After this until late 19th Century
mechanics became a discipline capable
of describing the motion of particles in
a clear and deterministic way.
(Light / Opticks was a
stream of particles -
Atomistic)
James Clerk Maxwell
1873 - Treatise on
electricity and
magnetism
He determined, that all
electromagnetism was linked,
and that the speed of light
could be determined
Hans Christian
Oersted / Michael
Farday
Thomas Young
1801
Light had
'wavelike'
characteristics
Interference Phenomena
"Adding in phase waves
gives mutual
reinforcement - Adding
out of phase waves
gives cancellation"
The OLD Quantum Theory
Michelson and Morley 1880
Spectra
Balmer
1885
Discovered that the
spectrum of Hydrogen,
could be described by a
simple mathematical formula
Anotações:
when light is shone through hydrogen gas, and then through a prism, the frequencies that have been absorbed by the gas are absent in the spectra. These frequencies could be predicted by a certain simple formula.
J.J Thomson
1897
Was the first to discover Electrons
In his model of the Atom -
Electrons, Neutrons and Protons.
His model was unsuccessful in
explaining Balmer's results
Tried to
understand
Balmer's result
The UltraViolet Catastrophe
Lord Rayleigh
1900
James Clerk Maxwell &
Ludwig Bolzmann concluded
that they cold reliably predict
the overall behaviour of a
complex system.
Lord Rayleigh used this
'statistical physics' applied
to a case of the black body
Anotações:
A black body is a hypothetical, whereby this 'body' can totally absorb all the radiation emitted at it, and then re-emit it, when it is being recorded.
In this assumption, a good approximtion could be made to a specially prepared oven. In doing so the answer is only dependant on the temperature of the black body.
The results from the Black
body experiment did not
match the calculations
Classical physics assumed
that radiation 'oozed' in and
out of the 'black body'
Max Planck
Submitted that energy was emitted or
absorbed from time to time in small
packets of energy of a definite size and
that the size of this energy packet was
directly proportional to the radiation's
frequency. THE BIRTH OF PLANCK's CONSTANT
Anotações:
This meant that high frequency radiation could only be emitted or absorbed in events involving a single quantum of significantly high energy, explaining why high frequency events seemed unusually supressed by comparison to the classical physics.
The Photoelectric effect
Albert Einstein
1905
In classical theory - Radiation absorbed by
a sheet of metal could release electrons if
the intensity or energy content of the beam
was sufficient to release the electron. In this
model the electron release would only be
related to the intensity, not the frequency
his experiment showed the opposite. Below
a certain frequency a beam may emit no
electrons however intense the beam is.
above that frequency a beam could emit
electrons however low the intensity
Anotações:
This puzzling behaviour became instantly understandable when you consider the beam of radiation as a stream of persisting quanta. An electron would be ejected if one of these quanta had collided with it and given up all its energy. The amount of energy in the quanta was directly proportional to the frequency, related by Planck's constant.
If the frequency were too low, then there would not be enough energy transmitted in the collision to enable the electrons escape.
The intensity of the beam, simply determined how many quanta were in the beam, and so how many electrons were involved in collisions and were able to escape. Increasing the intensity could alter the amount of energy in a single collision.
These Quanta became 'photons'
CRISIS
1925 + 1926
Heissenburg
Matrix Mechanics
Erwin Schrodinger
Wave Mechanics
The Nuclear Atom
Rutherford
1911
How come Alpha
particles were deflected
by a thin gold film
Anotações:
It was as astonishing as if a 15" naval round had recoiled on striking a sheet of toilet paper.
J.J Thompson's Plum Pudding model
(1897) of Atom's could make no sense
now
The Positive charge of the Gold Atoms, could not be
spread out as suggested, but must be concentrated at
the centre of the atom in order to provie enough
repulsion to deflect an alpha particle.
this gave way to the Solar System theory
Which led to yet more crisis - If electrons are constantly encircling an
atomic nucleus they are constantly changing direction. For this to work
they would have to radiate away some of their energy and gradually
decline to the centre of the nucleus. Atoms would be unstable and this
process of decay the spectra should no longer be as defined as the
spectra that balmer had uncovered
The Bohr Atom
Niels Bohr
1913
Bohr took on some of the principle
thinking that Plank had used.
Rather than to suppose that
radiation could ooze in and out on
a sliding scale he assumed a system
that was rather more discrete.
Electrons he proclaimed could only
exist at certain permitted orbits.
(against the position a classical
physicist may take)
Using Planck's constant he proposed how
these radii could be determined. The
immediate consequence was that his theory
provided a more fitting model of an atom
THAT WAS STABLE. His mathematical
formula also led to the Balmer formula
discovered almost 30 years previously
Anotações:
once an electron was in an orbit, corresponding to the lowest permitted radius - it had nowhere else to go so no more energy could be lost. It may have got to this point by emitting energy as it dropped from higher orbits. This energy would be emitted as a single photon.
Compton Scattering
Arthur Compton
1923
He found that scattered x-rays had
their frequency changed by matter.
with a wave model this could not
be understood.
Anotações:
in the classical theory, it would be argued that the scattering process would be due to electrons in the atoms absorbing and re-emitting energy from the incident waves without altering the frequency
According to Planck's theory a change in
energy is equal to a change in frequency
which fitted, and thus Compton was able to
give a quantitive explanation for his findings
providing the most persuasive evidence to
date of the particle like character of
electromagnetic radiation.