SESSION PLAN

GRADE XII

TOPIC DUAL NATURE OF RADIATION & MATTER

SESSION 1

LEARNING INTENT 1. INTRODUCTION

2. PHOTOELECTRIC EMISSION

3. HERTZ’S OBSERVATION

PRE REQUESITE Recall the basic knowledge of how light behaves as an

electromagnetic wave, including concepts like wavelength,
frequency, and speed of light.

Light incident on the surface of a metal in the photoelectric effect

ENTRY
causes electrons to be ejected. The electron ejected due to the
photoelectric effect is called a photoelectron and is denoted by
e–. The current produced as a result of the ejected electrons is called
photoelectric current.

EXPLANATION  The photoelectric effect occurs when light shines on a material

and causes it to emit electrons. It’s a key experiment in physics
that shows how light interacts with matter and is foundational to
quantum mechanics.

 Hertz’s results provided the first experimental evidence for the

photoelectric effect, where light can eject electrons from a
material.

 Hertz observed that when he exposed the photoelectric plate to

ultraviolet (UV) light, the spark gap apparatus produced sparks
more easily.

This indicated that UV light was causing some change in the metal
plate, which in turn affected the spark production.

IMPORTANT RESULTS:

 Light of sufficiently high frequency can emit electrons from a

metal surface.
  There is a threshold frequency below which no electrons are
emitted.
 The emission of electrons is immediate and depends on the
frequency of the incident light rather than its intensity.

These discoveries were crucial for the development of quantum

mechanics and our understanding of light as both a wave and a
particle.

REINFORCEMENT Recapitulating all the keywords at the end of the class.

WRITTEN WORK Class Work - running notes and few numerical will be given .

1. List the consequences of the Photoelectric emission

PRACTICE QUESTIONNAIRE
2. The work function of caesium is 2.14 eV. Find the threshold
frequency for caesium, and the wavelength of the incident if the
photo current to zero by a stopping potential of 0.60 V. Given h =
6.63x10-34Js
NCERT
LEARNING ENABLED NOTES
1. The work function of the metal is 4.5 ev. Find the frequency of
light to be used to eject electrons from the metal surface with a
QUESTIONS (CBSE BANK) maximum kinetic energy of 6.606x1019 J

2. Photoelectrons are emitted from a surface when ultraviolet light of

wavelength 300nm is incident on it. The minimum negative potential
required to stop the emiision of the electrons is 0.54v Calculate :
(I) The energy of the incident photons
(ii) the maximum kinetic energy of the photoelectrons emitted
(iii) the work function of the metal express all answers in eV. (p.19)

HOME ASSIGNMENT NCERT Numerical given as H . W

 PHYSICS - SESSION PLAN

GRADE XII

TOPIC DUAL NATURE OF RADIATION & MATTER

SESSION 2

LEARNING INTENT 1. Laws of photoelectric emission and related graphical analysis

PRE REQUISITE Recall and Introducing basic concepts like

 Electromagnetic Waves: Familiarity with the basic properties of

electromagnetic waves, including wavelength, frequency, and the
speed of light and their relation.

 Energy Quantization

Introduce the photoelectric effect as the emission of electrons from a

ENTRY material when it is exposed to light.
· Photon Interaction: Explain that light can be thought of as consisting of
photons, which are particles carrying quantized energy.
· Threshold Frequency: Emphasize that electrons are emitted only if the
incident light has a frequency above a certain threshold, regardless of the
light’s intensity.

Effect of potential on photoelectric current.

EXPLANATION

Variation of photoelectric effect current with collector plate potential for

different frequencies of incident radiation.
 Variation of photoelectric effect current with collector plate potential for
different Intensity of incident radiation.

Variation of stopping V0 with frequency of incident radiation for a given

photosensitive material.

Laws Governing the Photoelectric Effect

1. For a light of any given frequency,; (γ > γ Th), the photoelectric

current is directly proportional to the intensity of light.
2. For any given material, there is a certain minimum (energy)
frequency, called threshold frequency, below which the emission of
photo electrons stops completely, no matter how high the intensity
of incident light is.
3. The maximum kinetic energy of the photo electrons is found to
increase with the increase in the frequency of incident light,
provided the frequency (γ > γ Th) exceeds the threshold limit. The
maximum kinetic energy is independent of the intensity of light.
4. The photo-emission is an instantaneous process.

REINFORCEMENT Recapitulating all the keywords at the end of the class.

WRITTEN WORK Class Work - running notes and few numerical will be given .

 Define Threshold frequency ?

PRACTICE
QUESTIONNAIRE  State the laws of photoelectric emission. ?

 what is cut off or stopping potential?

Quiz will be conducted

LEARNING ENABLED
NCERT
NOTES

Define the term ‘threshold frequency’ as used in photoelectric effect.

QUESTIONS (CBSE BANK) (ii) Plot a graph showing the variation of photoelectric current as a
function of anode potential for two light beams having the same frequency
but different intensities I1 and I2 (I1 > I2). (All India 2010)

Does the stopping potential in photoelectric emission depend upon the

intensity of the incident radiation in a photo cell. Comment on it.

1. The work function of a caesium metal is 2.31 eV.

HOME ASSIGNMENT Photoelectric emission occurs when light of frequency 6.4x1014
Hz in incident on the metal surface calculate: (i) the energy of
the incident radiation (ii) the maximum kinetic energy of the
emitted electron and (iii) the stopping potential of the surface

2. The photoelectric cut off voltage in a certain experiment is

1.5 v. What is the maximum K.E of photoelectrons emiited?

Prepare formulae sheet for this lesson

 PHYSICS - SESSION PLAN

GRADE XII

TOPIC DUAL NATURE OF RADIATION & MATTER

SESSION 3&4

Photoelectric effect and wave theory of light

LEARNING INTENT 1.

2. Einstein’s photoelectric equation: Energy quantum of

radiation
PRE REQUESITE Recall
Electromagnetic Spectrum: Familiarize with different types of
electromagnetic waves, including visible light, and their properties.
wave properties and behaviors

ENTRY Explaining the photoelectric effect and its importance

Threshold Frequency
Instantaneous Emission

Threshold frequency:
EXPLANATION
1. The maximum kinetic energy of the photo electrons is independent of
the intensity of radiation and a minimum frequency is required for the
photoelectric effect.

2. This minimum frequency is known as the threshold frequency.

The photoelectric effect cannot be described by wave theory for the

following reasons:

 Electrons are released after a tiny instant of time when light strikes a
material, according to wave theory.

 However, in the photoelectric effect, electron emissions occur

immediately and without delay.

 The energy of a wave grows as its intensity increases.

 In the photo electronic effect, however, increasing the intensity has no

influence on the energy of electrons released.

 All that occurs is an increase in the number of electrons expelled.

 According to the Einstein-Plank relation, we have E = hν …(1)
Once a photo-electron is outside the metallic surface, what will be its energy?
Since there is no electric field outside the metal surface, the energy of an
electron will be purely Kinetic in nature.
K.E. of the photo-electrons = (Energy obtained from the Photon) –
(The energy used to escape the metallic surface)

This energy is a constant for a given surface. We denote it by Φ. We call it

the work function and it is constant for a given substance. Thus we can write:

E. = hν – Φ … (2)
hν0 – Φ = 0 or hν0 = Φ ….(3)

using in (2), we have K.E. = hν – hν0 or K.E. = h(ν – ν0)

Also if V0 is the Stopping Potential, then

K.E. (max) = eV0; using this in equation (3), we have:

eV0 = h(ν – ν0) ……(4)

The values of ‘h’ are got from the photoelectric experiment by the above
equation.

REINFORCEMENT Recapitulating all the keywords at the end of the class.

WRITTEN WORK Class Work notes

1. Why can't the photoelectric effect be explained by the wave model?

PRACTICE
QUESTIONNAIRE 2. Why is photoelectric emission not possible at all frequencies?

3. If a photocell is illuminated with a radiation of 1240Ao, then stopping

potential is found to be 8 V. The work function of the emitter and the
threshold wavelength are
A) 1eV, 5200 A0 B) 2eV, 6200 A0

C) 3 eV, 7200 A0 D) 4eV, 4200 A0

LEARNING ENABLED NCERT and Running notes

NOTES
 Electrons are emitted from the cathode of a photocell of
QUESTIONS (CBSE BANK)
negligible work function, when photons of wavelength are
incident on it. Derive the expression for the de Broglie
wavelength of the electrons emitted in terms of the wavelength of
the incident light.

Calculate the kinetic energy of an electron having de Broglie

wavelength of 1Å

a) Why photoelectric effect can not be explained on the basis of

wave nature of light? Give reasons.

(b) Write the basic features of photon picture of electromagnetic

radiation on which Einstein’s photoelectric equation is based

HOME ASSIGNMENT NCERT exercise Q.No 6 The threshold frequency of a certain metal is 3.3
x10 14 Hz. If the light of frequency 8.2 x1014 Hz is incident on the metal,
predict the cut off voltage for Photoelectric emission.
 PHYSICS - SESSION PLAN

GRADE XII

TOPIC DUAL NATURE OF RADIATION & MATTER

SESSION 5

LEARNING INTENT 1. Particle nature of light : The Photon

PRE REQUESITE
Recall :
· key properties of light as a wave, including wavelength (λ),
frequency (ν), amplitude, and wave speed (v).
· Wave Equation: Be familiar with the wave equation v=νλ,
where v is the speed of light, ν is the frequency, and λ is the
wavelength.

ENTRY
Explaining that light was traditionally understood as an
electromagnetic wave, with properties such as wavelength (λ),
frequency (ν), and speed (v).
Introduce the photoelectric effect, where light striking a material
can eject electrons from its surface. Highlight key observations:

 Threshold Frequency: Electrons are emitted only if the

light frequency is above a certain threshold, regardless of
light intensity.
 Instantaneous Emission: Electrons are emitted almost
instantaneously when light of sufficient frequency is
applied.
 Kinetic Energy of Electrons: The kinetic energy of
emitted electrons depends on the frequency of the light,
not its intensity.

 A photon is an elementary particle. It is a quantum of

EXPLANATION light.
 The energy of a photon is given by E=hν
 Its momentum is p=hν/c
and speed is c, which is the speed of light.
 Irrespective of the intensity of radiation, every photon of
a frequency v has the same momentum
p=hν/c and energy E=hν

 The increase in the intensity of light only increases the

number of photons crossing an area per unit time. It does
not affect the energy of the radiation.
  A photon remains unaffected by electric and magnetic
fields. It is electrically neutral.
 A photon has a zero mass, i.e. it is massless.
 It is a stable particle.
 Photons can be created or destroyed when radiation is
emitted or absorbed.
 The total energy and momentum are conserved during a
photon-electron collision.
 A photon cannot decay on its own.
 The energy of a photon can be transferred during an
interaction with other particles.
 A photon is a spin-1 particle, unlike electrons which are
½ spin. It’s spin axis is parallel to the direction of travel.
It is this property of photons that supports the polarization
of light.

REINFORCEMENT Recapitulating all the keywords at the end of the class.

WRITTEN WORK Class Work notes

What is the mass of a photon?
PRACTICE QUESTIONNAIRE
What is the effect of the magnetic field on photons?

LEARNING ENABLED NOTES NCERT.

A) Calculate the energy and momentum of a photon in a
QUESTIONS (CBSE BANK) monochromatic beam of wavelength 331.5 nm.
B) How fast should a hydrogen atom travel in order to have the
same momentum as that of the photon in part a.

HOME ASSIGNMENT  A source of light is emitting photons. Does all the

photons emitted have the same energy? Is the source
monochromatic? Explain?
 PHYSICS - SESSION PLAN

GRADE XII

TOPIC DUAL NATURE OF RADIATION & MATTER

SESSION 6

LEARNING INTENT 1. Wave nature of matter and de Broglie equation

PRE REQUESITE Recall the concepts of Newton's laws of motion, momentum,

and energy.

ENTRY General discussion on Dual nature of radiation and matter.

 According to De Broglie’s hypothesis, there is symmetry in

EXPLANATION nature. If light and radiation act as particles and waves, then
matter will also have particle and wave properties. The dual
nature of matter was predicted by De Broglie
 λ=h /p = h /m v
‘λ Lambda’ denotes the particle’s wavelength,
while ‘p’ here represents the particle’s momentum. The de
Broglie connection is significant because it mathematically
indicates that matter can act in the manner of a wave.
 In layman’s terms, the de Broglie equation states that every
moving particle, whether microscopic or macroscopic, has its
unique wavelength.

 The wave aspect of the matter can be observed in

macroscopic things, indicating that they are composed of
waves. When it comes to more oversized items, the
wavelength shrinks as the object grows in size, eventually
becoming so tiny as to be imperceptible.
 This is why macroscopic objects in real life do not exhibit
wave-like qualities.
 Even the cricket ball you throw has a wavelength that is too
short for you to see. The Plank’s constant connects the
wavelength and the momentum in the equation.

REINFORCEMENT Recapitulating all the keywords at the end of the class.

WRITTEN WORK
Class Work notes
PRACTICE QUESTIONNAIRE The de-Broglie wavelength of an electron (mass 1×10−30kg,
charge=1.6×10−19 C) with a kinetic energy of 200eV is:
(Planck’s constant 6.6×10−34J):

A) 9.60×10−11m B) 8.25×10−11m

C) 6.25×10−11m C) 5.00×10−11m

What is de Broglie Equation? What is the formula to find the de

Broglie wavelength?

LEARNING ENABLED NOTES Ncert and Class Work notes

1. An electron and alpha particle have the same de-Broglie
QUESTIONS (CBSE BANK) wavelength associated with them. How are their kinetic energies
related to each other?
Two lines, A and B, in the plot given below show the variation
of de-Broglie wavelength, λ versus 1V√, Where V is the
accelerating potential difference, for two particles carrying the
same charge. Which one of two represents a particle of smaller
mass ? (All India 2008)

2. Show graphically, the variation of the de- Broglie wavelength

(λ) with the potential (V) through which an electron is
accelerated from rest.
3. A proton and an electron have same kinetic energy. Which one
has greater de-Broglie wavelength and why?

HOME ASSIGNMENT  Show that the wavelength of EM radiation is equal to de-

Broglie wavelength of its quantum