# Electromagnetic Spectrum - PowerPoint

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"Electromagnetic Spectrum - PowerPoint"

```					     Electromagnetic Spectrum
• The speed of electromagnetic waves is 3 x 108 m/s
• As we will see later, light was shown to have
wave-like properties
• We have studied waves in water last semester
• In water waves, the water itself moves at right
angles to the direction of the wave
• The question before us is with light, what moves?
Electromagnetic Spectrum
• Maxwell argued that light must be an
electromagnetic wave
• Hertz was able to produce EM waves using a
spark gap with oscillating electrons at a frequency
of about 109 vibrations/second
• These waves were shown to have a velocity of 3 x
108 m/s
• They could be reflected, refracted and showed
interference, just like light
Electromagnetic Spectrum
• Long before people knew what light was,
by doing interference measurements, people
showed that visible light had wavelengths
ranging from 4 x 10-7 m to 7.5 x 10-7 m
• Recall that f=c where c is the velocity of
light
• That means the frequencies of visible light
range from 4 x 1014 Hz and 7.5 x 1014 Hz
Electromagnetic Spectrum
• So, we know about two ranges of
frequencies or wavelengths of EM radiation
• What about others?
• We now know that there is a very large
range of frequencies and wavelengths of
EM radiation in nature
• We can display them in a chart
Electromagnetic Spectrum

We can produce many of these ranges by accelerating
electrons, whether in an antenna or by rapidly stopping
moving electrons as in an x-ray tube.
Measure the Speed of Light
Energy in EM Waves
• W have determined that the energy density
in an electric field is 0E2/2
• We also learned that the energy density in a
magnetic field is B2/20
• So, the energy per unit volume in space
containing an EM wave is
2
1    2 1B
u  0 E 
2       2 0
Energy in EM Waves
• We can do some manipulations using some
of the relationships we have derived earlier
to get some other forms for the energy
density
• The energy is shared equally between the
electric and magnetic fields
2
2    B0
u  0 E         EB
0   0
Poynting Vector
• We can look at the energy transported
across unit area per unit time by an EM
wave
Poynting Vector

V  Ax  Act
2
U  uV  0 E Act
U
S      0 cE 2
At
Poynting Vector

U
S        0 cE 2
At
To get the average value
2    1 2
E        E0 for sine waves
2
Poynting Vector

1     2   1 c 2 E 0 B0
S  0cE 0       B0 
2         2 0      20
Information Transmission
• We can use EM waves to carry information
by a technique known as modulation
• We are going to add two signals together
• Recall when we added waves together last
fall, the resulting wave is just formed by
• This is known as amplitude modulation
Information Transmission

This is amplitude
modulation and is used for
kHz) and for TV pictures
Information Transmission

This is frequency
modulation and is used for
FM Radio (88-108 MHz)
and for TV audio
Information Transmission

Here the antenna is
responding to the electric
field portion of the EM
wave.
Information Transmission

Here the antenna is
responding to the magnetic
field portion of the EM
wave.
Information Transmission

Tune the radio by
adjusting the LC circuit to
match the frequency of the
incoming EM wave.

```
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