wave nature of radiation and matter

8/14/2019 Wave Nature of Radiation and Matter

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01/21/09 Department Of Physics -NITSrinagar

ScientificMethod

Includes several interdependent steps:* Observations * Controlled Experiments

* Qualitative and Quantitative Reasoning Mathematical Modeling * Predictions

Verification/Falsification of Theories

Speculation and conjecture also have a placein science; but ultimately, an acceptablescientific theory must be verified by relevantobservations and experiments.


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Physics: Theory

Experiment Physics is a systematic attempt to

understand natural phenomena inas much depth and detail as

possible, and use this knowledge topredict, modify and controlphenomena.

Theories are constructed to explain

experimental facts Trying to answer the question

HOW (and not WHY) thingshappen in nature


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Evolution ofPhysics

Physics has a history of synthesizingmany phenomena into a few theories:

Newton

Maxwell

Motion

HeatSound

Light Electricity

Magnetism

Gravity


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Recap: ClassicalMechanics

Newtons laws of Motion:

Second Law Measures The Force

F=ma

Equation of motion:

M d2x/dt2 = Force

F=Mg [Gravitational Force]

F=Q[E+VxB] { LorentzForce}Job: x(t), given x(0).

Deterministic


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Relativity

Theory of how physicsTheory of how physics

will appear to differentwill appear to differentobservers who areobservers who are

moving at somemoving at some

uniform velocity withuniform velocity withrespect to one another.respect to one another.


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What is Light?

Newton: Light was made up of particles

(corpuscles)

He was right, but the reasoning thathe used to come to that decision was

erroneous.


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Wave Theory OfLight

Huygens Concept of LightWaves.

Maxwell's Equations

Interference Diffraction

Polarization

Light waves part ofElectromagnetic Spectrum.

Now we use light for

communication


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Particle Effect

Einstein (1905): The photoelectriceffect was thought to be the mostdirect and convincing evidence ofthe existence of photons and the'corpuscular' nature of light and

electromagnetic radiation.


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The Nobel Prize inPhysics 1921

for his services to TheoreticalPhysics, and especially for his

discovery of the law of thephotoelectric effect.

Einstein (1879 1955)


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OtherExperiments

Compton Effect.

X-ray Production.

Pair-Production.


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What is Light?Waves!

Partial Reflection 4% of the photons

are reflected by the front surface of aglass slab while the other 96% are

transmitted into.Difficulty: How does a single photon

know which of the two possiblepaths it should take when it hits the

glass surface?How does it make up its mind?DEEP MYSTERY!!


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What is Light??

Newtons corpuscular theory was notsatisfactory. Partial Reflection could beexplained by a theory of waves, but the wavetheory could not explain Photoelectric Effect.

So there was a period inwhich one had to know whichexperiment one was analyzingin order to tell if light was

waves or particles. This stateof confusion was called theWave-Particle Duality oflight !


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Quantum Optics

Light was waves on Mondays,Wednesdays and Fridays; it was particleson Tuesdays, Thursdays and Saturdays;

and on Sundays, we think about it!

Quantum Optics reconciles the particleproperties of light with the wave nature.


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Wave Properties ofParticles

In 1924, Louis de Broglie postulated thatbecausephotons have wave and particlecharacteristics, perhaps all forms of matterhave both properties

For instance, for a photon:

De Broglie suggested that this formula istrue for any particle! Thus, the frequencyand wavelength of matter waves can bedetermined. I.e. de Broglie wavelength of aparticle is

h

mv

hc E hf

E hc hp

c c thus or

h

p


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Wave Properties of

Particles The frequency of matter wavescan also be determined

E

h

E hf


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The Davisson-Germer

Experiment They scattered low-energy electrons from a nickel target

They followed this with extensive diffraction

measurements from various materials

The wavelength of the electrons calculated fromthe diffraction data agreed with the expected deBroglie wavelength

This confirmed the wave nature of electrons

Other experimenters have confirmed the wave nature ofother particles


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The Electron Microscope

The electron microscopedepends on the wavecharacteristics of electrons

Microscopes can onlyresolve details that areslightly smaller than thewavelength of theradiation used to

illuminate the object The electrons can be

accelerated to highenergies and have smallwavelengths


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Birth of QuantumMechanics

Electron Theory of Matter (1900):

Electrons + Nucleus Failure ofClassical Laws

Quantum Mechanics (1926-27Schroedinger, Heisenberg, Born):

Nonrelativistic

Invented to explain the strange andclassically inexplicable behavior ofatoms and molecules thespectrum of light they emitted


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The Wave Function

In 1926 Schrdinger proposed a waveequation that describes the manner inwhich matter waves change in space andtime

Schrdingers wave equation is a keyelement in quantum mechanics

Schrdingers wave equation is generallysolved for the wave function,

The wave function depends on theparticles position and the time

i Ht


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The WaveFunction

The value of ||2 at some

location at a given time isproportional to the probabilityof finding the particle at thatlocation at that time


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QuantumMechanics

1. The question how a photon choosesits path given some alternative isoperationally meaningless.

All we can predict from theory is thatout of 100 photons that come down,an average of 4 will be reflected bythe front surface.


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The UncertaintyPrinciple

When measurements are made, theexperimenter is always faced withexperimental uncertainties in the

measurements

Classical mechanics offers no fundamental barrier toultimate refinements in measurements

Classical mechanics would allow for measurementswith arbitrarily small uncertainties


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The UncertaintyPrinciple Quantum mechanics predicts that a barrier to

measurements with ultimately small uncertaintiesdoes exist

In 1927 Heisenberg introduced the uncertaintyprinciple

If a measurement of position of a particle is made withprecision x and a simultaneous measurement of linearmomentum is made with precision p, then the productof the two uncertainties can never be smaller than h/4


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The UncertaintyPrinciple

Mathematically,

It is physically impossible to

measure simultaneously the exactposition and the exact linearmomentum of a particle

Another form of the principle deals

with energy and time:

4

hpx x

4

htE


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Thought Experiment the Uncertainty Principle

A thought experiment for viewingan electron with a powerfulmicroscope

In order to see the electron, at

least one photon must bounce offit During this interaction,

momentum is transferred from thephoton to the electron

Therefore, the light that allowsyou to accurately locate theelectron changes the momentumof the electron

Q


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QuantumMechanics

QM demanded a wholechange in philosophy:

Determinism of classical physics QMpredictability which can at best beprobabilistic, with an intrinsic indeterminacy

(Uncertainty Principle) Conceptual Problems...


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QuantumMechanics

Physics has beenreduced to calculatingonly the probability of an

event, and not predictingexactlywhat will happen!!

Nature permits us to

calculate onlyprobabilities.

Quantum


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QuantumMechanics

This crazy model of the world can explainall the simple properties of light --Light goes in straight lines;When light reflects off a mirror, the

angle of incidence is equal to the angleof reflection;Refraction;Interference;

Diffraction;and so on...


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Where to?

Superposition principle Schroedingers cat paradox

Against common sense buthas no experimental doubtabout it.

Use the superposition stateto build a Quantum

Computer?!Immense parallel-processingmachine


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Where to?Atomic Physics.

Interaction betweenmatter /radiation

Super fluids

SuperconductorsSemiconductor Physics

Condensed matter Physics

Nuclear and particle PhysicsNano-Physics

Computational Physics


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Our Group:

Testing Quantum Mechanics atLow Energy .

Studying Nuclear Structure. Extreme case of neutron and

proton rich light nuclei. Our Recent Contribution Phys Rev C 77 014303 APS


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01/21/09Department Of Physics -NIT

Srinagar

Moral of thestory

Theory ExperimentApplications

Scientific Methods continue!


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01/21/09Department Of Physics -NIT

Srinagar

Than

ks

wave nature of radiation and matter

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