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EM Spectrum
Physics 102
Professor Lee Carkner
Lecture 27
PAL #26 EM Waves
Radio tuner with L = 10 nH to tune in f =
90.3 MHz
f = 1/2p(LC)½
LC = (1/2pf)2
C = (1/2pf)2(1/L)
C = (1/(2)(p)(90.3X106))2(1/10X10-9)
C =
What value of L will tune in 106.5 MHz?
L = (1/2pf)2(1/C)
L = (1/(2)(p)(106.5X106))2(1/3.11X10-10)
L =
Direction of B Vectors
 If the antenna is vertical the E vectors go up and down
and the direction of propagation is north
 B vectors are at right angles to E vectors and so are
horizontal or east and west
If you have a LC oscillator receiving
electromagnetic waves with a fixed L
and a variable C, how would you
change C to double the frequency?
A)
B)
C)
D)
E)
Quarter C
Half C
Double C
Quadruple C
You cannot get exactly double f by
changing C
The E and B fields of an electromagnetic wave
are oriented
A) parallel to the direction of travel and to each
other
B) perpendicular to the direction of travel and
to each other
C) parallel to the direction of travel and
perpendicular to each other
D) perpendicular to the direction of travel and
parallel to each other
E) Always 180 degrees from each other and the
direction of travel
If the frequency of the wave emitted by a
radio transmitter is doubled, what
happens to the wavelength and the
speed?
A)
B)
C)
D)
E)
wavelength is halved, speed is halved
wavelength is halved, speed is same
wavelength is same, speed is same
wavelength is same, speed is doubled
wavelength is doubled, speed is
halved
Energy in EM Wave

Each has an energy density:

uB = ½(1/m0)B2
The total energy density is the sum of each
term

This energy density is not very useful

Called the intensity, I
Intensity of Flashlight
Intensity
The value of u depends on where the EM
wave is in its cycle

The energy density of each field is the same

The intensity depends on how much energy
the wave delivers, which depends on the
energy density and the speed:
I = uc = (½e0Erms2 + ½e0Erms2)c
I = ce0Erms2
Units: joules per second per square meter (W/m2)
Intensity of Light
We normally don’t know much about the E field of
a light wave, but rather we know something about
the source of the wave

As the waves travel out the power is spread out
over a sphere of radius r (r is the distance away):
I = Ps / 4pr2

Light (like sound) falls off with an inverse square
law
Inverse Square Law
Radiation Pressure

If someone shines a flashlight on you, the
light is trying to push you away

EM pressure is due to the fact that light
has momentum which can be
transmitted to an object through
absorption or reflection
Momentum Transfer
The change in momentum due to light is
given by:
Where Dp is the momentum change and DU
is the energy change

For reflection the momentum change is twice
as much:
Light Pressure
From Newton’s second law
The amount of energy delivered in time Dt is:
where I is the intensity and A is the area
Since pressure (pr) is force per unit area the
pressure becomes:
pr = I/c (total absorption)
pr = 2I /c (total reflection)
Example: Light Sail
Radiation pressure can be used to power a
spacecraft

The sail can gather light from a star to propel
the spacecraft

Light sail powered craft need no engines or
fuel

EM Waves and Energy
The energy of an EM wave is proportional to the
frequency
where h = Planck’s constant = 6.63 X 10-34 J s

Radio waves and X-rays produce different effects
They kinds of EM radiation we can observe
depends on:

Our eyes
Atmospheric Transmission
O2, N2
Absorption
Gamma
+
X-ray blocked
H2O, CO2
Absorption
Infrared blocked
Sensitivity of Your Eye
Light from the Sun
When you look at the sun (note, don’t look at
the sun), it seems to be producing a uniform
orange-yellow light

Your eyes can only see the visible portion
The sun’s visible light is thermal (blackbody)
radiation

The sun produces more yellow and red light
than blue, so the sun looks orange
Solar Emission Spectrum
Radio Waves from the Sun
Solar Spectrum
Sun emits most strongly at visible
wavelengths

Produces stronger IR at cooler regions

Produces high energy radiation in outer
layers

Also produces low energy radiation in
magnetic loops

Next Time
Read 24.10
Homework, Ch 22, P 21, Ch 24, P 53, 57
Final exam:
Monday, 6-8 pm, Room 102 Science
Please see me if you have a conflict