STAGE 2 PHYSICS
READING
Electricity and Magnetism
Key Ideas pg 73-89
pg 101-104
Photoelectric Effect
Key Ideas
Intended Student Learning
Photons
Using graphical and algebraic methods, solve
problems that require the use of Kmax  hf  W .
Describe how microscopic observations of the building
up of an image produced by light of very low intensity
demonstrate the arrival of localised bundles of energy
and momentum called ‘photons’.
Calculate the energy and momentum of the photons in
various regions of the electromagnetic spectrum.
Describe how two-slit interference patterns build up
over time when light of very low intensity is used.
Photoelectric Effect
When light of sufficiently high frequency is incident on
matter, it may be absorbed by the matter, from which
electrons are then emitted. This is called the
‘photoelectric effect’.
Describe an experimental method for investigating the
relation between the maximum kinetic energy of the
emitted electrons (calculated from the measured
stopping voltage) and the frequency of the light
incident on a metal surface.
The intensity of the incident light affects the number,
but not the energy, of emitted electrons.
Describe how Einstein used the concept of photons
and the conservation of energy to explain the
photoelectric effect.
The minimum frequency f o at which electrons are
emitted varies with the type of material and is called
the ‘threshold frequency’.
Deduce the equation Kmax  hf  W , where K max is
the maximum kinetic energy of the emitted electrons.
The work function W of a surface is the minimum
energy required to remove an electron from it.
Plot experimental values of maximum kinetic energy
versus frequency, and relate the slope and horizontal
and vertical intercepts to the equation Kmax  hf  W .
The work function W is related to the threshold
frequency by W  hf o .
Using graphical and algebraic methods, solve
problems that require the use of Kmax  hf  W .
Photons
The particle model of light states that when light interacts with matter, it can behave as if it were
made up of a stream of particles known as photons. This model can explain many properties of light
that cannot be explained by the wave model, such as the photoelectric effect and production of xrays
Photons are localised bundles (quanta) of energy and momentum. Individual photons have a
frequency and wavelength; they do not however have any mass. Because of this, the general
equations that apply to particles do not apply to photons
Equations
E=
p=
v=
h=
Questions
1. Light of 525nm is made up of photons find:
a. The frequency of the light
b. The momentum of the photons
c. The energy of the photons
Particle or Wave?
The particle property of light was first demonstrated by conducting Young’s double slit experiment
using low intensity light and a sensitive photographic film. Classical physics predicted very dim, but
fully formed bands on the film. What was observed however looked like randomly scattered
individual dots of light. Each dot corresponds to a single photon reaching the film. If left for an
extended period of time a complete two slit interference pattern is produced.
The Photoelectric Effect
The photoelectric effect is the instantaneous emission of electrons when light strikes the surface of a
metal.
The light used must have a frequency above the threshold frequency for that particular metal.
Electrons are bound to a metal by a particular amount of energy characteristic to that metal. To
produce the photoelectric effect, the incident light must therefore have enough energy to free
electrons from the metal.
The threshold frequency is the lowest frequency such that the incident light has enough energy to
cause electrons to be released.
The work function is the smallest amount of energy binding electrons to the metal. It is given the
symbol W. Therefore, for a particular metal, W = h.f0, where f0 is the threshold frequency.
Experimental Method
•
The potential difference between anode and cathode is
initially zero
•
When monochromatic light is shone on the metal
cathode, electrons may be emitted. Some of these will
travel across the gap to the anode. Thus, a current is
produced in the circuit.
•
The potential difference is increased, which slows the
electrons down as they cross the gap. The work done on
these electrons is given by W = ΔV.q.
•
Therefore only those electrons with kinetic energy greater than this will make it across the
gap
•
The potential energy is increased until the current decreases to zero.
•
Just before this, the only electrons that are able to cross the gap are those with the
maximum possible kinetic energy
•
The maximum kinetic energy can then be calculated:
•
𝐾𝑚𝑎𝑥 = 𝑊 = ∆𝑉. 𝑞
•
∴ 𝐾𝑚𝑎𝑥 = ∆𝑉. 𝑞
•
These steps are repeated using different light sources and/or
different metals. The results for each metal are then plotted
on a graph of maximum energy versus frequency of light
Experimental Observations
1. The maximum kinetic energy of the emitted electrons depends on the frequency of light and
the metal used. The greater the frequency of light, the greater the maximum kinetic energy
of the electrons. The relationship between maximum kinetic energy, frequency of light and
the metal is 𝐾𝑚𝑎𝑥 = ℎ. 𝑓 − 𝑊
2. There is a minimum frequency of light that will eject photoelectrons (f0 for each metal used)
3. Increasing the intensity of light increases the magnitude of the current produced. However it
does not affect the stopping voltage. This is because although the intensity of light increases
the number of electrons emitted (due to more photons), it does not affect the maximum
kinetic energy (as frequency remains the same)
4. Electrons are emitted the instant that light is shone on the metal
Einstein’s Explanation of the Photoelectric Effect
In order to explain the photoelectric effect Einstein developed the following concepts.



A single photon passes its energy to a single electron
A photon will transfer all or none of its energy
An electron can only absorb the energy of one photon at a time
Einstein used these concepts to explain the following properties of the photoelectric effect:

Electrons are emitted the instant that light is shone on the metal.
When a photon collides with an electron, it instantly passes its energy to the electron

The greater the intensity of light, the more electrons are emitted
The greater the intensity of light, the more photons will strike the metal. This means that
more electrons will be emitted (as each photon can cause one electron to be emitted).

The kinetic energy of the electron does not depend on the intensity of light.
The intensity of the light determines the number of photons, but not the energy of the
photons (which depends on the frequency). Each electron will only absorb the energy of one
photon. Thus, an increased intensity will result in more electrons being emitted, but each
electron will have the same energy as before.

Emitted electrons may have a range of kinetic energies.
Different electrons within a metal will be bound by different amounts of energy (greater or
equal to W). The greater this amount of energy, the more energy is required to release the
electron. A photon must have energy at least equal to W in order to release the least bound
electron. If the photon has more energy than W it will be able to release electrons that are
more strongly bound. Any energy greater than the binding energy will be converted into the
kinetic energy of the emitted electron.