Download G482 Electrons , Photons and Waves

G482 Electrons , Photons and
Waves - Quantum Physics
Quantum Physics
• Energy of a photon
• The photoelectric effect
• Wave–particle duality
• Energy levels in atoms
Energy of a Photon
What is a Photon ?
a quantum/packet/of electromagnetic
energy
Energy of a Photon
How do you calculate the energy of a photon ?
The Photoelectric Effect
Describe the Photoelectric Effect
A photon is absorbed by an electron in a metal surface causing an electron to be
emitted.
Energy of photon = hf
Energy is conserved.
Only electrons with energy above the work function energy will be emitted.
Only light with a frequency greater than the threshold frequency will cause emission
Energy=work function + Max KE of electron.
The work function is the minimum energy required to release an electron from the
surface.
Number of electrons emitted depends on light intensity.
( Energy of Electron does not)
The Photoelectric Effect
Describe the Photoelectric Effect Experiment
A clean zinc plate is mounted on the cap of a gold leaf electroscope
where the plate is initially charged negatively .
Shine a UV light on the plate and watch the gold leaf collapse as
charge leaves the plate, indicating the emission of electrons
The Photoelectric Effect
Photoelectric Effect Graphical Analysis
The Photoelectric Effect
Photoelectric Effect Graphical Analysis
As intensity goes up
it does not affect
the energy of the
electrons.
As frequency goes
up it does not affect
the number of
electrons emitted
As intensity goes up
more electrons
emitted
The Photoelectric Effect
Photoelectric Effect Numerical Analysis
To convert from eV to J multiply by 1.6 x 10 -19
The Photoelectric Effect
Describe the LED Experiment to find h
Adjust the potential divider to low/zero voltage
Connect flying lead to one LED
Increase voltage until LED just lights/strikes
Repeat several times and average to find Vmin
Repeat for each LED
Shield LED inside opaque tube to judge light more accurately
Wave Particle Duality
When do waves behave like particles ?
In the photoelectric effect – the energy of the light does not
depend on the amplitude of the wave but on its frequency.
Wave Particle Duality
When do particles behave like waves ?
Travelling electrons are diffracted by graphite producing
a series of diffraction rings. Maximum diffraction occurs
when the de Broglie wavelength of the electrons is
similar to the gap between layers of carbon atoms.
Wave Particle Duality
What is the de Broglie wavelength for an electron ?
Travelling electrons are observed to behave as
waves/show wavelike properties
Where the electron wavelength depends on its
speed/momentum
Energy Levels in Atoms
What is an Emission Line Spectrum?
light emitted from excited isolated atoms produces a
line spectrum
a series of sharp/bright/coloured lines
against a dark background
Energy Levels in Atoms
What is an Absorption Line Spectrum?
in an absorption spectrum a series of dark lines appears
against a bright background/within a continuous
spectrum
Energy Levels in Atoms
Why are spectra important?
They reveal that electrons within
atoms can only occupy certain
energy levels.
Explaining Line Spectra
Energy Levels in Atoms
An energy-level diagram for the hydrogen
atom
Numerical and Graphical Analysis
Electrons have discrete energy
levels within atoms - they
cannot have any energy .
hf or hc/λ = E1 – E2