Class 12 Physics NCERT REPRINT 2026-27

Chapter 11: Dual Nature of Radiation and Matter

Complete concise study notes, cathode rays and specific charge, work function, photoelectric effect, Einstein's photoelectric equation, photon characteristics, de Broglie matter waves, solved examples 11.1–11.3, and exercises 11.1–11.11.

01 / Exam-Focused Notes

Chapter 11: Dual Nature of Radiation and Matter

Topics covered: Electron Emission methods, Photoelectric Effect 4 Empirical Laws, Hertz & Hallwachs observations, Stopping Potential ($V_0$) & Cut-off Frequency ($ν_0$), Einstein's Photoelectric Equation ($Kmax = hν - φ_0 = eV_0$), Millikan's $h$ verification, Photon Particle Properties ($E=hν, p=h/λ$), and de Broglie Matter Wave Hypothesis ($λ = h/p = 1.227/√V nm$).

100% SYLLABUS
Chapter Navigation • NCERT Sections 11.1 – 11.8
11.1-11.2Electron Emission 11.3-11.4Photoelectric Effect 11.6-11.7Einstein's Eq & Photons 11.8de Broglie Matter Waves
11.1 & 11.2
NCERT Sections

Introduction, Cathode Rays & Electron Emission

Discovery of Electron & Work Function

11.2.1 Specific Charge ($e/m$), Electron Discovery & Work Function ($φ_0$)

  • Historical Foundations:
    • William Crookes (1870) discovered cathode rays in low-pressure gas discharge tubes ($\sim 0.001 mm Hg$).
    • J. J. Thomson (1897) measured the specific charge of the electron: $em = 1.76 × 1011 C kg-1$, independent of cathode material or gas, establishing the electron as a universal fundamental constituent of matter.
    • R. A. Millikan (1913) oil-drop experiment determined the elementary quantum of charge: $e = 1.602 × 10-19 C$.
  • Work Function ($φ_0$):
    The minimum energy required by a free electron to escape from a metal surface against the attractive pull of positive surface ions. $$1 eV = 1.602 × 10-19 J$$ (Lowest for Caesium $φ_0 = 2.14 eV$, highest for Platinum $φ_0 = 5.65 eV$).
  • Three Methods of Electron Emission:
    1. Thermionic Emission: Imparting thermal energy to electrons by heating the metal filament.
    2. Field Emission (Cold Emission): Applying an intense external electric field ($\sim 10^8 V m-1$) to pull electrons out.
    3. Photoelectric Emission: Illuminating the photosensitive metal surface with light of suitable frequency ($ν ≥ ν_0$).
11.3 & 11.4
NCERT Sections

Photoelectric Effect & Experimental Observations

Hertz, Hallwachs & Lenard Observations

11.4.1 Experimental Setup & The 4 Laws of Photoelectric Emission

  • Experimental Apparatus: Evacuated quartz tube containing emitter plate ($C$) and collector plate ($A$), illuminated by monochromatic light through a quartz window, with variable potential and commutator.
  • Four Experimental Laws of Photoelectric Effect:
    1. Current vs Intensity: For a given photosensitive material and frequency $ν > ν_0$, the photoelectric current ($i_p$) is directly proportional to the incident light intensity ($I$).
    2. Stopping Potential ($V_0$) vs Intensity: For a fixed frequency, saturation current increases with intensity, but the stopping potential $V_0$ (and hence $Kmax = e V_0$) is strictly independent of light intensity.
    3. Threshold Frequency ($ν_0$): Below a characteristic threshold frequency $ν_0 = φ_0/h$, no electrons are emitted regardless of intensity. Above $ν_0$, stopping potential $V_0$ increases linearly with frequency: $$V_0 = (he)ν - φ_0e \quad [Straight line with universal slope h/e]$$
    4. Instantaneous Emission: Photoelectric emission occurs instantaneously with no detectable time lag ($< 10-9 s$), even for exceedingly dim light.
$$Kmax = 12 m vmax^2 = e V_0$$
Fig 11.3 & 11.5: Photoelectric Current vs Collector Potential & Stopping Potential vs Frequency
Collector Potential V → Photoelectric Current I -V₀ Intensity I₃ (High) Intensity I₂ Intensity I₁ (Low) (a) Constant Frequency (ν): V₀ is constant Frequency ν → Stopping Potential V₀ ν₀ (Metal A) Metal A (Slope = h/e) ν₀' (Metal B) Metal B (b) Universal Linear Slope: dV₀/dν = h/e
Failure of Classical Wave Theory

11.5 Failure of Classical Wave Theory to Explain Photoelectric Effect

Feature Classical Wave Theory Prediction Experimental Reality (Quantum Fact)
Effect of Intensity Greater intensity means larger electric field amplitude $E$, so $Kmax$ should increase with intensity. $Kmax$ is completely independent of intensity; only photocurrent increases.
Threshold Frequency ($ν_0$) Sufficiently intense light of any frequency should deliver enough energy over time to eject electrons (no threshold). Strict cut-off frequency $ν_0$ exists; no emission occurs for $ν < ν_0$ no matter how intense the beam.
Time Lag Continuous wave energy is spread over millions of surface atoms; it should take hours for an electron to accumulate work function energy. Emission is instantaneous ($< 10-9 s$), as a single photon delivers all its energy to a single electron in one collision.
11.6 & 11.7
NCERT Sections

Einstein's Photoelectric Equation & Photon Particle Picture

Einstein's Quantum Formulation

11.6.1 Einstein's Photoelectric Equation & Millikan's Verification

  • Einstein's Postulate (1905): Light consists of discrete energy packets (quanta) called photons, each carrying energy $E = hν$. In photoelectric effect, one photon is absorbed entirely by one electron.
$$Kmax = hν - φ_0 = h(ν - ν_0) = h cλ - φ_0$$ $$e V_0 = hν - φ_0 \implies V_0 = (he)ν - φ_0e$$
  • Millikan's Verification (1916): Millikan measured the slope of $V_0$ versus $ν$ for alkali metals, finding the universal slope $h/e$, directly yielding Planck's constant $h = 6.626 × 10-34 J s$.
Properties of Photons

11.7.1 The 5 Fundamental Properties of Photons

  1. Interaction: In every interaction with matter, radiation behaves as particles called photons.
  2. Energy & Momentum: Each photon has energy $E = hν = hcλ$, linear momentum $p = Ec = hνc = hλ$, and travels at speed $c$ in vacuum.
  3. Intensity Independence: Increasing light intensity increases only the photon flux (number of photons emitted per second), while each individual photon's energy and momentum remain constant.
  4. Electric Neutrality: Photons have zero rest mass ($m_0 = 0$), carry no electrical charge, and are unaffected by electric and magnetic fields.
  5. Collisions: In photon-particle collisions (e.g. Compton scattering), total energy and momentum are conserved, but the total number of photons may not be conserved (absorption or creation).
Example 11.1

Laser Photon Energy and Emission Rate

Monochromatic laser light of frequency $ν = 6.0 × 1014 Hz$ emits power $P = 2.0 × 10-3 W$.
(a) What is the energy of a photon in the beam?
(b) How many photons per second are emitted on average?

(a) Energy per photon: $$E = hν = (6.63 × 10-34 J s) × (6.0 × 1014 s-1) = 3.98 × 10-19 J = 2.48 eV$$
(b) Rate of photon emission ($N$): $$N = PE = 2.0 × 10-3 W3.98 × 10-19 J = 5.0 × 1015 photons s-1$$
Example 11.2

Caesium Work Function, Threshold Frequency & Wavelength

Work function of caesium is $φ_0 = 2.14 eV$.
(a) Find threshold frequency $ν_0$.
(b) Find incident wavelength $λ$ if photocurrent is stopped by $V_0 = 0.60 V$.

(a) Threshold frequency ($ν_0$): $$ν_0 = φ_0h = 2.14 × 1.602 × 10-19 J6.63 × 10-34 J s = 5.16 × 1014 Hz$$
(b) Incident wavelength ($λ$): $$E = e V_0 + φ_0 = 0.60 eV + 2.14 eV = 2.74 eV = 2.74 × 1.602 × 10-19 J = 4.39 × 10-19 J$$ $$λ = h cE = (6.63 × 10-34) × (3.0 × 10^8)4.39 × 10-19 = 4.54 × 10-7 m = 454 nm$$
11.8
NCERT Section

Wave Nature of Matter: de Broglie Waves

de Broglie Hypothesis

11.8.1 Matter Waves & de Broglie Wavelength ($λ = h/p$)

  • de Broglie Hypothesis (1924): Nature loves symmetry. If radiation exhibits wave-particle duality, moving material particles must also possess an associated wave character (matter waves).
$$λ = hp = hm v = h√2 m K$$
  • For an Electron Accelerated through Potential Difference $V$ Volts: $$K = e V \implies λ = h√2 m e V = 1.227√V nm = 12.27√V \AA$$
  • Macroscopic vs Microscopic Objects:
    • For macroscopic bodies (e.g. $m = 0.12 kg, v = 20 m/s \implies λ \sim 10-34 m$), matter wavelength is far too minute to produce measurable diffraction.
    • For electrons ($m = 9.11 × 10-31 kg, v \sim 10^6 m/s \implies λ \sim 0.1 nm$), wavelength is comparable to crystal atomic lattice spacing, enabling electron diffraction and electron microscopy.
Example 11.3

de Broglie Wavelength: Electron vs Macroscopic Ball

Find de Broglie wavelength for:
(a) Electron moving at $v = 5.4 × 10^6 m/s$.
(b) Ball of mass $150 g$ moving at $v = 30.0 m/s$.

(a) Electron ($m = 9.11 × 10-31 kg$): $$p = m v = 9.11 × 10-31 × 5.4 × 10^6 = 4.92 × 10-24 kg m s-1$$ $$λ = hp = 6.63 × 10-344.92 × 10-24 = 0.135 nm = 1.35 \AA \quad (comparable to X-rays)$$
(b) Ball ($m = 0.150 kg$): $$p = 0.150 × 30.0 = 4.50 kg m s-1$$ $$λ = 6.63 × 10-344.50 = 1.47 × 10-34 m \quad (unmeasurably small)$$
02 / Interactive Assessment

Concept Check & Practice Quiz

15 targeted MCQs covering work function, photoelectric laws, stopping potential graphs, Einstein's equation, photon properties, and de Broglie matter waves with in-depth explanations.

15 MCQS
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03 / Textbook Solutions

NCERT Exercises 11.1 – 11.11

Stepwise solutions for exercise questions in NCERT Class 12 Physics Chapter 11 Dual Nature of Radiation and Matter Reprint 2026-27.

11 QUESTIONS
Ex 11.1X-ray Maximum Frequency & Minimum Wavelength

11.1 Find the (a) maximum frequency, and (b) minimum wavelength of X-rays produced by $30 kV$ electrons.

(a) $νmax = 7.24 × 1018 Hz$ • (b) $λmin = 0.0414 nm = 0.414 \AA$.
(a) Maximum Frequency ($νmax$):
$e V = hνmax \implies νmax = e Vh = (1.602 × 10-19 C) × (30 × 10^3 V)6.63 × 10-34 J s = 7.24 × 1018 Hz$.
(b) Minimum Wavelength ($λmin$ / Duane-Hunt Limit):
$λmin = cνmax = 3.0 × 10^8 m s-17.24 × 1018 s-1 = 4.14 × 10-11 m = 0.0414 nm$.
Ex 11.2Caesium Photoemission Parameters

11.2 Work function of caesium is $φ_0 = 2.14 eV$. Light of frequency $ν = 6.0 × 1014 Hz$ is incident. Find: (a) $Kmax$, (b) Stopping potential $V_0$, and (c) Maximum speed $vmax$.

(a) $Kmax = 0.34 eV = 5.54 × 10-20 J$ • (b) $V_0 = 0.34 V$ • (c) $vmax = 3.49 × 10^5 m/s$.
(a) $Kmax$:
$hν = (6.63 × 10-34) × (6.0 × 1014)1.602 × 10-19 eV = 2.483 eV$.
$Kmax = hν - φ_0 = 2.483 - 2.14 = 0.343 eV = 5.50 × 10-20 J$.
(b) Stopping Potential: $e V_0 = Kmax \implies V_0 = 0.343 V ≈ 0.34 V$.
(c) Maximum Speed:
$vmax = √2 Kmaxm = √2 × (5.50 × 10-20)9.11 × 10-31 = 3.48 × 10^5 m s-1$.
Ex 11.3Maximum Kinetic Energy from Stopping Potential

11.3 The photoelectric cut-off voltage in an experiment is $1.5 V$. What is the maximum kinetic energy of photoelectrons emitted?

$Kmax = 1.5 eV = 2.40 × 10-19 J$.
$Kmax = e V_0 = (1.602 × 10-19 C) × (1.5 V) = 2.403 × 10-19 J = 1.5 eV$.
Ex 11.4He-Ne Laser Photon Flux & Momentum

11.4 He-Ne laser ($λ = 632.8 nm$) emits power $P = 9.42 mW$.
(a) Find energy and momentum of each photon.
(b) Find number of photons per second arriving at target.
(c) Speed of a hydrogen atom having the same momentum.

(a) $E = 3.14 × 10-19 J = 1.96 eV, p = 1.05 × 10-27 kg m/s$
(b) $N = 3.0 × 1016 photons s-1$ • (c) $v_H = 0.63 m/s$.
(a) $E = h cλ = (6.63 × 10-34) × (3.0 × 10^8)632.8 × 10-9 = 3.14 × 10-19 J = 1.96 eV$.
$p = hλ = 6.63 × 10-34632.8 × 10-9 = 1.05 × 10-27 kg m s-1$.
(b) $N = PE = 9.42 × 10-3 W3.14 × 10-19 J = 3.0 × 1016 photons s-1$.
(c) $m_H = 1.67 × 10-27 kg \implies v_H = pm_H = 1.05 × 10-271.67 × 10-27 = 0.628 m s-1 ≈ 0.63 m s-1$.
Ex 11.5Planck's Constant from Stopping Potential Slope

11.5 Slope of cut-off voltage versus frequency curve is $4.12 × 10-15 V s$. Calculate Planck's constant $h$.

$h = 6.60 × 10-34 J s$.
$$Slope = he \implies h = Slope × e = (4.12 × 10-15 V s) × (1.602 × 10-19 C) = 6.60 × 10-34 J s$$
Ex 11.6Cut-off Voltage Calculation

11.6 Threshold frequency is $ν_0 = 3.3 × 1014 Hz$. If incident light has $ν = 8.2 × 1014 Hz$, predict the cut-off voltage $V_0$.

Cut-off voltage $V_0 = 2.03 V$.
$$V_0 = he(ν - ν_0) = 6.63 × 10-341.602 × 10-19(8.2 × 1014 - 3.3 × 1014) = (4.138 × 10-15) × (4.9 × 1014) = 2.028 V ≈ 2.03 V$$
Ex 11.7Threshold Condition for Photoemission

11.7 The work function of a metal is $4.2 eV$. Will this metal give photoelectric emission for incident radiation of wavelength $330 nm$?

No photoemission occurs ($Ephoton = 3.76 eV < φ_0 = 4.2 eV$).
$$E = h cλ = (6.63 × 10-34) × (3.0 × 10^8)(330 × 10-9) × (1.602 × 10-19) eV = 3.76 eV$$ Since $E = 3.76 eV < 4.2 eV$, the photon energy is less than the work function; therefore, no photoelectric emission will occur.
Ex 11.8Threshold Frequency Determination

11.8 Light of $ν = 7.21 × 1014 Hz$ ejects electrons with $vmax = 6.0 × 10^5 m/s$. Find threshold frequency $ν_0$.

Threshold frequency $ν_0 = 4.74 × 1014 Hz$.
$$Kmax = 12 m vmax^2 = 12 × (9.11 × 10-31) × (6.0 × 10^5)^2 = 1.64 × 10-19 J$$ $$hν_0 = hν - Kmax = (6.63 × 10-34 × 7.21 × 1014) - 1.64 × 10-19 = 4.78 × 10-19 - 1.64 × 10-19 = 3.14 × 10-19 J$$ $$ν_0 = 3.14 × 10-19 J6.63 × 10-34 J s = 4.74 × 1014 Hz$$
Ex 11.9Work Function from Argon Laser Wavelength

11.9 Argon laser light $λ = 488 nm$ gives stopping potential $V_0 = 0.38 V$. Find the work function $φ_0$.

Work function $φ_0 = 2.16 eV = 3.46 × 10-19 J$.
$$E = h cλ = (6.63 × 10-34) × (3.0 × 10^8)(488 × 10-9) × (1.602 × 10-19) eV = 2.54 eV$$ $$φ_0 = E - e V_0 = 2.54 eV - 0.38 eV = 2.16 eV = 3.46 × 10-19 J$$
Ex 11.10de Broglie Wavelength of Macroscopic Bodies

11.10 What is the de Broglie wavelength of:
(a) Bullet of mass $0.040 kg$ travelling at $1.0 km/s$?
(b) Ball of mass $0.060 kg$ moving at $1.0 m/s$?
(c) Dust particle of mass $1.0 × 10-9 kg$ drifting at $2.2 m/s$?

(a) $λ = 1.66 × 10-35 m$ • (b) $λ = 1.11 × 10-32 m$ • (c) $λ = 3.01 × 10-25 m$.
(a) Bullet: $λ = hm v = 6.63 × 10-34 J s0.040 kg × 1000 m/s = 1.66 × 10-35 m$.
(b) Ball: $λ = 6.63 × 10-340.060 × 1.0 = 1.11 × 10-32 m$.
(c) Dust particle: $λ = 6.63 × 10-341.0 × 10-9 × 2.2 = 3.01 × 10-25 m$.
(All three wavelengths are astronomically smaller than atomic sizes and thus physically unmeasurable).
Ex 11.11Photon Wavelength Equality Proof

11.11 Show that the wavelength of electromagnetic radiation is equal to the de Broglie wavelength of its quantum (photon).

For a photon of frequency $ν$ and electromagnetic wavelength $λ$: $$Energy E = hν = h cλ$$ $$Linear momentum p = Ec = hλ$$ According to the de Broglie relation for momentum p: $$λde Broglie = hp = h(h/λ) = λ$$ Hence, the de Broglie wavelength of a photon is identically equal to the wavelength of the electromagnetic wave of which it is a quantum.
04 / Rapid Reference

Chapter Summary & Points to Ponder

Core quantum formulae, physical quantities and dimensions table, and official NCERT Points to Ponder for Chapter 11.

100% SYLLABUS

Work Function & Threshold

φ₀ = h ν₀ = h c / λ₀

Minimum energy needed to liberate an electron from metal surface. Lowest for Caesium ($2.14 eV$).

Einstein's Photoelectric Eq.

Kmax = e V₀ = hν − φ₀

Linear dependence on frequency; completely independent of light intensity.

Stopping Potential Slope

Slope = h / e • Intercept = −φ₀ / e

Universal straight line slope measured by Millikan to determine Planck's constant $h = 6.626 × 10-34 J s$.

Photon Momentum & Energy

E = hν • p = h / λ = hν / c

Zero rest mass ($m_0 = 0$), travels at speed $c$, electrically neutral, energy conserved in collisions.

de Broglie Matter Waves

λ = h / p = h / (m v)

Associates wave character with moving particles. Macroscopic $λ \sim 10-34 m$; electron $λ \sim 0.1 nm$.

Electron Wavelength Formula

λ = 1.227 / √V (nm)

For an electron accelerated from rest across potential difference $V$ Volts.

Physical Quantity Symbol Dimensions SI Unit Formula / Remarks
Planck’s constant h [M L² T⁻¹] J s $E = hν = 6.626 × 10-34 J s$
Stopping potential V₀ [M L² T⁻³ A⁻¹] V $e V_0 = Kmax$
Work function φ₀ [M L² T⁻²] J ; eV $φ_0 = hν_0 = 1.602 × 10-19 J/eV$
Threshold frequency ν₀ [T⁻¹] Hz ($s-1$) $ν_0 = φ_0 / h$
de Broglie wavelength λ [L] m $λ = h / p = h / √2 m K$
NCERT Official

Points to Ponder

  1. Bound Nature of Free Electrons: Free electrons are free to move inside the metal lattice in a constant potential, but require minimum work function energy $φ_0$ to escape the surface.
  2. Energy Distribution: Electrons inside a metal obey quantum Fermi-Dirac statistics and Pauli's exclusion principle; electrons with higher initial energies escape with maximum kinetic energy $Kmax$.
  3. Discrete Light-Matter Interactions: Photoelectric effect proves that energy exchange between radiation and matter occurs in localized discrete packets of magnitude $hν$.
  4. Stopping Potential as Decisive Test: The independence of stopping potential on light intensity and linear dependence on frequency provides conclusive proof of the photon model over classical wave theory.
  5. Phase vs Group Velocity of Matter Waves: The phase velocity $v_p = ω/k$ of a de Broglie wave exceeds $c$ and has no physical significance; the group velocity $v_g = dω/dk$ represents the true particle velocity.
05 / Practice Tests

3-Tier Practice Tests

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