Transcript Quantum Theory Chapter 27

Quantum Theory
Chapter 27
According to Hertz Maxwells theory
showed that all of optics was explained
by electromagnetic theory, but…….
Two small problems remained
1. Wave theory could not describe the spectrum
of light emitted from a hot body, such as steel
in a photo.
2. Ultraviolet light discharged electricallycharged metal plates.(this was called the
photo electric effect)
Solving these two little problems required the
development of quantum theory.
• These so called “hot” )bodies produced such
thermal energy(from vibrating particles) that
they glowed.(radiate)
• The hotter they become the more energy they
put off. And the brighter the light becomes.
The frequency at which the maximum amount
of light is emitted is directly proportional to
the temperature in Kelvins.
• As temp increases so does power. The amount
of power emitted each second is proportional
to the absolute temp raised to the 4th power.
Ex. Our sun is a yellow star is has less power and
lower temp than a white star. This is true even
though our sun radiates each square meter of
Earth with 1000J per second.
Look as the spectrum on page 556 of your text.
Max Planck assumed the energy of all vibrating
atoms gave off specific frequencies and
introduced this revolutionary equation in
1900.
E = nhf
f= frequency of vibration
h= Plancks constant (7X10 -34 J/Hz)
n= must be an integer
The energy of an incandescent body is
quantified, Max Planck
• Planck also stated ”All atoms do not radiate
electromagnetic waves when they are vibrating,
instead they only emitted when their vibration
energy underwent a change”
• He also put forth that the changes only occurred
in integer valued "quanta” ie. The change in
whole values of hf=E
• This was the first time the scientific community
realized that the physics of Newton was not valid
on all conditions.
Photoelectric effect
• By definition: The emission of electrons when
electromagnetic radiation falls upon a
material. (more on page 557)
• Regardless of how bright or dim the light is, to
allow any emission of electrons the energy
(light) on a metal must exceed the threshold
frequency. Fo.
• This threshold frequency varies with each
metal.
• Recall that electromagnetic radiation theory states, the more
intense the radiation, regardless of the frequency, the
stronger the electric and the resulting magnetic field. Thus for
low intensity radiation the electrons would need to absorb
radiation for a long period of time to reach the threshold
frequency and be ejected.
• The photon theory on the other hand explains the
photoelectric effect by proposing that light and all other
forms of radiation consist of discrete bundles of energy
(Quanta) and the energy of each photon depends on the
frequency of the light.
In comes Einstein
• In 1905 Einstein stated, “the light and other
forms of radiation must come in discrete bundles
of energy, later named photons.
• The energy of each photon would depend on the
frequency of the light.
• He explaind the existance of the threshhold
Planck spoke of and stated that the excess energy
not used to release electrons was shown in the
formula KE= hf-hfo
yes, kinetic energy
• How can Einstein theory be tested? Using a
cathode tube like the on pictured on pg 558. The
max KE of the electrons at the cathode is equal to
at the anode
• KE = -q Vo
• Vo is the magnitude of the stopping potential in
J/C and q is the charge of an electron in C.
• Since a Joule is so large for atomic systems we
use electron volts.
• 1eV =1.6x10-19J
• A graph of the kinetic energies of electrons ejected
from a metal verses the frequencies of the incident
photons is always a straight line.
• If you put max KE on the y-axis and Frequency on
the x-axis of a graph the slope of the line yields
Planck constant (h)
• The graph differs for each metal but always produces
the same results.
• The threshold frequency needed to release the most
weakly bound electrons is known as the work
function. W= hfo
The Compton Effect
• Even though a photon has no mass it has
kinetic energy?
ρ = hf/c
=h/λ
why? c is the velocity of light and v= λf
Yes, this is showing that you can indeed have
momentum without mass, neat huh?
• The energy of a photon is given by E= hc/ λ
• If a wave length increases the photon loses both
energy and momentum. This means that a tiny
shift in wave length in scattered photons can
cause a loss of momentum/energy. This energy is
conserved by the electrons in the metal in equal
amounts to what is lost by the photons.
• Thus, Compton prove that even though Photons
have no mass they do obey the conservation of
energy theorem.
• Assign page 570 reviewing concepts 1-10 all,
• Problems 1-13 same page.
A particle that behaves like a wave?
• Victor de Broglie in 1923 showed that
electromagnetic waves had particle like
properties.
Since
ρ = mv = h/λ
λ= h/ρ= h/mv
Remember KE= qV= 1/2mv2
Wrap up
• This little “non-mass” particle exhibits diffraction and
interference and it spreads out like a wave.
• Electromagnetic partials called photons act just like
what Einstein theorized light would, both as a wave
and a particle and thus exhibit duality.
• SORT OF LOKE HAVING YOUR CAKE AND EATING IT
TOO!T
• However this one is special, it has no mass but
exhibits both momentum and energy?
The Heisenberg Uncertainty principle
The Heisenberg Uncertainty principle states that
one cannot ever know exactly the location and
momentum of a photon of light. It simply cannot
ever be exactly measured.
Why?
1. you measure where a particle of light is
2. by the time you decide its placement is, ,it has
moved.
COOL HUH?
Assign pg 571 14-21 all