Applide AScience Physics

1. Photoelectric
effect
Course: Diploma
Subject: Applied Science Physics
Unit: V
Chapter: II

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3. Photoelectric effect
• The phenomenon of emission of electrons from a
metallic surface by the use of light (or radiant) energy
is called photoelectric effect. The phenomenon was
discovered by Lenard. For photoelectric emission, the
metal used must have low work function, e.g., alkali
metals. Cesium is the best metal for photoelectric
effect.

4. Laws of Photoelectric effect
• The no. of electrons emitted per second i.e. photo
current is proportional to the intensity of incident
light.
• If frequency of incident radiation is below threshold
frequency, no photo electric emission will take place.
• The max. velocity or max. K.E of photoelectrons
depends on the frequency of radiation not on
intensity. K.E. Increases with the increase in
frequency.

5. Characteristics of Photoelectric effect
• (I) Effect of 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; but
the stopping potential remains the same.
• This means that the intensity of incident light affects
the photoelectric current but leaves the maximum
kinetic energy of photoelectrons unchanged.

6. • (ii) Effect of Frequency : When the intensity of
incident light is kept fixed and frequency is increased,
the photoelectric current remains the same; but the
stopping potential increases.
• If the frequency is decreased, the stopping potential
decreases and at a particular frequency of incident
light, the stopping potential becomes zero. This value
of frequency of incident light for which the stopping
potential is zero is called threshold frequency If the
frequency of incident light is less than the threshold
frequency no photoelectric emission takes place.
• Thus, the increase of frequency increases the
maximum kinetic energy of photoelectrons but leaves
the photoelectric current unchanged.

7. • (iii) Effect of Photo metal : When frequency and
intensity of incident light are kept fixed and photo
metal is changed, we observe that stopping
potentials versus frequency (v) graphs are parallel
straight lines, cutting frequency axis at different
points.
• (iv) Effect of Time : There is no time lag between the
incidence of light and the emission of photoelectrons.

8. EINSTEIN'S PHOTOELECTRIC
EQUATION
• According to Plank's quantum theory, light is emitted
from a source in the forms of bundles of energy
called photons. Energy of each photon is .
• Einstein made use of this theory to explain how photo
electric emission takes place.
• According to Einstein, when photons of
energy fall on a metal surface, they transfer
their energy to the electrons of metal.
E h
E h
2

9. • When the energy of photon is larger than the minimum
energy required by the electrons to leave the metal
surface, the emission of electrons take place
instantaneously.
• The chance that an electron may absorb more then one
electron is negligible because the number of photons is
much lower than the electron.
• After absorbing the photon, an electron either leaves the
surface or dissipates its energy within the metal in such
a short interval that it has almost no chance to absorb
second photon.
• An increase in intensity of light source simply increases
the number of photon and the number of photo electrons
but no increase in the energy of photo electron.

10. • However, increase in frequency increases the energy of
photons and photo electrons. According to Einstein's
explanation of photoelectric emission, a photon of energy
'E' performs two operations:
1. Removes the electron from the surface of metal
2. Supplies some part of energy to move photo electron
towards anode
• Since minimum amount of energy to remove electron from
a surface is equal to work function, we can write Einstein
equation as:

11. • Energy Supplied = Energy Consumed in ejecting an
electron + maximum Kinetic energy of electron
• h f = KE + W
• KE = h f – W
• h fo = Wo
• Equations are identical and are known as Einstein's
photoelectric equations.

12. Photocell
• Introduction:-
• A photocell is a practical application of the
phenomenon of photoelectric cell.
• Definition:-
• Photoelectric cell or photocell, device whose
electrical characteristics (e.g., current, voltage, or
resistance) vary when light is incident upon it.
OR
• The photo electric cell also known as phototube is an
electron tube in which the electrons initiating an
electric current originate by photo electric emission.

13. CONSTRUCTIPON OF PHOTOCELL
• Principle:-
• The working of photocell is based upon the
photoelectric effect.
• Construction:-
• It consists of a cathode and an anode in an evacuated
glass tube connected to appropriate terminals of the
battery as shown in the figure.
• The material of the cathode is selected to suit to the
frequency range of the incident radiation over which
the cell Is operated.

14. • For example, sodium or potassium cathode emits
photoelectrons fro visible light, cesium coated
oxidized silver emits electrons for the infrared light
and some other metals respond to ultraviolet
radiations.
• Working:-
• When light of frequency greater than threshold
frequency of the cathode falls on cathode plate,
photoelectrons are emitted.
• These are attracted towards the anode and due to this
flow of charges, current flows in the circuit.
• The number of electrons emitted depends upon the
intensity of light.

15. • When intensity of light is increased, the value of
current also increases.
• If light is switched off, the current flowing in the
circuit also stops.

16. Applications OF Photocell
• To count vehicles passing a road.
• To count items running on a conveyer belt.
• To open doors automatically in a building such as
banks or other commercial buildings or offices.
• To operate burglar alarms.
• To produce sound in movies.
• Photocells have myriad uses, especially as switches
and sensors.
• They are a common fixture in robotics, where they
direct robots to hide in the dark, or to follow a line or
beacon.

17. • Automatic lights that turn on when it gets dark use
photocells, as well as streetlights that switch on and
off according to whether it is night or day.
• They are used as timers to measure the speeds of
runners during a race.
• Photocells may be used in the place of variable
resistors and photovoltaic cells.
• Some circuit applications include light meters and
light controlled relays.

18. Examples.
1. Calculate the energy of a photon of blue light with a
frequency of 6.67 x 1014 Hz. (State in eV) [2.76eV]
2. Calculate the energy of a photon of red light with a
wavelength of 630 nm. [1.97eV]
3. Barium has a work function of 2.48 eV. What is the
maximum kinetic energy of the ejected electron if the
metal is illuminated by light of wavelength 450 nm?
[0.28 eV]
4. When a 350nm light ray falls on a metal, the maximum
kinetic energy of the photoelectron is 1.20eV. What is
the work function of the metal? [2.3 eV]

19. 5. A photon has 3.3 x 10-19 J of energy. What is the
wavelength of this photon?
6. What is the energy of one quantum of 5.0 x 1014 Hz
light?

20. REFERENCE BOOKS AUTHOR/PUBLICATION
ENGINEERING PHYSICS S S PATEL (ATUL PRAKASHAN)
MODERN ENGINEERING
PHYSICS
A S VASUDEVA
ENGINEERING PHYSICS K. RAJGOPALAN

21. Image Reference links
• http://0.tqn.com/y/chemistry/1/W/7/z/photoele
ctric_effect.jpg
• https://coraifeartaigh.files.wordpress.com/2010
/04/image04.gif?w=500