The document discusses electromagnetic waves and their relationship with electric and magnetic fields, highlighting that a changing magnetic field produces an electric field and vice versa. It explains the properties of electromagnetic radiation, its ability to travel through space without a medium, and presents various applications such as communication systems, medical imaging, and astronomical observations. Furthermore, it touches on the electromagnetic spectrum, frequency allocation, and the generation and reception of radio waves.

1. Electromagnetism
Electromagnetism
Electromagnetic Waves
Electromagnetic Waves
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2. The Electromagnetic Connection
The Electromagnetic Connection
• A
A changing
changing magnetic field produces an electric field, and a
magnetic field produces an electric field, and a
changing electric field produces a magnetic field.
changing electric field produces a magnetic field.
• Electric and Magnetic fields can produce forces on charges
Electric and Magnetic fields can produce forces on charges
• An
An accelerating
accelerating charge produces electromagnetic waves
charge produces electromagnetic waves
(radiation)
(radiation)
• Both electric and magnetic fields can transport energy
Both electric and magnetic fields can transport energy
– Electric field energy used in electrical circuits, e.g., released in
lightning
– Magnetic field carries energy through transformer, for example
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3. Electromagnetic Radiation
Electromagnetic Radiation
• Interrelated electric and magnetic fields traveling through space
Interrelated electric and magnetic fields traveling through space
• All electromagnetic radiation travels at
All electromagnetic radiation travels at c
c = 3
= 3
10
108
8
m/s
m/s in
in
vacuum –
vacuum – the
the cosmic speed limit!
cosmic speed limit!
– real number is 299792458.0 m/s exactly
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4. What’s “Waving” in EM waves?
What’s “Waving” in EM waves?
• What medium transports sound waves?
What medium transports sound waves?
– Can there be sound waves in the vacuum of outer space?
• What medium transports water waves?
What medium transports water waves?
• What medium transports radio waves?
What medium transports radio waves?
• A topic of considerable debate in the late 1800’s and early
A topic of considerable debate in the late 1800’s and early
1900’s
1900’s
• Led to the concept of the “luminiferous ether” – an invisible
Led to the concept of the “luminiferous ether” – an invisible
“jello” that was thought to vibrate electromagnetically
“jello” that was thought to vibrate electromagnetically
• Experiments that sought this ether didn’t find it!
Experiments that sought this ether didn’t find it!
• This was quite a surprise
This was quite a surprise
Electromagnetic waves travel through empty space!
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5. Examples of Electromagnetic Radiation
Examples of Electromagnetic Radiation
• AM and FM radio waves (including TV signals)
AM and FM radio waves (including TV signals)
• Cell phone communication links
Cell phone communication links
• Microwaves
Microwaves
• Infrared radiation
Infrared radiation
• Light
Light
• X-rays
X-rays
• Gamma rays
Gamma rays
• What distinguishes these from one another?
What distinguishes these from one another?
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6. Uses of Electromagnetic Waves
Uses of Electromagnetic Waves
• Communication systems
Communication systems
– One-way and two-way
• Radar
Radar
• Cooking (with microwaves)
Cooking (with microwaves)
• Medical Imaging (X rays)
Medical Imaging (X rays)
• “
“Night Vision” (infrared)
Night Vision” (infrared)
• Astronomy (radio,
Astronomy (radio, 
wave, IR, visible, UV, gamma)
wave, IR, visible, UV, gamma)
All that we experience through our eyes is conveyed by
electromagnetic radiation…
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7. The Electromagnetic Spectrum
The Electromagnetic Spectrum
• Relationship between frequency, speed and
Relationship between frequency, speed and
wavelength
wavelength
f
f ·
·
 = c
= c

f
f is frequency,
is frequency, 
 is wavelength,
is wavelength, c
c is speed of light
is speed of light
• Different frequencies of electromagnetic radiation
Different frequencies of electromagnetic radiation
are better suited to different purposes
are better suited to different purposes
• The frequency of a radio wave determines its
The frequency of a radio wave determines its
propagation characteristics through various media
propagation characteristics through various media
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8. US Frequency Allocation – the FCC
US Frequency Allocation – the FCC
(300 MHz has a wavelength of 1 meter)
“Radio” frequency-space is allocated to the hilt!
Here’s a sample region from 300–600 MHz
International allocation gets tricky
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9. Generation of Radio Waves
Generation of Radio Waves
If charges oscillate back and forth, get time-varying magnetic fields too
If charges oscillate back and forth, get time-varying magnetic fields too
Note that the magnetic fields are perpendicular to the electric field vectors
Note that the magnetic fields are perpendicular to the electric field vectors
B
+
+
+



+









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10. Polarization of Radio Waves
Polarization of Radio Waves
B
E
Transmitting
antenna
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11. Reception of Radio Waves
Reception of Radio Waves
Receiving antenna works best
when ‘tuned’ to the
wavelength of the signal, and
has proper polarization
Electrons in antenna are “jiggled”
by passage of electromagnetic wave
B
E
Optimal antenna length is one quarter-wavelength (/4)
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12. Questions
Questions
Why are car radio antennas vertical?
Why are car radio antennas vertical?
Why are cell phone antennas so short?
Why are cell phone antennas so short?
How do polarizing sunglasses work?
How do polarizing sunglasses work?
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