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Chapter 18

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Title: Chapter 18


1
Chapter 18 The Electromagnetic Spectrum and
Light
  • Jennie L. Borders

2
Section 18.1 Electromagnetic Waves
  • Electromagnetic Waves are transverse waves
    consisting of changing electric fields and
    changing magnetic fields.
  • An electric field in a region of space exerts
    electric forces on charged particles.
  • A magnetic field in a region of space produces
    magnetic forces.

3
Electromagnetic Waves
  • Electromagnetic waves are produced when an
    electric charge vibrates or accelerates.
  • This is a transverse wave because the particles
    are vibrating perpendicular to the direction of
    the wave.

4
Electromagnetic Waves
  • Electromagnetic waves can travel through a
    vacuum, or empty space, as well as through
    matter.
  • The transfer of energy by electromagnetic waves
    traveling through matter or across space is
    called electromagnetic radiation.

5
Speed of Electromagnetic Waves
  • The speed of light in a vacuum is 3.0 x 108 m/s.
  • In a vacuum, all electromagnetic waves travel at
    the same speed.
  • Electromagnetic waves vary in wavelength and
    frequency.

6
Electromagnetic Wave Speed
  • Formula for speed of a wave
  • v l x u
  • For electromagnetic waves, v is always the speed
    of light which is represented by a c.
  • c l x u
  • c speed of light (3.0 x 108 m/s)
  • l wavelength (m)
  • u frequency (Hz)

7
Sample Problem
  • A radio station broadcasts a radio wave with a
    wavelength of 3.0m. What is the frequency of the
    wave?
  • c l x u u c/l
  • c 3.0 x 108 m/s u 3.0 x 108 m/s / 3.0m
  • l 3.0m u 1.0 x 108 Hz
  • u ?

8
Practice Problems
  • The radio waves of a particular AM radio station
    vibrate 680,000 times per second. What is the
    wavelength of the wave?
  • A global positioning satellite transmits a radio
    wave with a wavelength of 19cm. What is the
    frequency of the radio wave?

c l x u l c/u l 3.0 x 108 m/s / 680,000 1/s
440m
19cm 0.19m c l x u u c/l u 3.0 x 108
m/s / 0.19m 1.6 x 109 Hz
9
Wave or Particle?
  • Electromagnetic radiation behaves sometimes like
    a wave and sometimes like a particle.
  • Evidence for a wave includes the fact that light
    can produce constructive and destructive
    interference.

10
Wave or Particle?
  • Evidence for a particle includes the fact that
    light causes the photoelectric effect.
  • The emission of electrons from a metal caused by
    light striking the metal is called the
    photoelectric effect.

11
Photons
  • In 1905, Albert Einstein proposed that light, and
    all electromagnetic radiation, consists of
    packets of energy called photons.
  • Each photons energy is proportional to the
    frequency of the light.

12
Intensity
  • Photons travel outward from a light source in all
    directions.
  • The intensity of light decreases as photons
    travel farther from the source.

13
Section 18.1 Assessment
  • How fast does light travel in a vacuum?
  • What makes electromagnetic waves different from
    one another?
  • What happens to the intensity of light as photons
    move away from the light source?
  • What is the wavelength of an AM radio wave in a
    vacuum if its frequency is 810 kHz?

810 kHz 810,000Hz c l x u l c/u l 3.0 x
108 m/s / 810,000 1/s 370m
14
Section 18.2 The Electromagnetic Spectrum
  • A scientist observed that when light is passed
    through a prism the temperature of the light is
    lower at the blue end and higher toward the red
    end.
  • He also concluded that there was invisible
    radiation beyond the visible spectrum.

15
Electromagnetic Spectrum
  • The full range of frequencies of electromagnetic
    radiation is called the electromagnetic spectrum.
  • The electromagnetic spectrum includes radio
    waves, infrared rays, visible light, ultraviolet
    rays, X-rays, and gamma rays.

16
Radio Waves
  • Radio waves are used in radio and television
    technologies, as well as in microwave ovens and
    radar.
  • In AM radio, the stations have signals with
    varying amplitudes.
  • In FM radio, the stations have signals with
    varying frequencies.

17
Radio Waves
  • The shortest radio waves are called microwaves.
  • Microwaves also carry cell phone conversations.

18
Infrared Rays
  • Infrared rays are used as a source of heat and to
    discover areas of heat differences.
  • Thermograms are color-coded pictures that show
    variations in temperature.

19
Visible Light
20
Ultraviolet Rays
  • Ultraviolet radiation has higher frequencies than
    violet light.
  • Ultraviolet rays have applications in health and
    medicine, and in agriculture.

21
X-Rays
  • X-rays have high energy and can penetrate matter
    that light cannot.
  • X-rays are used in medicine, industry, and
    transportation to make pictures of the inside of
    solid objects.

22
Gamma Rays
  • Gamma rays have the shortest wavelengths in the
    electromagnetic spectrum, about 0.005nm or less.
  • Gamma rays are used in the medical field to kill
    cancer cells and make pictures of the brain, and
    in industrial situations as an inspection tool.

23
Section 18.2 Assessment
  • List the kinds of waves included in the
    electromagnetic spectrum, from longest to
    shortest wavelength.
  • How are AM radio waves different from FM radio
    waves?
  • What type of electromagnetic waves are
    microwaves?

24
Section 18.3 Behavior of Light
  • Materials can be transparent, translucent, or
    opaque.
  • A transparent material transmits light, which
    means it allows most of the light that strikes to
    pass through it.
  • A translucent material scatters light.
  • An opaque material either absorbs or reflects all
    of the light that strikes it.

25
Interactions of Light
  • When light strikes a new medium, the light can be
    reflected, absorbed, or transmitted.
  • When light is transmitted, it can be refracted,
    polarized, or scattered.

26
Reflection
  • An image is a copy of an object formed by
    reflected (or refracted) waves of light.
  • Regular reflection occurs when parallel light
    waves strike a surface and reflect all in the
    same direction.
  • Diffuse reflection occurs when parallel light
    waves strike a rough surface, and reflect in many
    different directions.

27
Refraction
  • A mirage is a false or distorted image.

28
Polarization
  • Light with waves that vibrate in only one plane
    is polarized light.

29
Scattering
  • Scattering means that light is redirected as it
    passes through a medium.

30
Section 18.3 Assessment
  • Explain the differences among opaque,
    transparent, and translucent materials.
  • List and explain three things that can happen to
    a light wave when it enters a new medium.
  • What is the difference between diffuse reflection
    and regular reflection?
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