Download 1 – Foundations of Quantum Theory

Foundations of
Quantum Theory
Maxwell Planck
• 1894 – Turned his attention
to Black Body Radiation
The Problem According to Kirchoff
• “how does the intensity (Energy/(second x unit
area)) of the electromagnetic radiation emitted
by a black body (a perfect absorber) depend on
the frequency of the radiation (i.e., the color of
the light) and the temperature of the body?”
• Planck used a mathematical interpretation
of the problem to predict the physical
application
• His math matched the experimental
results perfectly (Now that’s good
Physics!!!)
• The conclusion of the research (not his
conclusion) was interpreted to be that the
energy could only be released in quantised
(packet) form now referred to as a photon
Planck's Quantum idea:
 challenged the classical wave
theory of light by proposing that
electromagnetic waves do not
transmit energy in a continuous
manner, but instead, energy is
transmitted in small packages or
bundles.
Planck's Quantum idea:
 challenged the classical physics of
Newton, since it proposed that an
object is not free to vibrate with
any random energy; the energy is
restricted to certain discrete
values.
• "My unavailing attempts to somehow
reintegrate the action quantum into classical
theory extended over several years and
caused me much trouble." - Max Planck
• Classical theory views light as a wave
• Some argue that the father of Quantum
theory was Einstein because Planck didn’t
really realize what the math said (he kept
trying to make light a wave when his
calculations seem to refer to a packet of
energy whose size varies with frequency)
E = hf
• h – Planck’s Constant h= 6.626 x 10-34 m2kg/s
• f – frequency of the photon (Hz)
Reminder: f=c/
• This formula states that the energy released by
an object at a certain frequency is a multiple of
the energy of each packet of light (the frequency
by Planck’s constant)
Blackbody Radiation
1. At a given temperature, a spectrum of different
wavelengths is emitted, of varying intensity, but
there is a definite intensity maximum at one
particular wavelength.
2. As the temperature increases the intensity
maximum shifts to a shorter wavelength (higher
frequency)
Photoelectric Effect
What is it?
• effect which involves electrons being separated
from a substance as the substance is exposed to
electromagnetic radiation.
Condition for the Photoelectric Effect
• When a photon hits a material and
that energy is absorbed by an
electron:
▫ If the energy absorbed is greater than the
electron binding energy (energy holding
it there) then the electron takes off
▫ We sometimes call these photoelectrons
Note:
• Some substances just reemit the
radiation while some will dislodge
electrons
• If you increase the intensity of the
light (more photons), the number of
electrons ejected will increase but not
the energy given each electron
• Any extra energy is converted to the
kinetic energy of the particle
•Ephoton = W + Ek
Compton Effect
• The Compton Effect: The scattering
of low energy photons after colliding
electrons with high-energy photons
• The scattering photons have lower
energy and lower frequency
• High energy photons are usually Xrays
• Compton attributed particle like
momentum to high energy photons
Reinforces particle nature of photons
• Attributes a momentum like property to photons
• Since momentum is generally reserved for
particles, photons act like particles
▫ Therefore, we can apply conservation of
momentum and specifically cons of momentum
for an elastic collision
• Compton’s genius was to give light momentum by
using mass-energy equivalence
𝑝𝑝ℎ𝑜𝑡𝑜𝑛
ℎ
=
𝜆
Exray = E'xray + Eelectron
hf = hf' +
2
0.5mv
Pair Production
Using Mass Energy Equivalence
• When a photon collides with a heavy nucleus,
the photon disappears
• Since energy is equivalent to mass, two particles
are produced with equal mass but opposite
charge
▫ An electron and a positron