Transcript Dual Nature of Light

Einstein used Planck’s ideas to try to explain the
photoelectric effect.
Einstein thought that electrons bound in a
metal, are held with different amounts of
energy.
Einstein explained the idea using what he called
a ‘potential well’.
The y-axis shows the
total energy of the
electrons
The electrons are
bound to the metal
when their energy is
negative
When their energy is
zero, they are free
but stationary
When their energy is
positive, they have
kinetic energy
The minimum energy
required to emit a
electron from the
metal is called the
work function, W
The work function is
calculated at the
threshold frequency
As this is the
minimum frequency
required to eject an
electron
When a photon with
more energy than
the work function
hits the metal, the
energy over what’s
needed to eject it is
given to the electron
The difference
between W and hf is
the kinetic energy
The photon’s energy can be written as:
The maximum kinetic energy can now be
written as:
hf-W can be written as:
This is known as Einstein’s equation
For sodium, the work function is 2.3eV. Does
sodium exhibit the photoelectric effect for
yellow light of wavelength 580nm?
First calculate the energy of the photon
Then compare the energy of the photon and the
minimum energy required to eject an electron.
The photon has 2.1eV, which is less than the work
function (which is the minimum energy that the
photon must give the electron to eject it), so the
photoelectric effect will not occur for yellow light of
wavelength 580nm.
Ultraviolet light of wavelength 200nm is
incident on a clean silver surface. The work
function for silver is 4.7eV. What is:
the kinetic energy of the fastest moving ejected
electron
The threshold frequency for silver
The stopping voltage for silver
Ultraviolet light of wavelength 200nm is incident
on a clean silver surface. The work function for
silver is 4.7eV. What is:
the kinetic energy of the fastest moving ejected
electron
First calculate the energy of the incident photon
Then use Einstein’s equation to calculate the kinetic
energy
The kinetic energy of the fastest moving electrons is 1.5eV
Ultraviolet light of wavelength 200nm is
incident on a clean silver surface. The work
function for silver is 4.7eV. What is:
The threshold frequency for silver
Calculate f, using W = hfo
Ultraviolet light of wavelength 200nm is
incident on a clean silver surface. The work
function for silver is 4.7eV. What is:
The stopping voltage for silver
As calculated previously, the maximum kinetic
energy of the electrons is 1.5eV, then the
stopping voltage will be 1.5V
In 1923, Compton showed that X-ray photons
could collide with electrons and scatter, leaving
with a longer wavelength (less energy) than
before.
This is only possible if the photons were able to
transfer momentum and hence energy to the
electrons.
Maxwell suggested that photons do have
momentum given by
where c is the speed of light and E is the energy
of the photon.
As the energy of the photon is related to its
frequency by Planck’s equation, and
since c = f for waves, the momentum equation
can be written as
Light acts as both wave and particle:
It has a dual nature.
There have been experiments that can explain
both models, but only one model at a time.
Sir William Bragg’s explanation of light was as
follows:
‘On Mondays, Wednesdays and Fridays, light
behaves like waves, and on Tuesdays, Thursdays
and Saturdays like particles, and like nothing at all
on Sundays’
So even with current research and today’s
technology, we don’t really know what light is.