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Photoelectric Effect
Basically, the photoelectric effect is the ejecting
of electrons from a metal by shining light of a
particular frequency on it.
It doesn’t work for all wavelengths/frequencies
It was during 1887 that Hertz was conducting
the experiment in which the photoelectric
effect was discovered.
Light was shone onto the cathode, which at
certain frequencies emitted
electrons, which were
detected by the ammeter.
These emitted electrons
were called photoelectrons.
Named to show that light
was responsible for emitting
them.
The flow of these photoelectrons is called the
photocurrent.
In the initial set up of the experiment, the
cathode is made negative and the anode is made
positive.
When the photoelectrons are released from the
metal, they will naturally go to the positively
charged anode.
A sensitive ammeter will measure this
photocurrent flow
With this arrangement, the maximum current
flowing will be recorded by the ammeter.
All of the photoelectrons flowing across the gap
will have a certain amount of kinetic energy.
The amount depends on the location of the metal
that the electron came from
At or close to the surface – high energy
Well below the surface – low energy
When the anode and cathode had zero
potential, there was still a current being
recorded flowing across the gap.
Though not as high as when the anode was made
to be positive.
For electrons to be emitted, the light had to
have a certain frequency.
The minimum frequency that would cause this
is called the threshold or cut-off frequency, f0
Anything below this, would not provide enough
energy to eject electrons.
The German physicist Lenard found out the
following things:
The rate at which electrons are emitted depends on
the intensity of the light
High intensity – high current
Low intensity – low current
Cont’d..
Metals would only emit electron above certain
frequencies
Threshold frequency, f0
Electrons were emitted without any apparent time
delay
The time delay is actually about 10-9s
The intensity of light doesn’t affect the threshold
frequency.
Only frequency affects it
Lenard continued his investigations to find out
more about the kinetic energy of the electrons
being emitted.
In particular the maximum kinetic energy of the
released electrons
This was done by reversing the potential around
the circuit
The anode was made more and more negative
As this occurred, the current being recorded
was observed to decrease.
This showed that a decreasing number of
photoelectrons had enough energy to overcome
the opposing electric potential of the anode.
Electrons with little or no KE were stopped as soon
as the anode becomes negative
Once a certain potential was reached, no more
photoelectric current was recorded.
This point is called the stopping voltage, V0
The photoelectrons with the most KE being
stopped by Vo volts
Light with a higher frequency, was found to
have a higher stopping voltage.
Red light (low frequency)
No electrons
emitted
+
i = 0A
Bright Red light
No electrons
emitted
+
i = 0A
Green light. Electron emission starts
+
e-
μA
Small
current
Brighter Green light. Electron emission starts
+
e-
μA
Bigger
current
Violet light. Electron emission continues
+
e-
μA
Small current
Brighter Violet light. Electron emission continues
μA
+
e-
Bigger
current
Dim Light
+
-


e
-
Small
current
Small reversed voltage
Dim Light
+
e
--
No
current
More negative reversed voltage
Photoelectric effect animation
http://www.ifae.es/xec/phot2.html
A convenient unit of energy when dealing with
electrons is the electron volt (eV).
An electron gains 1eV of energy when it is
accelerated across a potential difference of 1
volt.
1 eV = 1.6 × 10-19 J
To convert from J to eV:
divide the energy (in Joules) by 1.6  10-19.
Since an electron gains 1eV of energy when it is
accelerated across a potential difference of 1
volt, it can be said that:
An electron accelerated through a potential
difference of 50V, will gain 50eV of kinetic energy
Should a stopping voltage of 10V be required,
the maximum kinetic energy of any electron
would be 10eV.
Convert 10eV into J:
Convert 650J into eV & MeV:
The voltage at which the current is reduced to
zero, the ‘Stopping voltage’ Vo, gives a measure
of the maximum kinetic energy of the emitted
photoelectrons.
Ek max = ½mvmax2= eVo
e - Charge on an electron
V0 – stopping voltage
m – mass of an electron = 9.11 x 10-31kg
vmax – maximum speed of the electron
A photoelectron is released though doesn’t
make it to ‘the other side’ as the stopping
voltage is 6.0V.
What is the maximum kinetic energy of this
photoelectron and at what speed must it be
travelling?
If the stopping voltage is 6.0V, the maximum kinetic
energy must be 6.0eV
After further work by Philip Lenard between
1899 and 1902, Albert Einstein concluded that
light does in fact behave like a particle.
The wave model could not explain the
photoelectric effect.