Chapter 18: Electromagnetic Spectrum

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Chapter 18 Electromagnetic Spectrum Light
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18.1 Electromagnetic Waves

• Question What do x-ray machines, microwave
  ovens, and heat lamps have in common with police
  radar, TV, and radiation therapy???

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Electromagnetic Waves

• Answer They all use WAVES to transport energy
  from one location to another!!!

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Electromagnetic Waves

• Electromagnetic Waves- (EM) transverse waves
  consisting of changing electric fields and
  changing magnetic fields

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Electromagnetic Waves

• Can carry energy from one place to another
• Produced by constantly changing fields
• Magnetic and electric fields travel at right
  angles to each other

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EM Waves

• Electromagnetic waves are produced when an
  electric charge vibrates or accelerates.
•  As fields regenerate, their energy travels in
  the form of a wave.
• Unlike mechanical waves, EM waves do not need a
  medium to travel through!
• EM waves can travel through a vacuum (or empty
  space) or matter.

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EM Radiation

• Electromagnetic Radiation- the transfer of energy
  by electromagnetic waves traveling through matter
  or across space.

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THE SPEED OF EM WAVES

• Question Why do you see lightning before you
  hear thunder?

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Speed of EM Waves

• Answer Because light travels faster than sound!
•  
• But how much faster is light???

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How long would it take you to drive from San
Francisco to New York?
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Speed of Light Analogy

• Scientists have discovered that light and all
  electromagnetic waves travel at the same speed
  when in a vacuum ? 3 x 108 m/s!
• Consider driving non-stop at 60 mph from NYC to
  San Francisco.
• This trip would take you 50 hours
• Light travels this distance in less than 0.02
  second!!!

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Speed of EM Wave

• Speed of EM wave wavelength x frequency
• Wavelength is inversely proportional to frequency
• As the wavelength increases, the frequency
  decreases

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Differences between EM Waves

• Even though all EM waves travel at the same
  speed, it does not mean they are all the same!
• EM waves vary in wavelength and frequency

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18-2 The EM Spectrum 
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Prism Experiment

• In 1800, William Herschel used a prism to
  separate the wavelengths present in sunlight. He
  produced a band of color red, orange, yellow,
  green, blue, indigo and violet.

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EM Spectrum

• The full range of frequencies of electromagnetic
  radiation is called the electromagnetic spectrum
• Which includes the following parts
• radio waves, infrared rays, visible light,
  ultraviolet rays, X-rays, and gamma rays.

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EM Spectrum

• Each kind of wave is characterized by a range of
  wavelengths and frequencies.

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Radio Waves

• Radio waves have the longest wavelengths in the
  EM spectrum, from 1mm to 1000s of km. They
  also have the lowest frequencies, 300,000 mHz or
  less.
• used in radio, TV, microwaves, and radar
•  

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Radio Waves

• In a radio studio, music and voices are changed
  into electronic signals that are carried by radio
  waves.
• AM radio stations broadcast by amplitude
  modulation, the amplitude of the wave is varied
• FM radio stations broadcast by frequency
  modulation, the frequency of the wave is varied
• A station is lost when its signal becomes too
  weak to detect, an FM station is more likely to
  be lost because FM signals do not travel as far

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Difference between AM FM
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More Applications of Radio Waves

• Radio Waves also include the application of
• Television
• Radio waves also carry signals for TV, including
  the information for pictures
• Microwaves
• Radar (Radio Detection and Ranging)
• Short bursts of radio waves that reflect off
  objects they encounter and bounce back, being
  detected by a radio receiver

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Infrared Radiation

• Infrared waves have higher frequencies than radio
  and wavelengths that vary from about 1mm to
  750nm
• Used as a source of heat and to discover areas of
  heat differences
• Invisible to our eye
• Warmer objects give off more infrared than
  cooler, a device called a thermograph create
  thermograms (color-coded picture) that show
  temperature variation
• Thermograms can be used to find places where a
  building loses heat, search and rescue teams use
  infrared cameras to locate victims

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Infrared Radiation
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Visible Light

• The visible part of the EM spectrum is light the
  human eye can see.
• Each color of the visible spectrum corresponds to
  a specific frequency and wavelength (ROYGBIV)

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Ultraviolet (UV) Rays

• The wavelengths of ultraviolet rays very from
  400nm to about 4nm, and higher frequencies than
  violet light.
• In moderation, UV rays help your skin produce
  vitamin D
• Excessive exposure can cause sunburn, wrinkles,
  and skin cancer
• Used to kill microorganisms
• Plant nurseries use UV to help plants to grow
  during winter

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UV Radiation
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X-Rays

• X-rays have very short wavelengths from about
  12nm to 0.005nm , and have higher frequencies
  than UV
• Have high energy and can penetrate matter that
  light cannot
• Used in medicine (pictures of bones), industry
  (test sealed lids), and transportation (contents
  of truck trailers)

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Gamma Rays

• Gamma rays have the shortest wavelengths about
  0.005nm or less, and have the highest frequencies
  and therefore the most energy and the greatest
  penetrating ability
• Used in the medical field to kill cancer cells,
  in brain scans, and in industrial situations such
  as inspecting pipelines for sign of damage
• Overexposure can be deadly

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Gamma Rays Radiotherapy
The normal cells receive a lower dose of gamma
radiationthan the cancer cells, where all the
rays meet. Radiotherapy aims to kill the cancer
cells while doing as little damage as possible to
healthy normal cells.
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18.3 Behavior of Light
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Question to Ponder

• What would you see if you were snorkeling in warm
  ocean waters over a coral reef? You might see
  fish of bright colors, clown fish, sea stars,
  etc. Why can you see these animals SO CLEARLY???
  Why can you see the reef through the water but
  not through the bottom of the boat that brought
  you to the reef???

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Light Materials

• Without light, nothing is visible!
• When you look at the reef animals, what you are
  really seeing is LIGHT
• You can see the reef through the water, because
  LIGHT passes through the water between the reef
  and your eyes.
• You cant see the reef through the bottom of the
  boat because LIGHT doesnt pass through the boat!

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Behavior of Light

• How light behaves when it strikes an object
  depends on many factorsincluding the material it
  is made of.
• Materials can be
• Transparent
• Translucent
• Opaque

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Transparent

• Transparent material through which you can see
  clearly, transmits light
• Most light is able to pass through
• Examples water, windows

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Translucent

• Translucent you can see through the material,
  but the objects you see through it does not look
  clear or distinct.
• Scatters Light
• Examples some types of jello, certain bars of
  soap, frosty windows

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Examples of Translucent
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Opaque

• Opaque material either absorbs or reflects all
  of the light that strikes it.
• NO light is able to pass through
• Examples fruit, wooden table, metal desk

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Interactions of Light

• When light encounters matter, some or all of the
  energy in the light can be transferred to the
  matter. And just as light can affect matter,
  matter can affect light.
• When light strikes a new medium, the light can
  be
• Reflected
• Absorbed
• Transmitted

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Reflection

• When you look in a mirror, you see a clear image
  of yourself.
• An image is a copy of an object formed by
  reflected (or refracted) waves of light.
• Two types of reflection
• Regular Reflection
• Diffuse Reflection

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Regular Reflection

• Regular Reflection occurs when parallel light
  waves strike a surface and reflect all in the
  same direction
• Occurs when light hits a smooth, polished surface
• Mirrors or surface of a still body of water (page
  547, figure 18)

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Diffuse Reflection

• Diffuse Reflection occurs when parallel light
  waves strike a rough, uneven surface, and reflect
  in many different directions
• Paper has a rough surface, (page 547, figure 18)
• Rough surfaces causes diffuse reflection of the
  light that shines on it

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When Light is TRANSMITTED

• Reflection occurs because there is no
  transmission of light (light is not able to pass
  through to the new material)
• However, when light is transmitted different
  things can happen. Light can be
• Refracted
• Polarized
• Scattered

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Refraction

• Refraction ability of light to refract, or bend
  when it passes at an angle from one medium into
  another.
• Two easily observable examples that occur when
  light travels from air into water
• Underwater objects appear closer and larger than
  they really are
• Can make an object such as a skewer (or pencil)
  appear to break at the surface of the water (page
  548, figure 19)

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Refraction
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Refraction Can Create a Mirage

• Refraction can sometimes cause a mirage.
• Mirage a false or distorted image.
• Mirages occur because light travels faster in hot
  air than in cooler, dense air
• On a sunny day, air tends to be hotter just above
  the surface of a road than higher up
• Mirages also form this way above the hot sand in
  deserts

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Examples of Mirages
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What is Polarization?
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Polarization

• Light is an EM Wave ? EM waves vibrate in TWO
  planes
• Light waves that vibrate in only one plane is
  called polarized light.
• Polarizing filters transmit light waves that
  vibrate in only one direction or plane (page 548,
  figure 20)
• Unpolarized light vibrates in ALL directions

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Polarization
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Scattering

• Earths atmosphere contains many molecules and
  other tiny particles. These particles can
  scatter light.
• Scattering light is redirected as it passes
  through a medium (page 549, figure 21)

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Scattering explains a red/pink sunset!

• Scattering effect reddens the sun at sunset and
  sunrise
• Small particles in the atmosphere scatter
  shorter-wavelength (blue light) more than light
  of longer wavelengths
• By the time the sunlight reaches your eyes, most
  of the blue and even some of the green and yellow
  have been scattered
• Most of what remains for your eyes to detect are
  the longer wavelengths of light, orange and red

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Scattering of Light by Atmosphere