·
Definition: transmission outward in all directions of some emanation
·
e.g. electromagnetic waves, or, more simply, light
·
Henri
Becquerel (1896)
·
measured fluorescence of materials after being in the sun
·
found that uranium salts glow
even when they have not been in the light
·
they are radioactive.
·
Marie Curie refined and
purified these salts producing purer uranium, polonium, and radium
·
William
Crookes in the 1870s invented a vacuum tube in
which when electricity was pumped into a metal plate at one end (the cathode) it caused a glow in the
direction of a metal plate the anode)
at the other end.
·
Wilhelm
Röntgen discovered in 1895 that a cathode ray
tube also caused illumination of a coated paper screen up to 2 metres away.
·
J. J.
Thomson in 1897 at the Cavendish Laboratories at
·
tried to measure effects of cathode ray tubes
·
found that cathode rays could be generated from any element
·
found that they behaved like a stream of particles
·
Thomson believed the particles came out of chemical atoms.
·
He called cathode rays electrons.
Therefore, the “atom” had parts and was not an indivisible ultimate unit.
·
·
Ernest
Rutherford – 1911
·
from
·
student of J. J. Thomson at
·
later taught at
·
ultimately set up a laboratory at the
·
Set out to analyze the different “rays” that could be produced. Gave
them names from the Greek alphabet:
·
alpha rays
– later found to be the nucleus of helium atoms
·
beta
rays – turned out to be the same as cathode rays or electrons
·
gamma rays –
light of a small wave length, something like x-rays
·
To explore the structure of the atom,
·
Though most passed through the foil, some were deflected back.
·
·
A “black body” is
one that does not reflect light (or other radiation)
Planck’s
Quanta
·
Max
Planck – German physicist (work done in 1899-1900)
·
Realized that Maxwell’s (continuous)
wave equations led to the “catastrophe” because it allowed for infinitely small
amounts of energy.
·
A quantity divided by an infinitely small amount = an infinitely large
quantity.
·
If Planck used discrete
equations (as Boltzman did for statistical
mechanics), he could get around the division by zero problem.
h – the quantum
of energy
·
Planck found that energy could not be radiated at all in units smaller
than an amount he called h – the quantum of energy.
·
When he introduced the restriction h
into his equations, the ultraviolet catastrophe disappeared.
·
But what was the physical meaning of a smallest amount of energy?
Einstein
and the Photoelectric
Effect
·
Einstein took Planck’s constant, h,
to have serious physical meaning.
·
He suggested that light comes in discrete bits, which he called light quanta (now called photons).
·
This would explain how light can produce an electric current in a sheet
of metal.
·
Einstein’s Nobel Prize was for this work (not for relativity).
The
Bohr Atom
·
Niels Bohr (1885-1962)
The
Bohr
Atom and the Periodic Table
·
Bohr found that each “orbit” or “shell” had room for a fixed maximum
number of electrons.
·
2 in the first, 8 in the second, 18 in the third, 32 in the fourth,
etc.
·
This accounted for properties revealed by the Periodic Table
·
The Group number corresponds to the number of electrons in the outer
shell.
·
Louis de Broglie (1924) suggested that if
waves can behave like particles, maybe particles can behave like waves.
·
He proposed that electrons are waves of matter. The reason for the size
and number of electrons in a Bohr electron shell is the number of wave periods
that exactly fit.
Schrödinger’s
Wave Equations
·
Erwin
Schrödinger in 1926 published a general theory of “matter waves”
·
Schrödinger’s equations describe 3-dimensional waves using probability functions
·
Gives the probability of an electron being in a given place at a given
time, instead of being in an orbit
·
The probability space is the electron cloud.
Heisenberg’s Uncertainty
Principle
·
Werner Heisenberg
·
Schrödinger’s equations give the probability of an electron being in a
certain place and having a certain momentum.
·
Heisenberg wished to be able to determine precisely what the position
and momentum were.
·
To “see” an electron and determine its position it has to be hit with a
photon having more energy than the electron – which would knock it out of
position.
·
To determine momentum, a photon of low energy could be used, but this
would give only a vague idea of position.
·
Using any means we know to determine position and momentum, the
uncertainty of position, Dq,
and the uncertainty of momentum, Dp,
are trade-offs.
·
DqDp³ h/2p, where h is Planck’s constant
·
Note: the act of observing alters the thing observed.
Particles or Waves?
Are
the fundamental constituents of the universe
·
Particles – which have a position and momentum, but we just can’t know
it, or
·
Waves (or probability) – which do not completely determine the future,
only make some outcome more likely than others
·
Niels Bohr:
·
The underlying reality is more complex than either waves or particles.
·
We can think of nature in
terms of either waves or particles when it is convenient to do so.
·
The two views complement each
other. Neither is complete in itself.
Does
Quantum Mechanics Describe Nature
·
Einstein said no.
·
“God does not play dice.”
·
Is there no reality until we look?
·
If quantum mechanics is complete, radioactive decay doesn’t happen or
not happen until we measure it.
·
The cat is neither alive nor dead until we open the chamber.
·
Hugh Everett (1950s)
·
Every outcome that is possible happens, in different universes