Lecture 22: July 15 th 2009

Physics for Scientists and Engineers II , Summer Semester 2009

1

Lecture 22: July 15th 2009

Physics for Scientists and Engineers II


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


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


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


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


6

Voltage and Current Distribution

IV IV

IV IV


7

Driving the resonance frequency of the antenna

IV


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


9

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


10

Angular dependence of radiation intensity

S

2

2sin :Radiation ofIntensity

rSI avg

equally. mattersn orientatioradiation, receive toantennaan using when Similarly,

S


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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


12

Electromagnetic Spectrum


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).


14

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


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


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


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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.


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Reflection of Light/Wave

NormalIncident Ray Reflected Ray

1 '1

incidence of Angle reflection of Angle1

'1


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Specular and Diffuse Reflection

Specular reflection“Smooth surface”

Diffuse reflection

surfacein Variations


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The Wave under Refraction


21 '1

2Refracted Ray

Air

Glass

Angle of Refraction

12 vv

1v


21


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


22



1 '1

12 Refracted Ray

Air

Glass

Angle of Refraction

12 vv

1v


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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


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Snell’s Law of Refraction – not yet covered in class but needed for HW 16.

2211

2

1

1

2

1

2

sinnsinn

n

n

sin

sin

v

v

Snell’s Law of Refraction

Documents

Lecture 22: July 15 th 2009