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Chapter 18: Electromagnetic Spectrum

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Chapter 18: Electromagnetic Spectrum & Light Light & Materials Without light, nothing is visible! When you look at the reef animals, what you are really seeing is ... – PowerPoint PPT presentation

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Title: Chapter 18: Electromagnetic Spectrum


1
Chapter 18 Electromagnetic Spectrum Light
2
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???

3
Electromagnetic Waves
  • Answer They all use WAVES to transport energy
    from one location to another!!!

4
Electromagnetic Waves
  • Electromagnetic Waves- (EM) transverse waves
    consisting of changing electric fields and
    changing magnetic fields

5
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

6
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7
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.

8
EM Radiation
  • Electromagnetic Radiation- the transfer of energy
    by electromagnetic waves traveling through matter
    or across space.

9
THE SPEED OF EM WAVES
  • Question Why do you see lightning before you
    hear thunder?

10
Speed of EM Waves
  • Answer Because light travels faster than sound!
  •  
  • But how much faster is light???

11
How long would it take you to drive from San
Francisco to New York?
12
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!!!

13
Speed of EM Wave
  • Speed of EM wave wavelength x frequency
  • Wavelength is inversely proportional to frequency
  • As the wavelength increases, the frequency
    decreases

14
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

15
18-2 The EM Spectrum  
16
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.

17
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.

18
EM Spectrum
  • Each kind of wave is characterized by a range of
    wavelengths and frequencies.

19
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
  •  

20
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21
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

22
Difference between AM FM
23
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

24
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

25
Infrared Radiation
26
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)

27
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

28
UV Radiation
29
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)

30
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

31
Gamma Rays Radiotherapy
The normal cells receive a lower dose of gamma
radiation than 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.
32
18.3 Behavior of Light
33
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???

34
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!

35
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

36
Transparent
  • Transparent material through which you can see
    clearly, transmits light
  • Most light is able to pass through
  • Examples water, windows

37
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

38
Examples of Translucent
39
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

40
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

41
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

42
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)

43
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

44
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

45
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)

46
Refraction
47
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

48
Examples of Mirages
49
What is Polarization?
50
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

51
Polarization
52
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)

53
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

54
Scattering of Light by Atmosphere
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