4.5 Quantum Physics Flashcards
What are photons?
Photons are particles of light.
They are sometimes referred to as a quantum of
energy of EM radiation. ‘A quantum’ in this
context just means a set (finite) amount.
True or false: The energy of a photon is
proportional to the wavelength of the
light.
False.
It’s proportional to the frequency: E = hf
Energy is inversely proportional to the
wavelength.
What is ‘h’ in the equation E = hf? Give
units.
h is the Planck constant, measured in Js.
What quantity can be measured in
electron volts (eV)?
Energy.
Describe an experiment which can be
used to estimate the value of the Planck
constant.
An LED will only allow current to pass after a minimum voltage
has been put across it.
● At this voltage all the electrons will have the same energy as a
photon emitted by the LED (which you will know the frequency
of).
● Finding the threshold voltage by seeing when current flows in
the circuit can then be used to find h from: h = E/f
Describe an experiment which can be
used to estimate the value of the Planck
constant.
An LED will only allow current to pass after a minimum voltage
has been put across it.
● At this voltage all the electrons will have the same energy as a
photon emitted by the LED (which you will know the frequency
of).
● Finding the threshold voltage by seeing when current flows in
the circuit can then be used to find h from: h = E/f
Describe how to improve the accuracy of
the estimate of this experiment
● To improve the accuracy of this estimate, the experiment can be
repeated with a variety of different coloured LEDs, which each emit
different wavelengths of light.
● The values of wavelength and threshold p.d. for each can be
recorded, and a graph drawn of V against 1/λ.
● The gradient of this graph will be equal to hc /e.
● As the speed of light and the electron charge are known constants,
we can calculate the value of h from this.
What is the photoelectric effect?
The photoelectric effect is a phenomenon where
shining light with enough energy onto a metal
releases electrons (and can cause a current to
flow).
The electrons emitted are called photoelectrons.
Which features of the photoelectric effect
can’t be explained if light is a wave?
If light was a wave, then the energy of the electrons
released would increase with increasing intensity of the
light - but this isn’t the case. Instead the energy of the
electrons depends on frequency (and no electrons are
released below a certain threshold value, no matter how
intense the light is).
Which features of the photoelectric effect
can’t be explained if light is a wave?
If light was a wave, then the energy of the electrons
released would increase with increasing intensity of the
light - but this isn’t the case. Instead the energy of the
electrons depends on frequency (and no electrons are
released below a certain threshold value, no matter how
intense the light is).
How many photons does each
photoelectron absorb prior to emission?
Only 1.
If it doesn’t contain enough energy the electron
will re-emit the energy rather than being
released.
How does the photon model of light
explain the threshold frequency seen in
the photoelectric effect?
Each electron absorbs a single photon. This
single photon must have enough energy for the
electron to be released, if it doesn’t the energy is
re-emitted. The electron can’t build up energy as
it could if light was a wave.
What is the name given to the minimum
amount of energy an electron requires to
leave the surface of a metal?
The work function (or ‘work function energy’),
Write a word equation relating the
energy of an incident photon to the work
function and the kinetic energy of
released electrons.
Photon energy = work function + kinetic energy
True or false: The rate of emission of
photoelectrons is proportional to intensity
(provided the light is above threshold
frequency).
True.
Higher intensity means more photons, this
means more electrons can absorb energy and be
released.
