PHOTOELECTRIC EFFECT
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Photoelectric Effect
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What is it :
When metal surfaces are exposed to electromagnetic
radiation with sufficient energy they absorb the photons of
energy and emit electrons. This process is called the
photoelectric effect.
How did it all start?
Henrich Hertz was the first to discover this phenomena in
1887 when he was investigating radio waves.
In 1901 Max Planck showed that energy is quantized,
E=hf.
Albert Einstein explained the photoelectric effect in 1905.
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The effect of light on a metal
surface
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The photo-electric effect can be
demonstrated by means of an ultraviolet
lamp, a zinc plate, an electroscope and two
ordinary light bulbs of 40 W and 200 W.
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Photoelectric effect 5
An electroscope can be charged by
induction by holding a charged
acetate rod near the top plate.
Mobile negative charge in the metal
plate is repelled down to the leaf.
The leaf and central pole piece now
have the same type of charge so the
leaf rises.
Photoelectric effect 6
With the rod still nearby, the plate is
touched so more charge moves to the
plate through the person. The finger is
pulled away and then the charged rod
is removed. This is called CHARGING
by INDUCTION
You can charge the electroscope
positively by using a polythene rod
instead.
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Photoelectric effect 7
The process can be repeated whilst
a polished zinc plate is placed on
top of the electroscope.
ZINC PLATE
The same effect will be achieved
and the leaf will have been left in a
raised position. It will fall slowly
over time but not appreciably during
a short demonstration.
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Photoelectric effect 8
When the rod is removed, the extra
negative charge redistributes itself
evenly all over the leaf, central pole
and zinc plate.
Why does it do this?
ZINC PLATE
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Photoelectric effect 9
U-V photons
e
e e
e e
e
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Polished zinc
Start with an electroscope that
is charged negatively.
The U-V light causes
photoelectrons to be emitted.
These are repelled by the
surface and escape. Charge is
lost by the electroscope so the
leaf falls.
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Photoelectric effect 10
U-V photons
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Polished zinc
When all the extra charge has
gone, the leaf has fallen to its
resting position.
No further electrons will
escape because the surface will
not repel any liberated electron.
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Photoelectric effect 9
White light - a
mixture of all
VISIBLE colours
Polished zinc
The electroscope is charged
negatively.
The white light does not cause
photoelectrons to be emitted.
Charge is not lost by the electroscope. The leaf does
not fall no matter how bright(intense) the light
is or for how long it is shone onto the zinc.
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Photoelectric effect 12
Why?
Why do ultraviolet photons liberate
photoelectrons whilst visible light
photons do not?
Answer: none of the photons in white light
has enough energy to release even
one photoelectron
The energy of a photon is given by: E = hf
where h is Planck's constant and f is the frequency.
Also E = hc because c = fl (the wave equation)
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This means that the higher the frequency, the greater the
energy. Visible light contains frequencies that are too
for photoelectric emission.
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Alternatively, the shorter the wavelength, the greater the
energy of the photons.
Visible
wavelengths are too
long.
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Observations
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Ultraviolet light causes a negatively charged
electroscope to discharge – the leaves of the
electroscope collapse when UV light shines on it.
White light does not release e- from the zinc plate
even when irradiated with light of a much higher
intensity or for a longer period.
When the electroscope is positively charged
nothing happens because it is much more difficult
to remove e- from a positive object.
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When a glass plate is placed between the ultraviolet
source and the zinc plate, the electroscope stops
discharging.
CONCLUSION
1. Photoelectrons are emitted for a specific metal if the
frequency of radiation exceeds a certain limit (threshold
frequency, fo).
2.
The rate of photoelectron emission for a single frequency
radiation beam is proportional to the intensity of radiation
i.e. the more intense the radiation of the same
frequency the more photoelectrons are emitted.
3.
The emitted photoelectrons have kinetic energy ranging
from zero to a maximum.
4.
Maximum kinetic energy depends on frequency.
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5. The intensity of radiation has no effect on the
kinetic energy of the emitted photoelectrons.
6. Emission starts as soon as the surface is
irradiated with effective radiation.
7. Photoelectric current depends on intensity.
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Threshold frequency
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Each specific metal has a minimum frequency
called the threshold frequency for which
electrons will just be released from the metal.
The frequency of the incident light must be
equal to or greater than the threshold frequency
before electrons can be released.
More e- are liberated from a metal if light with a
higher frequency than that of the threshold
frequency of the metal strikes the metal surface
– an increase in intensity of this light increases
the number of e- that are liberated per second.
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Planck’s Quantum Theory
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In 1901, Max Planck, suggested that the
radiation of energy was not a continuous
process. Planck made the following
assumptions:
Energy is radiated in ‘packages’ or quanta.
Each quantum consists of a specific amount of
energy, E, which is directly proportional to the
frequency of the radiation: E  hf
A fraction of a quantum can never be radiated
nor absorbed, only whole numbers of quanta.
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After these investigations there
was a problem.
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Wave theory :
An electromagnetic wave produces an electric field, which exerts force
on the electrons on the surface of a metal. The force will push the
electrons from the surface.
Higher intensity of electromagnetic radiation results in a high electric
field which then produces a bigger electric force on the electrons. This
force will push off the electrons with a higher speed.
Emission should take place at any frequency because the electrons
would absorb energy from the incoming radiation until they have
energy enough to escape “So why threshold frequency?”
A The Quantum Theory (particle nature of light) was the answer
(Einstein, 1905)
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Einstein’s theory of the
Photoelectric effect
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EM radiation consists of small particles or lumps/packets of
energy called photons.
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Each photon carries energy proportional to its frequency.
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NB: There are free electrons in metals.
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When light is directed onto a metal surface a photon will collide
with a free electron.
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The interaction between a photon and an electron is a one to
one correspondence.
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The photon can then be reflected without a change in its kinetic
energy or it transfers all its kinetic energy to the electron.
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The electron gains all the kinetic energy from the
photon.
If the energy gained is sufficient the electron will
escape from the metal surface. This is the
process of photoelectric effect.
Part of the energy gained by the electron is used
to release it from the surface (i.e. to overcome
the force of attraction between the electrons and
the metal ions) and the rest of the energy is the
kinetic energy of the electron as it leaves the
metal.
The minimum energy required to overcome the
forces is called the work function (W).
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The magnitude of this energy
is a few electron volts.
The frequency that
corresponds to this energy is
the threshold frequency (fo).
The relation between the
work function and the
threshold frequency is given
by
W = hfo
Electrons are only emitted if
the frequency of radiation
greater than the threshold
frequency
(hf > W)
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Energy of photon
Energy of incident photon
= work function of the metal + maximum kinetic energy
of the released electrons.
hf = W + ½ mv2
where :
hf =
W=
the energy of each photon of frequency f
work function of the metal surface
½mv2 = maximum kinetic energy of the emitted
electrons
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Graph of Ek of photoelectrons vs
frequency of em-radiation
Maximum kinetic energy is measured in electron volts eV.
The threshold frequency (f0) of this material is 6,4 x1014 Hz.
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Graph of KE of electron and frequency of
incident light on metal
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WHY IS THE PHOTOELECTRIC
EFFECT SO IMPORTANT?
It helped explain the particle nature of
light.
 It is the basis of the quantum theory.
 It is used in photocells e.g. in solar
calculators, alarms and automatic
door openers
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The Dual Nature of Light
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What is light – a wave or a particle?
The wave theory cannot explain all the known facts in
connection with light.
Diffraction and interference can only be explained by the
wave theory.
The quantum hypothesis offers an excellent explanation
for the photo-electric effect but use the concept of
frequency to calculate the energy of a photon.
Light has both a wave- and particle nature.
The wave nature predominates during the propagation of
radiation, while the particle nature predominates during
the interaction with matter.
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Applications of the photoelectric
effect
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The photoelectric effect has many practical applications which include the
photocell, photoconductive devices and solar cells
A photocell A photocell is usually a vacuum tube with two electrodes.
One is a photosensitive cathode which emits electrons when exposed to
light and the other is an anode which is maintained at a positive voltage
with respect to the cathode. Thus when light shines on the cathode,
electrons are attracted to the anode and an electron current flows in the
tube from cathode to anode. The current can be used to operate a relay,
which might turn a motor on to open a door or ring a bell in an alarm
system
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The system can be made to be responsive to light, as
described above, or sensitive to the removal of light as
when a beam of light incident on the cathode is
interrupted, causing the current to stop. Photocells are also
useful as exposure meters for cameras in which case the
current in the tube would be measured directly on a
sensitive meter.
The photocell is at the centre of the many applications of
the photoelectric effect. It consists of a curved emitter and
a rod as collector, so as not to inhibit light from reaching
the emitter.
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The structure of a typical photocell is shown below:
The flash of a camera uses the photoelectric effect
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Photocells are used in garage door openers.
An example is shown in the diagram below:
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Spacecraft
The photoelectric effect will cause spacecraft
exposed to sunlight to develop a positive charge.
This can get up to the tens of volts. This can be a
major problem, as other parts of the spacecraft
in shadow develop a negative charge (up to
several kilovolts) from nearby plasma, and the
imbalance can discharge through delicate
electrical components. The static charge created
by the photoelectric effect is self-limiting, though,
because a more highly-charged object gives up its
electrons less easily
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Closely related to the photoelectric effect is the
photoconductive effect which is the increase in
electrical conductivity of certain non metallic materials
such as cadmium sulfide when exposed to light. This effect
can be quite large so that a very small current in a device
suddenly becomes quite large when exposed to light. Thus
photoconductive devices have many of the same uses as
photocells.
Solar cells, usually made from specially prepared silicon,
act like a battery when exposed to light. Individual solar
cells produce voltages of about 0.6 volts but higher voltages
and large currents can be obtained by appropriately
connecting many solar cells together.
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