2le reviewer

8/3/2019 2LE Reviewer

1/13

SLIDE NUMBER 6: Basic ideas About Light

I. Why talk about light?! Three great theories of 19th & 20th century physics

came from the need to resolve fundamental issues

about the nature of light (Relativity,

Electromagnetism, Quantum Mechanics)

! It helps us see and manipulate the very small! It provides information about remote objects

II. The Nature of Light

A. Particle theory I: corpuscles

Isaac Newton: explained the reflection and refraction of

light in terms of a stream of corpuscular bodiesallowed the development of Geometric

Optics (light propagating as straight rays)

B. Wave theory: light waves

Christian Huygens (late 1600s): described light in terms of

advancing wavefronts instead of streams of particles

Thomas Young and others (early 1800s): performed

experiments to demonstrate the wave nature of light,

particularly when encountering small obstacles

C. Particle theory II: photons

Max Planck: Explained emission of radiation (light) by

blackbodies in terms of energy quanta

Albert Einstein, 1905: Explained photoelectric effect using

photons! energy packets

D. Modern view

Light is both a wave and a particle

! The propagation of light is more completelydescribed by the wave theory (but can be

approximated to some extent by geometricoptics).

! The interaction of light with matter(absorption and emission) is best explained by

a quantum theory (i.e. photons).

III. The Speed of Light

! How did we realize that the speed of light is finite?! What is the speed of light?! Is the speed of light measurable/finite ?

Kepler : Speed of light infinite because vacuum of space didnot slow the speed of light down.

Galileo : started the measurement gameFlash from military artillery shows light travels

faster than sound. Speed of light not necessarily infinite.

Speed of light measured using lanterns:Suggestion 1638, experiments 1667

1. Two people stood at least a mile apart.2. Both had covered lanterns.3. When one person uncovered his lantern,

the other person had to uncover hislantern when he saw this.

4. Third person measured the time betweenwhen the first and second lanterns whereuncovered.

" Repeated experiments failed to accurately measureany time interval between when the first and

second lanterns were uncovered.

" They could only say that light travels very fastOlaus Roemer 1676 : speed of light measured using the

moons of Jupiter using one of Jupiter'she established that the speed of light is finite.

! Observed eclipse times (aboutonce every 1.76 days) of Iodeviated from predictions

cyclically

!Roemer realized deviationcaused by difference in Earth-

Jupiter distance and finitespeed of light

!According to Huygens: orbitaldiameter of Earth was about: 3

x 1011 m

Roemer observed a cumulative discrepancy of 22 minutes

Using Huygens estimate of distance, and Roemers idea

what value would have been computed for the speed oflight?

cRoemer = 3 x 1011 m / 22 minutes =3 x 1011 m / 1320 s

= 230,000 km/s
http://www.what-is-the-speed-of-light.com/light.htmlhttp://www.what-is-the-speed-of-light.com/light.html


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SLIDE NUMBER 7: Electricity, Magnetism, & Light

Guide Questions# What is electric charge? What are some properties

of charge?# What are electric and magnetic fields? How are

electric and magnetic fields produced?

# What is the relationship between light and otherelectromagnetic waves?

1. The electric charge#Charge is an intrinsic property of matter, the same way

mass is an intrinsic property of

matter.

#All matter is composed ofdiscretely charged particles

electrons and protons.

unit of electric charge : coulomb, C.

2. Properties of a chargeDichotomy of Charge: There are 2 kinds of charge,

positive and negative.

Unlike charges attract, like charges repel.

! The charged particles themselves arereferred to as electric charges.

Conservation of Charge:

# Neutral objects contain equal amounts of positiveand negative charges.

# Excess of one type of charge over the other resultsin the object having a net charge.

# Charges are not created or destroyed, onlytransferred between objects.

Quantization of Charge

Charge always appears in multiples of e

e = 1.602 x 10-19 C = the charge of a proton/electron

Benjamin Franklin & Electricity:

#Studied static electricity between different materials.#Found that objects could be positively or negatively

charged.

#battery, conductor, condenser, charge, discharge,uncharged, negative, minus, plus,electric shock, and electrician

Annihilation of Charges# Actually, charges can be destroyed (and even

created) but always in equal and opposite pairs.

# THE NET CHARGE OF THE UNIVERSE ISCONSTANT.

Back to Repulsion and Attraction

# Charles Coulomb studied the forces betweenelectrical charges.

# Coulombs Law is similar to Newtons Law ofGravitation:

Compare: k= 9 x 109 N-m2/ C2 vs G=6.67300 10-11N-m2/ kg2

2

21

r

qqkF"

3. The electric field# How does a distant charge know if other charges

have moved?# Michael Faraday conceived electric field lines or

Lines of Force to resolve his discomfort with the

action-at-a-distance concept.# Electric field lines point along the direction which

a test charge would experience a force.

# An electric charge sets-up an electric field in thespace around it.

# Other charges experience a force due to thatelectric field.

# Any changes to the position or magnitude of theoriginal charge translates to a change in the electricfield that propagates outward at the speed of light.


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4. The magnetic field# The magnetism associated with iron

(ferromagnetism), particularly magnetite was a

long known phenomenon.

# Similar to electric charges, there are two types ofmagnetic poles: north and south.*

* Nobody has ever been able to observe a single pole by

itself (a magnetic monopole).

5. ElectromagnetismHans Christian Oersted and Andre-Marie Ampere showed

that moving charges (electrical current ) could influence,

and be influenced by magnets.

# Moving charges create magnetic fields (B)Magnetic fields exert forces on other moving charges (andconductors carrying electrical current).

This electro-dynamic principle* makes the electric motor**

possible (as first constructed by Michael Faraday.)

* Also known as electromagnetic induction

**Electrical to Mechanical Energy

# Faraday constructed the first electric generator* viaelectromagnetic induction

# Moving a wire through a magnetic field generatesan electrical current. Moving a magnet around a

wire does the same.

Changing magnetic field will cause charges to move (thus

produce current).

SUMMARY# Electric charges create electric fields.# Moving electric charges create magnetic fields.# Changing (time-varying) magnetic fields create

electric fields.

James Clerk Maxwell

# Organized the existing concepts of electricity andmagnetism in a cohesive mathematical framework.

# Added his own discovery: changing (time-varying)electric fields create magnetic fields

# Maxwell did not form these equations.*# But by combining them, he predicted the existence

of traveling electromagnetic waves with a very

interesting property...

# Maxwell did not form these equations.*# But by combining them, he predicted the existence

of traveling electromagnetic waves with a very

interesting property...* He did make a slight correction to the last equation.

6. Light as an electromagnetic waveAccording to Maxwells equations, electromagneticwaves travel at the speed of light!

Maxwell concluded: Light is an electromagnetic wave.

The EM Spectrum

# Visible light is only a small segment of the verywide electromagnetic spectrum.

# The properties of different magnetic waves dependon their wavelength (frequency), but they allrepresent oscillating electric and magnetic fields.

0

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dtdldE B&'"$%

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5/13

SLIDE NUMBER 8:Blue Skies, Red Sunsets, Rainbows & Other Optical SpectaclesWhat happens when light hits an object?

A. The waves can be absorbed by the object.B. The waves can be transmitted through the object.C. The waves can be reflected off the object.D. The waves can be refracted through the object.E. For small objects, the waves can be scattered indifferent directions.

A.

B.

C.

D.

The specific behavior of light when it strikes anobject depends on its wavelength.

For visible light, we experience differentwavelengths as different colors (demo on

transmission & reflection).

Illustration 1: Reflecting the beauty of light

Illustration 2: Why are most leaves green?

Chlorophyll: RED and VIOLET light areABSORBED

Green light is REFLECTED

Illustration 3: Why pencils look bent when submerged in

water?

Snells law (Law of Refraction): n1 sin (01) = n2 sin (02)

n = index of refraction = c / v

nair~ 1.0 nwater= 1.33v = speed of light in the medium

Illustration 4: Why are there rainbows?

! White light is made up of various colors! Speed of light in vacuum (c) is the same for all

colors

! Speed of light in a medium (v) depends oncolor/wavelength

! Therefore n depends on wavelenght(DISPERSION)


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Secondary reflected light also form rainbow patterns

Illustration 4.2: Uniqueness of rainbows

Two observers standing apart from one another do not see

the same rainbow.

Illustration 4.3: Pot of Gold at the end of a rainbow?

Sorry Rainbows are suspended in mid air, hence it does notend anywhere in the ground

Illustration 5: Why is the sky blue? Scattering.

! Particles much smaller than wavelengths of lightscatter light in all directions.

! Blue (~450 nanometer wavelength) is scatteredover four times more strongly than red (~650nm).

! Small dust particles are Rayleigh scatterers.

Illustration 5.1: Why are clouds white

Illustration 5.2: Why are sunsets red?

! Light of lower frequency is scattered the least bynitrogen and oxygen molecules

! Thicker atmosphere presented to sunlight at sunsetthan at noon

! So more blue is scattered at sunset, so transmittedlight becomes redder

Longer Answer

! Tuning fork analog! Atoms, molecules and very tiny particles absorb

and reemit light at the same frequency

! The tinier the particle, the higher the frequency oflight it will scatter (think of bells: smaller bellstend to ring with higher notes than larger bells)

! Of the visible frequency light, violet is scatteredthe most, followed by blue, green, yellow, orange,

and red! Red is scattered only 1/10th as much as violet light! Although violet light is more scattered than blue,

our eyes are not very sensitive to violet light

! The lesser amount of blue predominates in ourvision so we see a blue sky

! If there are a lot of dust particles, light of lowerfrequency/higher wavelength is also scattered so

sky may be whitish blue

! Most ultraviolet light from sun absorbed by ozonelayer

! Remaining UV light scattered by atmosphericparticles and molecules

Q1: After a heavy rainstorm, the sky becomes a deeper blue.

Why?Q2: If molecules in the sky scatters low frequency light

(longer wavelength) more than high frequency (shorter

wavelength) light, how would the colors of the sky andsunsets appear?

Q3: Distant dark mountains are bluish in color. What is the

source of this blueness?

Q4: Why is the ocean blue?


7/13

SLIDE NUMBER 9: On Particles, Waves, & Wave-Particles

II. Brief Historical OverviewCorpuscular Theory of Light (1704)

" Isaac Newton proposed that light consists of astream of small particles, because it

travels in straight lines at great speeds is reflected from mirrors in apredictable way

Wave Theory of Light (1802)

" Thomas Young showed that light is a wave,because it

undergoes diffraction and interference(Youngs double-slit experiment)

II. Defining properties of particles & wavesParticles: Position x, Mass m, Momentum p = mv

Waves: Wavelength1, Amplitude A,Frequency f (inverse of period T)

number of cycles per second (Hertz)

f = c /1 T = 1/1

Waves vs Particles:

" A particle is localized in space, and hasroperties such as mass

" A wave is inherently spread out over many

" Waves superpose and pass throughcles

II. Wave theory of lightDiffraction

terference Fringes on a Screen

discrete

physical p

wave-lengths in space, and could have

amplitudes in a continuous range

(interference) each other, while parti

collide and bounce offeach other

I

Interference

In


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Double-Slit Experiment

ry of LightIV. Modern particle theo

A. Introduction" Any hot body radiates light

over the whole spectrum" The spectrum depends on both freqof frequenciesuency and

temperature

" Examples: light bulbs, the Universe.B The Blackbody Radiation

Definition: A blackbody

absorbs all radiation that falls on it

Spectrum

is an object which totally

y radiation versus

eratures

graph that deviates fromlly at short wavelengths

regarded as the

it radiation in

energy:

Plot of intensity of the blackbod

wavelength for various temp

Plot of intensity of the blackbody radiation versus

equency for various temperaturesfr

Ultraviolet Catastrophe

Classical theory predicts aexperimental data, especia

Plancks Quantum Postulate (1900)

Max Planck (1858-1947) is generally

father of quantum theory

" A blackbody can only emdiscrete packets or quanta, i.e., in

multiples of the minimum

E= hf

where h is a constant andfis thefrequency of the radiation


9/13

Photoelectric Effect: Response to Blue Light

" Result: A radiation law in extremely goagreement with experiment

Plancks Constant

" Experimentally determined to beh = 6.63 x 10-34 Joule sec

od

(Joule = kg m2 / sec2)

ct: What is it?

" A new constant of nature, which turns out tobe of fundamental importance in the new

quantum theory

C. Photoelectric effe

Light falling on metallic surface caneject electrons from surface.

Pho

The wave theory of light cannot explain these

observations

not frequency

When blue light is shone on the emitter plate,a current flows in the circuit

But for red light, no current flows in the circuit

Photoelectric Effect: Experimental Observations

" Only light with a frequency (f) greater than acertain threshold (f>fthresh) will produce a

current

" Current begins almost instantaneously (for f >fthresh), even for light of very low intensity

" Current is proportional to the intensity of theincident light

toelectric Effect: Problems with Wave Theory

"

" For waves, energy depends on amplitude and" This implies that a current should be produced

when say, high-intensity red light is used


10/13

D. Ein

steins Postulate (1905)

" own asphotons

(same as Plancks formula)

Light consists of particles, now kn

" A photon hitting the emitter plate will eject anelectron if it has enough energy

" Each photon has energy:E= hf

E. Everyday Evidence for Photons

" Red light is used in photographic darkroombecause

s

it is not energetic enough to break the

" Ultraviolet light causes sunburn but visiblephotons are more

nergetic

" ur eyes detect colour because photons ofdifferent chemical

reactions in retina cells

Oth E

V. Wave-particle duality" Determines the probability of an electron

arriving at a certain spot on the screen

" Electron as a wave: After many electrons,resembles the interference pattern of light

" which

behaviour

effect

spectral lines

halogen-silver bond in black and white films

light does not because UV

e

O

different energies trigger

er vidence for Photons: Atomic spectra

Electron as a particle: trying to detect

slit the electrons pass through causes them tobehave like particles

VI. Summary" Waves and particles exhibit very different

" Yet, light sometimes behaves like particles spectrum of blackbody radiation photoelectric

Ele er (a) 8 electrons, (b) 2ctron interference pattern aft

" And electrons sometimes behave likewaves

interference pattern of electrons" In quantum theory, the distinction between

waves and particles is blurred


11/13

SLIDE NUMBER 10:RelativityNewtons Laws vs. Maxwells Equations

Galilean transformation: Speed observed (v) = c - u

!Principia - Newton!Newtons laws Consistent with Galilean transformation!A dynamical theory of the electromagnetic field(1864) -

Maxwell

! Maxwells equations NOT consistent with Galileantransformation

! At least one had to be wrong.

II. Special Relativity

A. Postulates

I.

1. The Speed of Light is ConstantThe speed of light in vacuum is the same for all

observers

2. Principle of InvarianceThe laws of physics are the same for all inertial

reference systems

. Consequences of Special Relativity PostulatesB

1. W

independ

2. Velocity addition formula modified

3. Time Dilation

4. Length Contraction

e have to stop thinking of time and space as

ent of each other


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5. Relativity of Simultaneity

Set Up 1:

A concludes the two events (p and q) were simultaneousA & B have the same speed --- B agrees with Aif

Set Up 2:

A: still concludes the two events (p and q) were

ltaneoussimu

6. ESlid

III.

! Special relativity is only valid for constant velocityframes

! It took 10 years for Einstein to come up with asatisfactory theory of gravity.

Postulates

B: light hits p before it hits q, therend q) were NOT simultaneous

fore the two events (pa

A and B are both right; simultaneity is relative.

=mces: Visualization of the consequences of SR

2

General Relativity

! Special relativity had problems dealing withgravitation

! Principle of Equivalence: Inertial andgravitational mass are equivalent /indistinguishability of gravitational field and

accelerating re

! Principle of Relativity: The laws of physics arethe same in all reference systems

Consequences:

ference frame

1. Predicts that Gravity bends light

ath of light from distant quasar bent by gravitational field

f nearby galaxy $

bright outer images

P

o

four


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2. Correct Perihelion Shift of Mercuryeory - predicted a shift only of observed

alue

Einstein's predictions exactly matched the observation

3. Predicts the existence of Blackhole

If gravity can bend light then a very large gravitational

field can bend light so much that it can not escape

this is a black hole.

4. V fi

! se is not static it is expanding and has! le! Observatory in Mount Wilson, California

w? The Doppler Effect

1960 Ha

Beam ofat higher

. Time is slowed as the strength of gravitational fields

creases

. Gravitational

aves???

stronomers have

alized that a rare set

f double stars is madep of two pulsars1.

his unique discovery

ill allow them to testinstein's theory of

lativity in novel

ays, and to betternderstand the energy

s

enerate.

"Thi

sign icant discovery,"says Robert Massey of

the Royal Observatory,Greenwich, in London, UK. Einstein predicted the existenceof gravitational waves, but they have never been directly

observed. "There aren't many objects out there that could be

a copious enough source of gravitational waves, but this is

one of them," he says.

Source: Nature Science Update, 30 December 2003

EINSTEINs QUOTES

"I sometimes ask myself how it came about that I was theone to develop the theory of relativity. The reason, I think, is

that a normal adult never stops to think about problems of

space and time. These are things which he has thoughtabout as a child. But my intellectual development was

retarded,as a result of which I began to wonder about space

and time only when I had already grown up."

"Put your hand on a hot stove for a minute, and it seems like

an hour. Sit with a pretty girl for an hour, and it seems like

a minute. THAT'S relativity."

"Few are those who see with their own eyes and feel with

their own hearts."

"Gravitation can not be held responsible for people fa g

love"

ite, the universe and human

about the former."

Newton's th

v

s

eri cation of expanding universe

Our univer

been since it started about 14 billion years ago.

First observed by Edwin Hubb

!Ho

rvard

high energy gamma rays slightly red shiftedelevation

5

in

6

w

A

re

ou

T

wE

re

wu

beams that pulsar

g

s is a hugely

if

llin

in

"Two things inspire me to awe -- the starry heavens aboveand the moral universe within ."

"Only two things are infin

tupidity, and I'm not sures

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