM wave most often associated with heat.
• Example Infrared scopes and cameras convert
  infrared radiation into visible wavelengths,
  creating images of objects based on their
  temperature. Remote controls also use IR. IR is
  not visible by the unaided eye, but you can view
  IR in a mirror.

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• Visible light is the part which humans can see.
• If diffracted, white light spreads into the
  rainbow - ROYGBIV. Each color has its own
  wavelength and frequency.
• Red has the lowest frequency/energy and reaches
  further than the blue which has a higher
  frequency/energy.

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• Ultraviolet light, or UV light, consists of
  frequencies above those of visible light and
  below those of x-rays.
• Example Ultraviolet can be used to sterilize
  medical instruments and food by killing harmful
  bacteria.

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• X-rays and gamma rays have short wavelengths and
  high frequencies.
• X-rays are used for diagnosing fractures,
  locating dense tumors, and examining teeth.
• Gamma waves have the highest energy levels of all
  EM waves.
• Gamma rays are used to kill cancer cells and
  fight tumors.

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17.2 Quiz
Write QA

1. What two properties of EM waves change from one
   end of the spectrum to the other?
2. Describe two uses for microwave radiation.
3. How are EM waves used in dentistry and medicine?
4. Why are IR waves used instead of UV waves in
   remote controls?

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17.3
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• The production of light by materials at high
  temperatures is called incandescence. Stars,
  lightning, and glowing coals are all
  incandescent.
• Luminescence is the production of light without
  the high temperatures needed for incandescence.

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• The production of light by living organisms is
  called bioluminescence. Bioluminescent organisms
  produce light from chemical reactions instead of
  intense heat.
• Fluorescence occurs when a material absorbs EM
  radiation of one wavelength and gives off EM
  radiation of a different wavelength.

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Incandescent vs. Fluorescent Lighting
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• Incandescent lights are very bright but produce
  more infrared radiation than visible light. They
  waste a lot of energy as heat.
• Fluorescent lights are coated with a powder that
  absorbs ultraviolet light and gives off visible
  light. They are more efficient at converting
  electrical energy into light than incandescent
  bulbs.
• Light emitting diodes, or LEDs, are another type
  of artificial light. They are very efficient at
  converting electrical energy into light.

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17.3 Quiz
Write QA

1. Describe natural, nonliving sources of
   incandescent light.
2. What advantages does bioluminescence have over
   incandescence as a way for living organisms to
   produce light?
3. What are some advantages of incandescent
   lighting?
4. What are some disadvantages of incandescent
   lighting?

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17.4
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• When an electromagnetic wave encounters a medium,
  it can be reflected, transmitted, absorbed.
• Reflection is the bouncing back of a wave.
• Transmission is the passage of the EM wave
  through a medium.
• Absorption is the disappearance of an EM wave
  into the medium.

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• Materials can be classified by the amount and
  type of light they transmit.
• Transparent materials let most light through so
  that the objects behind the material are visible.
  Ex clear glass
• Translucent materials let light through, but they
  spread the light out so that objects cannot be
  clearly seen through them. Ex frosted glass
• Opaque materials do not let any light through.
  Ex black paper

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Polarization of light
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• Polarization is a way of filtering light so that
  all of its waves vibrate in the same direction.
  Polarizing filters allow only electric fields
  that are vibrating in a particular direction to
  pass through. (some sunglasses are polarized to
  block UV rays)

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• Different objects absorb and reflect different
  wavelengths.
• Ex a red object reflects mostly red wavelengths
  and absorbs most other wavelengths.
• White objects reflect all colors .
• Black objects absorb all colors.

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• Three colors of transmitted light, red, blue, and
  green, are called primary colors.
• They can be mixed to produce all possible colors
  of light.
• An equal mixture of all three can produce white
  light.
• In art, materials called pigments can also be
  mixed to produce colors.
• The primary pigments are blue, yellow, and red.

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Primary colors of light pigments
brown
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17.4 Quiz
Write QA

1. How do materials affect how light is transmitted?
   (Hint stained glass)
2. How does a polarizing filter reduce glare?
3. In order for an object to appear white, which
   wavelengths must the light contain?
4. If an object appears red, which colors of light
   is the object absorbing?