1) The document discusses topics related to photoelectricity, X-rays, and lasers including the photoelectric effect, Planck's quantum theory, Einstein's photoelectric equation, production of X-rays using a Coolidge tube, and characteristics of laser light. 2) It describes the three-level optical pumping system used in helium-neon lasers, which produces population inversion through excitation from the ground state to a higher excited state followed by relaxation into a metastable state. 3) Applications of these phenomena are discussed, such as uses of photoelectric cells in burglar alarms, medical and industrial applications of X-rays, and examples of laser applications in areas like engraving, cutting,

1. Chapter-3: Photoelectricity, X-Rays & LASERs
APS_22202_APPLIED PHYSICS

2. Photo Electricity:
➢ Photoelectric Effect -: It states that when light of suitable frequency is
incident on the metallic surface, then electrons are emitted from the
metal surface. This is called photoelectric effect.
➢ In photoelectric effect, maximum current that flows in the circuit is
called as saturation current.
Light Photoelectrons
(Emission of electron)
Metal
Surface

3. Photo Electricity:
➢ Planck’s Hypothesis or Planck’s Quantum Theory or Concept of
Photon -: It states that, energy is not emitted & absorbed continuously,
but is emitted or absorbed in discrete units or packets. These energy
packets are called as photons or quanta.
➢ The photons are electrically neutral & travel with speed of light.
➢ If υ is the frequency of light, the energy E associated with the photon is
directly proportional to υ.
𝐸 ∝ υ
𝐸 = 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 × υ
𝑬 = 𝒉υ
Where, h = Planck’s constant

4. Photo Electricity:
➢According to this theory, energy is always emitted or absorbed in an
integral multiple of hυ & not in fraction of hυ.
𝑬 = nhυ
Where, n = 1,2,3…..

5. Properties or Characteristics of Photon :
1) A photon travels at a speed of light.
2) Existence of photon is same as existence of electron.
3) The mass of a photon is,
𝒎 =
𝒉
𝒄𝝀
4) The momentum of a photon is,
𝒑 =
𝒉
𝝀
5) Photons travel in a straight line.
6) Photons are electrically neutral (non electric nature).
7) Photons can not be deflected by magnetic field (non magnetic nature).
8) Photons do not ionize.

6. Process of Photoelectric Emission :

7. Process of Photoelectric Emission :
➢ If a light of frequency υ is to be incident on a metal plate the photon of
energy hυ collides with the metal atom. During this collision the
energy possessed by the photon is absorbed by the metal atom. This is
absorption.
➢ Now energy possessed by an atom is hυ. An atom utilizes this energy
in two ways. Some part of energy is utilized to detach the electron
from the metal atom & remaining entire energy is used to throw the
electron in the atmosphere.
➢ The energy required to just separate the electron from the atom is
called photoelectric work function W0.

8. Einstein’s Photoelectric Equation:
➢ Energy of photon absorbed by the atom (hυ) is
1) Used to detach the electron (W0) &
2) Given to the electron in the form of K. E.
Thus,
ℎ𝝊 = 𝑊0 + 𝐾. 𝐸.
ℎ𝝊 = 𝑊0 +
1
2
𝑚𝑣2
1
2
𝑚𝑣2 = ℎ𝝊 − 𝑊0
𝑊ℎ𝑒𝑟𝑒, 𝑊0 = 𝑃ℎ𝑜𝑡𝑜𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑊𝑜𝑟𝑘 𝐹𝑢𝑛𝑐𝑡𝑖𝑜𝑛 = ℎ𝝊0
1
2
𝑚𝑣2 = ℎ𝝊 − ℎ𝝊0
𝑲. 𝑬. =
𝟏
𝟐
𝒎𝒗𝟐 = 𝒉(𝝊 − 𝝊𝟎)
This equation is called Einstein’s Photoelectric Equation.

9. Einstein’s Photoelectric Equation:
𝑲. 𝑬. =
𝟏
𝟐
𝒎𝒗𝟐 = 𝒉(𝝊 − 𝝊𝟎)
Where, m = mass of electron
v = Velocity of electron
h = Planck’s constant
υ = Frequency of light or photon
υ0 = Threshold Frequency
➢Cases-:
1) If υ < υ0 -: K.E. of photoelectrons is negative & noemission.
2) If υ = υ0 -: K.E. of photoelectrons is zero & emission just begins.
3) If υ > υ0 -: K.E. of photoelectrons is positive &emission takes place.

10. Definitions:
➢Threshold Frequency (𝝊0) -: The minimum frequency of the incident
light at which emission just begins is called threshold frequency.
➢Threshold Wavelength (λ0) -: The maximum wavelength of the incident
light at which emission just begins is called threshold wavelength.
➢Photoelectric Work Function (W0) -: The energy required to separate or
detach the electron from the atom is called photoelectric work function.
The value of photoelectric work function (W0) changes from metal to
metal.
➢Stopping Potential -: The negative potential given to the cell at which
photoelectric current becomes zero is called stopping potential.

11. Characteristics of Photoelectric Effect or Photocell:
1. A metal emits electrons only when the incident light has frequency
greater than critical frequency is called threshold frequency (υ0).
2. Photoelectric current is directly proportional to the intensity of light &
independent of frequency.
3. The velocity of photoelectron is directly proportional to the frequency
of light & independent of intensity.
4. For a given metal surface, stopping potential is directly proportional to
the frequency of light & independent of intensity.
5. This process is instantaneous.
6. The emission of photoelectrons emitted from the photocathode are
independent on temperature i.e. the photoelectric emission is different
from thermionic emission.

12. Light Dependent Resistor (LDR) or Photoresistor:
➢ Definition of LDR -: A type of semiconductor whose conductivity
changes with the intensity of light is called LDR.
➢ Principle -: The electrical resistance of LDR decreases as the intensity of
incident light increases.
➢ Symbol of LDR -:

13. Light Dependent Resistor (LDR) or Photoresistor:
➢ Working -: When light is incident on LDR, a photon is absorbed by the
material & electrons from valence band get excited & jump into the
conduction band. Hence, conductivity of material increases i.e. resistivity
of the material decreases.
➢ Application of LDR-: LDR is used
1) In camera for exposure control
2) In photocopy (Xerox) machine
3) In security alarms
4) As smoke detector
5) Automatic lighting control
6) Street light control

14. Photoelectric Cell -:
➢ Principle -: Light energy is converted into electricalenergy.
➢ Diagram -:

15. Photoelectric Cell -:
➢ Construction & Working-: Photoelectric cell consists of cathode &
anode enclosed in an evacuated glass bulb. The semi-cylindrical cathode
coated with the photosensitive material forms the inner side. The anode is
a rod of platinum. The cathode is connected to the negative terminal &
anode is connected to the positive terminal of battery through
milliammeter. When light is incident on the cathode, it emits
photoelectrons. These electrons are attracted by the anode. The
photoelectric current flows through the circuit.

16. Types of Photoelectric Cell -:
1) Photoemissive Cell -: The cell in which when light is incident on
cathode of a photocell, electrons are emitted is called photoemissive cell.
2) Photoconductive Cell -: The cell in which when light is incident on a
cell, its resistance decreases i.e. conductivity increases is called
photoconductive cell.
3) Photovoltaic Cell -: The cell in which light is directly converted into
electrical energy.

17. Applications of Photoelectric Effect :
1. It is used in lux meter to measure the intensity
2. It is used in burglar alarm
3. It used for automatic control of traffic signals.
4. Recording & reproduction of sound during shooting of a film.
5. Automatic switching of street lights
6. It is used in television sets, fire alarms.
7. It is used in detecting flaws in metals.
8. It used in exposure meter.

18. Applications of Photoelectric Effect :
Q. Explain the use of photoelectric cell in Burglar alarm.
Ans -: The infrared light from the source is incident on photocell. Then
photocell is in ON condition & photoelectric current flows continuously in
the circuit. This photocell is kept at entry or near valuables. When some one
come near the valuable, infrared gets interrupted & photoelectric current
immediately stops which automatically starts the electric alarm & entry of
thieves is detected.

19. X-Rays:
➢ Definition of X-Rays -: The electromagnetic radiation having very short
wavelength ranges from 10-10 m to 10-11 m is called X-rays. Wavelength
of X-rays is of the order 1A0.
➢ Minimum Wavelength of X-Rays (λmin) -: It is given by the formula
λ𝑚𝑖𝑛 =
ℎ𝑐
𝑒𝑉
=
12400
𝑉
𝐴0
=
12400 × 10−10
𝑉
𝑚
Where, h = Planck’s Constant
c = Velocity of Light
e = Charge on electron
V = Applied Voltage

20. Properties or Characteristics of X-rays:
1) Very short wavelength
2) Travels with speed of light
3) Affects the photographic plates
4) High penetrating power
5) Invisible to eyes.
6) Not deflected by electric or magnetic field
7) Kill some form of animal cells
8) Produce photoelectric effect

21. Production of X-Rays using Coolidge Tube:
➢ Principle -: When fast moving electrons are suddenly stopped then X-
Rays are produced.
➢ Diagram -:

22. Production of X-Rays using Coolidge Tube:
➢ Working-: Coolidge tube is highly evacuated hard glass bulb with
cathode & anode. The cathode i.e. metal filament is surrounded by
molybdenum metal cylinder kept at negative potential to the filament.
Hence, the electrons emitted from the filament are concentrated into fine
beam of electrons. The target T consists of copper block in which a piece
of tungsten is fitted. The target is placed at an angle of 450 with the path
of electron beam. When the filament is heated by an electric current, it
produces electrons. This beam of electrons is focused on the anode. The
electrons from the cathode are accelerated by application of high voltage
between the cathode & anode by using step up transformer. When fast
moving electrons are suddenly stopped by the tungsten anode they lose
their K.E. & X-rays are produced from the target. Some amount of K.E.
is converted into large amount of heat.

23. Applications of X-Rays:
➢ Engineering Applications -:
1) To detect the cracks in the body of an aeroplane or motor
2) To detect manufacturing defects in rubber tyres or tennis ball
3) To detect cracks in metal jobs
4) To distinguish between real diamond & duplicate diamond.
5) To detect cracks in the wall
6) To detect smuggling gold at air port & dock (ship) yard
7) To check quality of welded joints

24. Applications of X-Rays:
➢ Medical Applications -:
1) In surgery to detect bone fractures
2) To cure skin diseases & destroy tumors
3) To cure diseases like cancer.
4) To detect bullet’s position inside the body
➢ Scientific Applications -:
1) To study the structure of crystal
2) In chemical analysis & for determination of atomic number of chemical
elements.
3) To study structure of substances like cellulose, rubber, plastic.
4) For identification of chemical elements present in the solution
5) For analysis of structures of organic molecules

25. LASER:
L ight
Amplification by
S timulated
E mission of
R adiation

26. Characteristics or Properties of LASER Light:
1. COHERENT -: Laser source emits the waves are exactly in same
phase.
2. MONOCHROMATICITY -: The light waves emitted by the laser have
same wavelength.
3. UNDIRECTIONALITY -: The laser emits the light in only one
direction & produces sharp focus.
4. EXTREMELY INTENSE -: The laser light has extreme brightness.

27. Absorption orInduced Absorption (Stimulated
Absorption):
➢The process in which an atom in the ground state absorbs the energy of
incident photon (h𝝊) & gets excited towards higher energy level is called
stimulated absorption. i.e.
ℎ𝝊 = 𝐸2 − 𝐸1

28. Spontaneous Emission:
➢The process in which after completion of life time, the excited state
atom comes to the ground state by itself with the emission of photon
of energy (h𝝊) is called Spontaneous Emission.

29. Stimulated Emission:
➢The process in which the excited state atom is triggered due to the action of
incident photon, then excited state atom comes to the ground state with the
emission of another photon which is identical to the incident photon is called
Stimulated Emission.

30. Difference Between Spontaneous Emission &
Stimulated Emission:

31. Metastable State & Excited State:
➢Metastable State or Metastable Excited State -: The state in which atom
relaxes for long time (10-6 sec. to 10-3 sec.) & then comes down to the
ground state. This state is called metastable state.
➢Ordinary Excited State or Excited State -: The state in which atom
relaxes for very short time (10-8 sec.) & then comes down to the ground
state. This state is called ordinary excited state.

32. Population Inversion or Inverted Population:
➢ Population Inversion -: Making the population of higher energy
state more than that of the ground state is called population inversion or
invertedpopulation.
i.e. making N2 ˃˃ N1 as shown in followingfigure.
➢ A system in which population inversion is achieved is called active
system. In order to produce stimulated emission properly, population of
higher excited state should be greater than that of lower energy state.

33. Pumping & Their Types:
➢ Pumping-: The process of raising the atoms from lower energy state to
higher energy state is called pumping.
➢ Optical Pumping -: The process of raising the atoms from lower energy
state tohigher energy state using light medium is called optical pumping.
➢ Types or Methods of Pumping -:
1) Optical Pumping
2) Direct Electron Excitation (Electrical Pumping)
3) Inelastic atom-atom collision
4) Chemical Reaction (Chemical Pumping)

34. Engineering Applications of Laser:
1) Laser are used for engraving & embossing of printing plates.
2) Lasers are used in cutting, drilling & welding metals.
3) Lasers are used in computer printers.
4) Laser are used for 3D laser scanners.
5) Laser used in holography.
6) Lasers are used to find flaws or defects in a material.
7) Lasers are used in heat controlled treatment.
8) Laser through optical fiber is used to transfer data from one computer to
other.

35. Optical Pumping (Three Level Laser System) :

36. Optical Pumping (Three Level Laser System) :
➢ Proper lasing action can be produced using three energy level laser system
than that of two energy level laser system.
➢ Optical pumping i.e. photon of energy h𝝊13 is incident on ground state
atoms. Therefore ground state atoms get excited from energy level E1
to energy level E3.
➢ Atoms relax in energy level E3 for very very short time i.e. (10-8 sec.) &
hence atoms come down to the energy level E2. Energy level E2 is a
metastable state having life time 10-6 sec. to 10-3 sec. Hence atoms
relax in E2 for longer time.
➢ Hence, population of energy level E2 becomes more than that of E1 i.e.
making N2>>N1. Therefore required population inversion is done.
➢ If atom in E2 is triggered due to an action of incident photon of energy
h𝝊12 & follows the downward transition E2 to E1. During this
transition, atom emits the another photon which is identical with
incident photon i.e. laser radiations are produced.

37. Helium Neon Laser (He-Ne Gas Laser) -:
➢ Construction -:

38. Helium Neon Laser (He-Ne Gas Laser) -:
➢ Construction -:
➢ He-Ne laser consists of fused quartz tube having length of 80 cm &
diameter of 1.5 cm.
➢ This quartz tube is filled with a mixture of helium (He) & neon (Ne) gas.
➢ The mixture consists of 10 parts of He & 1 parts of Ne i.e. majority of
helium atoms (90%) & minority of neon atoms (10%).
➢ Perfect reflector is fixed at one end & partial reflector is fixed at other end
of the tube.
➢ The active material is excited by radio or high frequency generator.

39. Helium Neon Laser (He-Ne Gas Laser) -:
➢ Working -:

40. Helium Neon Laser (He-Ne Gas Laser) -:
➢ Working -:
➢ An electric discharge is produced in the gas due to the outside electrodes
of tube connected to radio or high frequency alternating current generator.
➢ Discharge excited He & Ne atoms collide with electrons in the metastable
states 20.61 eV & 20.66 eV respectively.
➢ Higher levels E4 & E6 of neon atom are close to excited energy levels H2
& H3 of helium atom, hence the probability of helium atoms transferring
their energy to excited neon atoms through collision is greater.
➢ Thus, the purpose of helium atom is to achieve population inversion.
Actual lasing atoms are neon atoms.
➢ The laser transition in Ne is from metastable state at 20.66 eV to an
excited state at 18.7 eV.
➢ He-Ne laser has narrow red beam which is used to read the bar codes.

41. Thank you