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Physics for Scientists and Engineers II , Summer Semester 2009
1
Lecture 22: July 15th 2009
Physics for Scientists and Engineers II
Physics for Scientists and Engineers II , Summer Semester 2009
2
Momentum and Radiation Pressure
c
2I
c
2SP and
2p :reflection completeFor
power)not pressure, is here P :(Note
)absorption (completec
I
c
S
c
1
A
1
A
1
A
FP
)pressure"radiation ("radiation by the surface on the exerted is P pressureA
)absorption (completep
:magnitude a has surface the todtransporte
p momentum the,S lar toperpendicu surface aby absorbed totally is T If
unit time.per p momentum and Tenergy carriesradiation neticElectromag
avg
avg
ER
ER
c
T
A
dtdT
c
T
dt
d
dt
dp
c
T
ER
ER
ER
ER
Physics for Scientists and Engineers II , Summer Semester 2009
3
Antennas
.radiation) (emenergy radiatesit s,accelerate particle charged aWhenever
fields. varying- timeproduce toneeded charges dAccelerate
radiation. neticelectromag produce to
fields varying- timeneed that weus tellequations sMaxwell'
fields magneticconstant Produce :currentsConstant
fields electricconstant Produce :charges Stationary
++
+
+
-
-
--
4
4
The Half Wave Antenna
Source of alternating voltage having frequency f = c /
Conducting rods
Physics for Scientists and Engineers II , Summer Semester 2009
4
Driving the resonance frequency of the antenna ----???
current).greater fields, electric(greater amplitudeshigher with occurs Resonance
).2L
c(f antenna theoffrequency n oscillatio-self the toequalfrequency a
with voltagegalternatinan applyingby n oscillatiodriven a createcan You
++
+
+
-
-
--
E
I
I
BS
E
BS
Snapshot in time:
Separation of charges creates an electricfield like that of a dipole: Given the name “Dipole antenna”
time.and spacein points allat EB
Physics for Scientists and Engineers II , Summer Semester 2009
5
Driving the resonance frequency of the antenna
++
+
+
-
-
--
E
I
I
B
flows)energy in which
direction theg(indicatin S
E
BS
Snapshot in time:
maximum). aat are rods theof endsouter at the
charges the0,I when (e.g., phase ofout 90 are and
time.and spacein points allat
BE
EB
Physics for Scientists and Engineers II , Summer Semester 2009
6
Voltage and Current Distribution
IV IV
IV IV
Physics for Scientists and Engineers II , Summer Semester 2009
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Driving the resonance frequency of the antenna
IV
Physics for Scientists and Engineers II , Summer Semester 2009
8
The “near field” (dipole field) behavior
++
+
+
-
-
--
E
I
I
B
E
BS
++
+
+
-
-
--
E
I
I
B
E
BS
S
ant.insignific becomesquickly and r
1 toalproportion of falls
field)(near field dipole The No! ???energy of flownet no
inward flowingEnergy
direction reverses vector Poynting
:reversescurrent theereafter,Shortly th
3
Physics for Scientists and Engineers II , Summer Semester 2009
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The “far field” behavior
2
0000
r
1 toalproportionintensity loosesradiation
r
1 toalproportion are B and E
phase.in aremanner in this produced fields electric and magnetic The
law Maxwell-Ampere B
law sFaraday'sdE
toaccording versa viceand field magnetic varying
- timeinduce fields electric varying- timedistanceslarger At
dt
dIIIsd
dt
d
Ed
B
Physics for Scientists and Engineers II , Summer Semester 2009
10
Angular dependence of radiation intensity
S
2
2sin :Radiation ofIntensity
rSI avg
equally. mattersn orientatioradiation, receive toantennaan using when Similarly,
S
Physics for Scientists and Engineers II , Summer Semester 2009
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Angular dependence of radiation intensity
equally. mattersn orientatioradiation, receive toantennaan using when Similarly,
SSender
SenderS
E
reception Good
reception No
SSender
B
reception No
B
E
antenna toparallel be should E
Physics for Scientists and Engineers II , Summer Semester 2009
13
The Nature of Light
Newton: Light = Stream of Particles (can explain reflection and refraction)
1678 Christian Huygens : Showed that a wave model of light can also explain reflection and refraction.
1801 Thomas Young: Experimental demonstration of the wave nature of light (Double slit experiment “Interference” effects)
Maxwell: Light = high frequency electromagnetic wave
Hertz: Confirms existence of electromagnetic waves …..but also discovers the photoelectric effect (contradicts the wave model, which predicts: more light intensity = more energy transferred to electrons)
Einstein 1905: Proposes that energy of light wave is present in particles called photons. E=hf is the energy of a photon (explains photoelectric
effect).
Physics for Scientists and Engineers II , Summer Semester 2009
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Measuring the speed of light
Galileo: Using lanterns between mountains didn’t work. The light is too fast.
1675: Ole Roemer found that the period of revolution of the moon Io around Jupiter depends on whether the earth is receding from or moving towards Jupiter (longer period when earth is receding). From these data Huygens calculated c > 2.3 x 108 m/s
1849: Fizeau’s method (toothed wheel): c = 3.1 x 108 m/s
Currently accepted value: c = 2.997 9 x 108 m/s
Physics for Scientists and Engineers II , Summer Semester 2009
15
Eclipse Time Delay of Io in the shadow of Jupiter
Roemer’s method
Sun
Iobeingeclipsedby Jupiter
Jupiter
In this position, the time of the eclipseis delayed by many minutescompared to the opposite positionof the earth.
delay
earthsun
t
Rc
2
Physics for Scientists and Engineers II , Summer Semester 2009
16
Ray Approximation in Geometric Optics
Geometrical Optics The study of the propagation of light under these assumptions:1) In a uniform medium light travels in a straight line. 2) Light changes direction in a medium with non-uniform optical properties, or at the interface between two media.
The “Ray approximation”:Wave moving through a mediumtravels in a straight line in thedirection of it’s rays.
Rays
Wave fronts
Physics for Scientists and Engineers II , Summer Semester 2009
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Ray Approximation in Geometric Optics
Encountering barriers (like an opening in a wall):
The ray approximation is valid if << d, where d is the size of the opening.
d
Wave continues in a straight line
d Diffraction occurs
Opening acts like a point source of a wave.
Physics for Scientists and Engineers II , Summer Semester 2009
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Reflection of Light/Wave
NormalIncident Ray Reflected Ray
1 '1
incidence of Angle reflection of Angle1
'1
Physics for Scientists and Engineers II , Summer Semester 2009
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Specular and Diffuse Reflection
Specular reflection“Smooth surface”
Diffuse reflection
surfacein Variations
Physics for Scientists and Engineers II , Summer Semester 2009
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The Wave under Refraction
NormalIncident Ray Reflected Ray
21 '1
2Refracted Ray
Air
Glass
Angle of Refraction
12 vv
1v
Physics for Scientists and Engineers II , Summer Semester 2009
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The Wave under Refraction
Incident Ray, Reflected Ray, and Refracted ray are all in one plane.
1 mediumin light of Speed
2 mediumin light of Speed
sin
sin
1
2
1
2 v
v
Physics for Scientists and Engineers II , Summer Semester 2009
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The Wave under Refraction
NormalIncident Ray Reflected Ray
1 '1
12 Refracted Ray
Air
Glass
Angle of Refraction
12 vv
1v
Physics for Scientists and Engineers II , Summer Semester 2009
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Index of Refraction – not yet covered in class but needed for HW 16
v
c
medium ain light of speed
in vacuumlight of speedn: Refraction ofIndex
1n :mediaother All
1n :For vacuum vacuum
n
n
nnn
n
nc
nc
v
v
fv
fv
fvfvfff
22111
2
2
1
2
1
2
1
2
1
221121
changes.ngth but wavele constant, remainsfrequency
:medium a enterslight When
Wavelength in vacuum
Wavelength in medium
n