Photoelectric effect

1. INTRODUCTION TO PHOTOELECTRIC EFFECT
THE PHOTOELECTRIC EFFECT WAS
DISCOVERED IN 1887, BY THE GERMAN
PHYSICIST HEINRICH RUDOLF HERTZ.
FROM THIS EFFECT WE CAN KNOW
ABOUT THE “EMISSION” OF
ELECTRONS FROM THE METAL
SURFACE UPON IRRADIATION WITH
LIGHT WAVES.
LATER IT WAS EXPLAINED BY EINSTEIN.

2.  LATER EINSTEIN PROPOSED :-
-INCIDENT LIGHT IS LIKE INDIVIDUAL PARTICLES, QUANTA.
-HE COINED IT NAME PHOTON FOR QUANTA.
-FOR A GIVEN FREQUENCY, EACH PHOTON CARIES THE ENERGY “E=HV”.
 INSIDE THE METAL ELECTRONS LOOSES MORE ENERGY FOR TRAVELING ONE SURFACE TO
ANOTHER SURFACE.
 SURFACE ELECTRON COMES OUT WITH MAXIMUM ENERGY.

3.  THE MODEL PROPOSED BY EINSTEIN:-
SO ELECTRONS LEAVE FRO THE SURFACE WITH SOME KINETIC ENERGY, IF THE PHOTON IS
SUFFICIENTLY TO KNOCK OUT THE ELECTRON THE SURFACE.
KE=HV-ᶲ
WHERE, ᶲ IS WORK FUNCTION OF THE METAL, ENERGY REQUIRED TO REMOVE
THE ELECTRON FROM THE SURFACE TO VACCUM LEVEL.
KE=HV-HV*
WHERE, HV*=ᶲ
 SO THERE IS MINIMUM LIGHT FREQUENCY CALLED “THRESHOLD FREQUENCY”(V*).
 FOR THE GIVEN METAL AT WHICH QUANTUM OF AN ENERGY IS EQUAL TO “WORK FUNCTION”.
 HIGH INTENSITY OF LIGHT CAUSES MORE EMISSION WITH THE SAME KINETIC ENERGY.
 EXCESS ENERGY IS KINETIC ENERGY OF THE PHOTO ELECTRON.

4. CONSTRUCTION OF PHOTOELECTRIC CELL
• Photoelectric cell consists of highly evacuated or gas filled glass tube , an emitter and collector.
• The light enters through a quartz window and falls on the semi cylindrical cathode c coated with photosensitive
metal.
• The anode is in the form of straight wire of platinum or nickel, coaxial with cathode.

5. WORKING OF PHOTO ELECTRIC EFFECT
 EMITTER IS CONNECTED TO NEGATIVE TERMINAL AND COLLECTOR IS CONNECTED TO POSITIVE TERMINAL
OF A BATTERY.
 A RADIATION OF FREQUENCY MORE THAN THRESHOLD FREQUENCY OF MATERIAL OF EMITTER.
 THE PHOTO ELECTRONS ARE ATTACHED TOWARDS THE COLLECTOR WHICH IS POSITIVE WITH RESPECT TO
THE EMITTER . THUS, THE CURRENT FLOWS IN THE CIRCUIT.
 IF THE INTENSITY OF INCIDENT RADIATION IS INCREASED , THE PHOTOELECTRIC CURRENT INCREASES

6. ENERGY OF PHOTON
 According to Einstein, each photon of a light waves of frequency has the energy E is given by,
E=hv.
• where E=energy of photon(joule).
h=planks constant – 6.626x10^-34
v= frequency of photon.
• It is the energy carried by single photon.The amount of energy is directly proportional to the
photons electromagnetic frequency and thus equivalent,is inversely proportional to the
wavelength.

7. PROPERTIES OF PHOTON.
 A photon does not have any mass.
 A photon does not have any charge and are not deflected in electric field or magnetic field.
 All the quantum numbers are zero for a photon.
 In empty space, the photon moves at speed of light.
 In the interaction of radiation with matter,radiation behaves as if it is made up of particles
called photons.
 The energy and momentum of a photon are related as E=p.c where p-magnitude of
momentum. and c-speed of light.
 Photon is called as virtual particles.

8. PHOTO ELECTRIC EFFECT.
 When a beam of light of sufficiently high frequency onto a metal surface then the light will
cause electons to leave the surface.
 The phenomenon of emission of electrons by the metals when they are exposed to light of
suitable frequency is called as the photo-electric effect and emitted electrons is called as
photoelectrons.
 The minimum frequency of light which causes electrons to be emitted from a metal surface
is known as threshold frequency,if no electrons are ejected ,that means the frequency of the
light is less than the threshold frequency.

9. EINSTEIN PHOTOELECTRIC EQUATION.
 Einsteins photoelectric function- According to quantum theory, radiation is considered as shower of particles
called photons.
 Energy of photon absorbed by the atom is :
1. Used to detach the electron and
2. K.E is given to electron.
o hu=W0+K.E.
o hu=W0+1/2 MV^2
o W0= Photo electric work function=hu0.
o K.E=h(u-u0).
o Where M=mass of electron, V=velocity of electron , h= Plancks constant.
o u=Frequency of radiation, uo=Threshold frequency.

10. o Significance-
1.If u<u0- Kinetic energy is negative. i.e.., No emission.
2.If u=u0 –Kinetic energy is zero. i.e.., Emission just begins.
3. If u>u0 –Kinetic energy is positive. i.e..,Emission take place.
At this point the frequency is called as threshold frequency.
 Work Function, Wo
 Minimum amount of energy which is necessary to start photo electric emission.
 It is a property of material.
 Different materials have different values of work function.

11. What is threshold frequency ?
Threshold frequency is defined as the minimum frequency of light which causes electrons to
be emitted from a metal surface.
If no electrons are ejected ,this means that the frequency of the light is less than the
threshold frequency.

12. Why does threshold frequency exist ?
The threshold frequency is the frequency of light that carries enough energy to dislodge
an electron from an atom .This energy is entirely consumed in the process . therefore,
the electron gets no kinetic energy at the threshold frequency and it is not released
from the atom.

13. How to find threshold frequency ?
To find the threshold frequency we can use the formula
>> E=hf ;
E = energy of the photon
h = plank’s constant
f = threshold frequency

14. we can find the energy of photon by using
Einstein’s equation:
We can find energy of a photon by using
E = hf = hc / lamda
C = Speed of light in a vaccum
Lamda = wave length of photon
or
electromagnetic radiation

15. Difference between
threshold frequency and threshold wavelength

16. THE MAXIMUM KINETIC ENERGY OF
PHOTOELECTRON
• According to Einstein's photoelectric equation,
the maximum kinetic of the photoelectrons is
Kmax =hf− Φ
where h= Plank's constant,
f= frequency of the incident radiation,
Φ = work function for the material.
• Thus, the maximum kinetic energy of the
photoelectrons depends on the frequency of incident
light and nature of the emitter plate material.

17. • Thus, the maximum kinetic energy of the
photoelectrons depends on the frequency of
incident light and nature of the emitter plate
material.
• Einstein’s modal predicts a linear relationship
between Kmax and the light frequency f ,which is
confirmed by experimental observation .
• Cutoff frequency fc and cutoff wavelength λc :fc
=Φ/h and λc = hc/ Φ
• Stopping potential Kmax = e ΔVs

18. CONCEPT OF STOPPING POTENTIAL AND RETARDING
POTENTIAL
• Retarding potential
It is defined as negative potential of the collecting electrode at which the number of electron reaches
at the collecting electrode decreases.
• Stopping potential
It is defined as negative potential of the collecting electrode at which the no electron reaches at
collecting electrode.
It means at this potential entire kinetic energy of the photo electrons converted into potential energy.
So mathematically, If ‘Vo’ is the stopping potential then
1/2mV^2 max = eVo
eVo = potential energy of an electron

19. FACTORS AFFECTING PHOTO ELECTRIC EFFECT
 Material type
 Intensity
 Frequency
 Voltage
 Time lag

20. MATERIAL TYPE
• Different metal start emitting electron at different wavelength or frequency.
• If energy of frequency is more than work function of particular metal then electron will emit from that
metal.
Sodium Calcium Platinum

21. INTENSITY
• Intensity of light means the energy incident per unit area per second.
• For a given frequency, if intensity of incident light is increased, the photoelectric current increases and
with decrease of intensity, the photoelectric current decreases.
• It does not affect stopping potential.
Low Intensity High Intensity
V=Vo V=Vo

22. FREQUENCY
• Minimum frequency required to emit electron from metal called Threshold frequency.
• If we increasing the frequency further then that energy use by electron as Kinetic energy.
• Stopping potential is entirely depends on frequency.
Low Frequency High Frequency
V=V1 V=V2

23. STOPPING POTENTIAL VERSUS CURRENT CURVE
Constant frequency
Varying Intensity
Varying frequency
Constant Intensity

24. CODE IN PYTHON TO SHOW FREQUENCY VERSUS
ENERGY GRAPH FOR SODIUM METAL
Output should like above figure
Energy vs. Frequency

25. APPLICATIONS OF PHOTOELECTRIC EFFECT
• Photo-cell is the most important application. It is most commonly found in the solar panels. It works on the
basic principle of the light striking the cathode which in produces current .
• Photomultiplier tube makes use of photo electric effect to convert light intensity into electrical current.
• Photoelectric effect also founds applications in photocopies , light meter , photodiodes, phototransistors.
• Scintillators : A scintillators is a device that emit light when it attracts radiation from either source in the lab
or a cosmic source.

26. ADVANTAGES AND DISADVANTAGES OF PHOTOELECTRIC EFFECT
ADVANTAGES:
 Excellent radio graphic images: no scatter radiation.
 Enhances natural tissue contrast . Depends on 3 power of the atomic number ; so it magnifies the different
in tissues composed of different elements , such as bone and soft tissues .
 Lower energy photons : total absorption . Dominant up to 500kev

28. DISADVANTAGES:
 Excellent radio graphic images: maximum radiation exposure.
 All the energy is absorbed by the patient whereas in other reactions only part of the incident photons
energy is absorbed.