Atomic Particles

Atoms are made of protons, neutrons and electrons

99.999999999999% of the atom is empty space Electrons have locations

described by probability functions

Nuclei have protons and neutrons

nucleus

mp = 1836 me

Atomic sizes

Atoms are about 10-10 m Nuclei are about 10-14 m Protons are about 10-15 m The size of electrons and

quarks has not been measured, but they are at least 1000 times smaller than a proton

What is Light?

Properties of lightReflection, Refraction

A property of both particles and waves Interference and Diffraction

Young’s double slitsA Property of Waves Only

PolarisationA Property of Waves Only

Classical Physics

Light is a waveYoung’s Double Slit Experiment Faraday’s experimentsMaxwell’s equations

Jt

EB

t

BE

B

E

000

0

1

0

Line-Emission Spectrum

ground state

excited state

ENERGY IN PHOTON OUT

Bohr Model

e- exist only in orbits with specific amounts of energy called energy levels

Therefore…

e- can only gain or lose certain amounts of energy

only certain photons are produced

Bohr Model

12

3456 Energy of photon

depends on the difference in energy levels

Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom

Other Elements

Each element has a unique bright-line emission spectrum.

“Atomic Fingerprint”

Helium

Bohr’s calculations only worked for hydrogen!

The Birth of the QuantumMax Planck

The energy contained in radiation is related to the frequency of the radiation by the relationship

n is a positive integer called the quantum number f is the frequency of the oscillation

A discreet packet of energy, later to become known as “a photon”

nhfE

Implications of Planck’s Law

The energy levels of the molecules must be discreet

Only transitions by an amount E=hf are allowed

The implication is that light is discreet or quantised

These quantum levels are now known as number states

43210

energy4hf3hf2hf1hf0

energy n

Photoelectric effect

When light strikes the When light strikes the cathodecathode, electrons , electrons are emittedare emittedElectrons moving between the two plates Electrons moving between the two plates constitute a currentconstitute a current

Photoelectric Effect Explanation

Einstein: the quanta of energy are in fact localised “particle like” energy packets

Each having an energy given by hf Emitted electrons will have an energy given by

Where is known as the “work function” of the material hft where ft is the threshold frequency for the metal.

hfKmax

Louis de Broglie1892 - 1987

Wave Properties of Matter

In 1923 Louis de Broglie postulated that perhaps matter exhibits the same “duality” that light exhibits

Perhaps all matter has both characteristics as well For photons,

h

c

hf

c

Ep

mv

h

p

h

Which says that the wavelength of light is related to its Which says that the wavelength of light is related to its momentummomentum

Making the same comparison for matter we find…Making the same comparison for matter we find…

Quantum Theory

Particles act like waves?!The best we can do is predict the

probability that something will happen.

Heisenberg Dirac Schrodinger

SchrödingerErwin

(1887 – 1961)

“The task is not so much to see what no-one has yet seen, but to think what nobody has yet thought, about that

which everybody sees.”

Schrodinger’s cat

After consultation with Einstein, Schrodinger proposed a thought experiment in which he highlighted the apparent inconsistencies between the so-called Copenhagen interpretation of Quantum Mechanics and the reality of macroscopic measurements.

He proposed that a cat be placed in a sealed box. The release of a poison is then subject to the probabilistic decay of a radioactive isotope. If the isotope decays, the poison is released. If no decay occurs, the poison is not released.

The result is that the cat is in a superposition of states between being dead, and being alive. This is very unintuitive.

Quantum mechanics

Wave-particle dualityWaves and particles have interchangeable

propertiesThis is an example of a system with

complementary properties

The mechanics for dealing with systems when these properties become important is called “Quantum Mechanics”

Measurement disturbes the system

The Uncertainty Principle

The Uncertainty Principle Classical physics

Measurement uncertainty is due to limitations of the measurement apparatus

There is no limit in principle to how accurate a measurement can be made

Quantum Mechanics There is a fundamental limit to the accuracy of a

measurement determined by the Heisenberg uncertainty principle

If a measurement of position is made with precision x and a simultaneous measurement of linear momentum is made with precision p, then the product of the two uncertainties can never be less than h/2

xx p

The Uncertainty Principle

Virtual particles: created due to the UP

E t

In Search of the Higgs Boson

Higgs boson is “cosmic molasses” – the Holy Grail of particle physics

Interactions with the Higgs Field are theorized to give all the particles their masses

LHC detectors should be able to confirm or disprove initial hints for Higgs at E=115 GeV