Download 6.1.1

Did the Greeks Invent
Everything?!?
6.1.1 Photons, Photoelectric Effect, and Particle
Nature of Light
The Birth of Classical Physics
• The ancient Greeks were certainly not the first to
wonder about and investigate nature, but they were
the first to leave written records of their ideas.
• They recorded ideas regarding a vast number of
subjects from Astronomy to Zoology.
• They conceptualized the building blocks of matter –
which they called the ‘atom’ -- literally means
‘cannot be cut’.
585 BCE – 1900 CE
• Beginning with the Greeks and
continuing until around 1900,
the system of “Classical
Physics” slowly emerged with
these fundamental ideas at its
core…
– Newton’s Laws define mechanics
– Light is a WAVE
– Atoms make up matter and are
indivisible
• In around 1900, physicists were
announcing that physics is a
COMPLETE SCIENCE and can
explain EVERYTHING!
Well… almost everything…
• Physicists in this era were still unable to
explain THREE important phenomenon
Blackbody Radiation
Wave Theory: A blackbody will emit
radiation that directly relates to its
temperature. The higher the
temperature, the more frequencies
can be emitted.
Experiment: The intensity of radiation
does not keep increasing but forms a
curve of intensities below the
expected amount.
Well… almost everything…
• Physicists in this era were still unable to
explain THREE important phenomenon
Bright Line Spectrum
Wave Theory: When you heat an
element it should radiate a
continuous range of colors related
to that temperature (spectrum).
Experiment: Each element emits only
very specific colors in a ‘bright line
spectrum’
Well… almost everything…
• Physicists in this era were still unable to
explain THREE important phenomenon
Photoelectric Effect
Wave Theory: Increasing intensity should
give the same number of faster
electrons. Increasing frequency should
give more electrons per second.
Experiment: Increasing intensity gives
more electrons per second. Increasing
frequency gives faster electrons or
makes the effect stop working.
Planck’s Constant
• Max Planck invented the concept of the
QUANTUM to explain the problem of
Blackbody Radiation.
• Derived the constant 6.63 x 10-34 J·s to describe
the scale of energy increments allowed in
atoms.
• Energy going into and out of atoms is
QUANTIZED. By why?
Photoelectric Effect
• Albert Einstein uses the concept of the
quantum to explain the Photoelectric Effect.
• Invents the idea of the PHOTON which has an
energy related to Planck’s Constant and its
frequency.
Equation
E photon  hf 
hc

Einstein’s Model
Each photon
has a chance
to liberate
So, we have phenomenon
that
show
that
an electron
light MUST be a wave…
beam of light
but the photon must have enough
energy (the right frequency)
ex. DIFFRACTION and INTERFERENCE
a brighter beam
would mean MORE
photons
…but the PHOTOELECTRIC EFFECT
metal plate shows that light
MUST be made up of particles…
• For Einstein’s explanation
toIT?!?
be correct light is
SO WHICH IS
NOT aEM
wave!
It must
be ismade
of waves
particles.
Radiation
(light)
neither
nor The
particlesparticles
carry abut
certain
amount of
has properties
of energy
BOTH. related
to the light’s frequency.
Photon Properties
•
•
•
•
Massless
Carry energy and momentum
Travel at the speed of light
Have particle-like collisions – COMPTON EFFCT
Before
incoming
photon
After
electron
e-
e-
v
v=0
electron
gains KE
outgoing photon
lower E, lower f
longer λ
Example #1
• What is the energy of a photon with a
wavelength of 4.5 x 10-7 meters? What type of
photon is it?
Ephoton = hf = hc/λ
Ephoton = (6.63 x 10-34 J·s)(3 x 108 m/s) / (4.5 x 10-7 m)
Ephoton = 4.4 x 10-19 J
Visible Light
Example #2
• If a photon has 7.0 x 10-3 joules of energy, what
frequency does it have? What type of photon
is it?
Ephoton = hf = hc/λ
7.0 x 103 J = (6.63 x 10-34 J·s) f
f = 1.1 x 1031 Hz
Gamma Ray
Wave/Particle Duality
• 1924 Louie de Broglie: because light has
properties of particles, it might be possible for
matter to exhibit the properties of waves.
• This was proven to be true by ELECTRON
DIFFRACTION experiments demonstrated by
Davisson and Germer in 1927.
• While particles do seem to have a particle nature,
this is only evident for FAST-MOVING, SUBATOMIC PARTICLES.
Mass/Energy Duality
• In 1905, Einstein publishes his paper on Special
Relativity.
• One of several proposals from this paper is that:
MASS  ENERGY AND ENERGY  MASS.
Equation
E  mc
2
End of 6.1.1 - PRACTICE