Electromagnetic Waves

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Chapter 24

• Electromagnetic Waves

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Scottish physicist James Clerk Maxwell showed
mathematically in the 1860s that light must be a
combination of electric and magnetic fields.
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Properties of Electromagnetic Waves

• an electromagnetic wave is a transverse wave
• electromagnetic waves can travel through a vacuum
  or a material substance
• the speed of electromagnetic wave in a
• vacuum is c 3.00 X 108 m/s!

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The Speed of Light
Distance that light travels in a year ?
speed of light in a vacuum c 3 x105 km/s
(186,000 mi/s) so one light second 3x105 km and
one light year 9.46x1012 km
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The Finite Speed of Light
The distance from the Earth to the moon is 1.3
light seconds away i.e. it takes light traveling
at a velocity of 3x105 km/s 1.3 seconds to reach
the moon. Thus, you are seeing the moon as it
was 1.3 seconds ago since it took that amount of
time to travel to the earth.
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Astronomical Distance Scales are Astronomical!
Proxima Centauri 4.2 light years away
The Sun 8.3 light minutes away
Andromeda Galaxy 2.4 million light years away
Galaxies in the Hubble Deep Field as far as
billions of light years away
The finite speed of light gives us a fossil
record of the history of the Universe!!!!
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If you pass white light through a prism, it
separates into its component colors.
R.O.Y. G. B.I.V !!
Different wavelengths correspond to different
colors!
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What is Color?

• Objects dont have an intrinsic color!
• Color of objects due to how much light is
  reflected, absorbed, or emitted.
• There is no real color only what our eye
  perceives!

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The Electromagnetic Spectrum
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Visible light is only one type of electromagnetic
radiation emitted by stars
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A Pyroelectric Thermometer
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The Energy Carried by Electromagnetic Waves
Electric Energy Density
Magnetic Energy Density
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The Energy Carried by Electromagnetic Waves
Total Energy Density

or
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The Energy Carried by Electromagnetic Waves
Average Energy Density
or
where
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The Energy Carried by Electromagnetic Waves
Intensity
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Example
A neodymium glass laser emits short pulses of
high-intensity electromagnetic waves. The
electric field has an rms value of Erms 2.0 X
109 N/C. Find the average power of each pulse
that passes through a 1.6 X 10-5 m2 surface that
is perpendicular to the laser beam. (p. 744)
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Doppler Effect
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Polarization
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Malus Law
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Example
What value of ? should be used so that the
average intensity of the polarized light
reaching the photocell is one-tenth the average
intensity of the unpolarized light? (p. 748)
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