EXPERIMENTAL STUDY OF PHOTO ELECTRIC EFFECT
Figure below shows the experimental set up for studying the photoelectric effect.
The arrangement consists of an evacuated glass or quartz tube inclusive a photosensitive cathode C and metallic A.
A transparent window W is sealed onto the glass tube which can be covered with different filters to obtain the desired frequency.
The anode and cathode are connected to a battery through a potential divided by which potential difference between anode and cathode can be changed.
The reversing switch RS tends to make anode positive or negative with respect to cathode.
The P.D between anode and cathode is measured by the voltmeter V while photoelectric current is indicated by the micro ammeter.
1.
1.Effect of intensity of light on photo electric current
1.Effect of intensity of light on photo electric current
The anode A is maintained at positive potential with respect to cathode C and a radiation of suitable frequency (above threshold frequency) is incident on cathode C.
As a result photo electric current is set up.
Keeping the frequency of incident radiation and accelerating potential fixed the intensity of the incident radiation is changed in steps.
For each value of intensity of radiations the corresponding value of photo electron current is noted.
If we plot a graph between intensity of radiation and photoelectric current it is found to be a straight line passing through the origin O as shown in figure below
This shows that photoelectric current is directly proportional to the intensity of incident radiation
The intensity of radiation can be changed by changing the distance between cathode C and the source of radiation.
Effect of potential of anode with respect to cathode on photoelectric current
We keep the anode at some positive accelerating potential with respect to cathode C and illuminate the cathode with radiation of fixed frequency f above threshold frequency and fixed intensity I
If we increase the positive potential on anode gradually, it is found that photo electric current also increases a stage comes when the photo electric current becomes maximum.
If we increase the positive potential on anode further the photo electric current does not increase.
This maximum value of photo electric current is called saturation current and corresponds to the photo electrons emitted by the cathode reach the anode A
Now saturation current is of higher value as shown in figure below
This is expected because the greater the intensity of incident radiation the greater is the photo electric current
Stopping potential
Stopping potential is the minimum retarding potential at which photoelectric current becomes zero Or is the potential difference when no electrons are able to reach the anode.
It is also known as stopping voltage or cut-off potential.
Stopping potential is a measure of the maximum kinetic energy of the photo electrons.
Since potential difference V
For stopping Voltage Vo
For an electron
At V0, even the photo electrons having maximum kinetic energy K.E max (i.e. fastest photo electrons) cannot reach the anode A.
Therefore, the stopping potential V0 is a measure of the maximum kinetic energy K.E max of the photo electrons.
eV0 is the work done by the retarding force to stop the photo electron with maximum kinetic energy and is therefore equal to K.Emax.
At V0, it is found that the photoelectric current cannot be obtained even if we increase the intensity of radiation. It is same for different intensities I1, I2 and I3 of incident radiation.
3. Effect of frequency of incident radiation on stopping potential.
We now study the relation between the frequency f of the incident radiation and the stopping potential V0.
For this purpose, we take the radiations of different frequencies but of the same intensity.
For one frequency say f1, of the incident radiation, we plot the graph between photoelectric current and potential of anode A with respect to cathode C at a constant intensity of incident radiation
Keeping the intensity of incident radiation the same, we repeat the experiment for frequency f2 of the incident radiation.
The following is the resulting graph
Observations from the graphs
1. The value of stopping potential is different from radiation of different frequencies
2. The value of stopping potential is move in low higher frequency. This implies that the value of maximum kinetic energy depend on the frequency of incident radiation.
The greater the frequency of incident radiation, the greater is the kinetic energy of emitted photo electrons.
3. The value of saturation current depends on the intensity of incident radiation but is independent of the frequency of incident radiation.
If we draw a graph between the frequency of incident radiation (f) and the stopping potential (V0) at constant intensity of radiation, it will be a straight line AB as shown in figure below
From the graph
At fo, stopping potential V0 = 0. It means that at fo, the photo electric current is just zero (i.e. photo electrons and emitted with zero velocity) and there is no retarding potential.
V0 = 0
This limiting frequency fo is called threshold frequency for the cathode material.
It is a minimum frequency of the incident radiation which is just sufficient to eject photo electrons (i.e. with zero velocity) from the surface of a metal.
Stopping potential is directly proportional to the frequency of incident radiation.
V0 α f
V0 α f
The greater the frequency of incident radiation, the higher is the stopping potential and vice versa.
Experiments show that photo electric emission is an instantaneous process.
As soon as light of suitable frequency (equal to or greater than fo) is incident on the surface of the metal, photo electrons are emitted from the metal surface.
The time delay is less than 10-9 second
LAWS OF PHOTOELECTRIC EMISSION
The above experimental study of photoelectric effect leads to the following laws of photoelectric emission.
i. For a given metal, there exists a certain minimum frequency of incident radiation below which no emission of photo-electrons takes place. This cut off frequency is called threshold frequency fo.
ii. For a given metal and frequency of incident radiation (>fo) the photo electric current is directly proportional to the intensity of incident radiation.
iii. Above the fo, the maximum kinetic energy of the emitted photo-electron is independent of the intensity of the incident radiation but depends only upon the frequency of the incident radiation.
iv. The photoelectric emission is an instantaneous process.
The above laws of photoelectric emission cannot be explained on the basis of light or radiation. This gave death blow to the wave theory of light or radiation.
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