Dual nature means radiation and matter can show both wave-like and particle-like behaviour.
Light shows particle behaviour in photoelectric effect, while moving electrons show wave behaviour through de Broglie waves.
Tiny particles changed modern Physics by proving that light and matter do not behave in one fixed way. Important Questions Class 12 Physics Chapter 11 help students practise work function, photoelectric effect, Einstein’s equation, photon energy, stopping potential, and de Broglie wavelength. CBSE 2026 questions from this chapter often test graph interpretation, direct formula use, and conceptual differences between wave theory and photon theory.
Key Takeaways
- Work Function: It is the minimum energy needed to remove an electron from a metal surface.
- Photoelectric Effect: It proves that light transfers energy in packets called photons.
- Stopping Potential: It measures the maximum kinetic energy of emitted photoelectrons.
- Matter Waves: Moving particles show wave nature through de Broglie wavelength.
Important Questions Class 12 Physics Chapter 11 Structure 2026
| Concept |
Exam Use |
Key Formula or Rule |
| Work Function |
Minimum energy for electron emission |
φ₀ = hν₀ |
| Photoelectric Effect |
Explains light as photons |
Kmax = hν - φ₀ |
| Stopping Potential |
Measures maximum kinetic energy |
eV₀ = Kmax |
Important Questions Class 12 Physics Chapter 11 Overview
Important Questions Class 12 Physics Chapter 11 focus on how light and matter show dual behaviour. The chapter explains electron emission, photoelectric effect, Einstein’s photoelectric equation, photons, and wave nature of matter.
Q1. What is dual nature of radiation and matter?
Dual nature means radiation and matter can show both wave and particle behaviour. Light shows wave behaviour in interference and particle behaviour in photoelectric effect.
Moving particles like electrons show wave behaviour through de Broglie wavelength.
Q2. Why is Dual Nature of Radiation and Matter important for CBSE 2026?
Dual Nature is important because it links modern Physics with quantum ideas. It includes direct numericals and concept-based graph questions.
CBSE 2026 can test work function, stopping potential, threshold frequency, photon energy, and de Broglie relation.
Dual Nature of Radiation and Matter Class 12 Important Questions on Electron Emission
Dual nature of radiation and matter class 12 important questions often begin with electron emission. Students should know why metals need external energy to emit electrons.
Q3. What is work function in Class 12 Physics?
Work function is the minimum energy required to remove an electron from a metal surface. It is denoted by φ₀.
Its unit is electron volt or joule. One electron volt equals 1.602 × 10⁻¹⁹ J.
Q4. Why can free electrons not leave a metal surface normally?
Free electrons cannot leave normally because positive metal ions pull them back. The electron needs extra energy to overcome attraction.
This minimum extra energy equals the work function of the metal.
Q5. What are the three types of electron emission?
The three types are thermionic emission, field emission, and photoelectric emission. Each method gives electrons enough energy to leave metal.
Thermionic emission uses heat, field emission uses strong electric field, and photoelectric emission uses light.
Q6. What is thermionic emission?
Thermionic emission is electron emission caused by heating a metal. Heat gives free electrons enough energy to escape.
It occurs when thermal energy exceeds the work function.
Q7. What is field emission?
Field emission is electron emission caused by a very strong electric field. The field pulls electrons out of the metal.
NCERT gives the required field order as 10⁸ V m⁻¹.
Q8. What is photoelectric emission?
Photoelectric emission is electron emission caused by light of suitable frequency. The emitted electrons are called photoelectrons.
This emission occurs only when photon energy exceeds the work function.
Photoelectric Effect Class 12 Important Questions
Photoelectric effect class 12 questions test definitions, observations, and graph logic. Students must connect intensity, frequency, current, and stopping potential.
Q9. What is photoelectric effect?
Photoelectric effect is the emission of electrons from a metal surface due to incident light. The light must have frequency above threshold frequency.
The emitted electrons are called photoelectrons.
Q10. Who discovered photoelectric emission?
Heinrich Hertz discovered photoelectric emission in 1887. He observed stronger sparks when ultraviolet light fell on the detector loop.
His experiment showed that light helped charged particles escape from metal.
Q11. What did Hallwachs observe using a zinc plate?
Hallwachs observed that ultraviolet light discharged a negatively charged zinc plate. An uncharged zinc plate became positively charged.
This proved that negatively charged particles left the zinc plate.
Q12. What did Lenard observe in photoelectric effect?
Lenard observed that ultraviolet light produced current in an evacuated tube. The current stopped when ultraviolet light stopped.
This showed that light emitted electrons from the metal plate.
Q13. What is threshold frequency?
Threshold frequency is the minimum frequency required for photoelectric emission. It is denoted by ν₀.
Below ν₀, no photoelectrons emit, even when light intensity is high.
Q14. What is the effect of intensity on photocurrent?
Photocurrent increases linearly with intensity when frequency stays above threshold frequency. Higher intensity means more photons per second.
More photons release more photoelectrons per second.
Q15. Does intensity affect stopping potential?
Intensity does not affect stopping potential for a fixed frequency. It only changes saturation current.
Stopping potential depends on maximum kinetic energy, which depends on frequency.
Q16. What is saturation current?
Saturation current is the maximum photoelectric current. It occurs when all emitted photoelectrons reach the collector.
Increasing positive collector potential beyond this point does not increase current.
Einstein Photoelectric Equation Class 12 Important Questions
Einstein photoelectric equation class 12 questions are high-scoring. Always write photon energy, work function, and maximum kinetic energy clearly.
Q17. What is Einstein’s photoelectric equation?
Einstein’s photoelectric equation is Kmax = hν - φ₀. It explains maximum kinetic energy of emitted photoelectrons.
Here, hν is photon energy and φ₀ is work function.
Q18. How is stopping potential related to maximum kinetic energy?
Stopping potential relates to maximum kinetic energy as eV₀ = Kmax. It stops even the fastest photoelectrons.
The stopping potential is measured in volts.
Q19. Write Einstein’s equation using threshold frequency.
Einstein’s equation using threshold frequency is Kmax = h(ν - ν₀). This follows from φ₀ = hν₀.
Photoelectric emission occurs only when ν is greater than ν₀.
Q20. Caesium has work function 2.14 eV. Find threshold frequency.
The threshold frequency is 5.16 × 10¹⁴ Hz. Use φ₀ = hν₀.
- Given Data:
φ₀ = 2.14 eV
h = 6.63 × 10⁻³⁴ J s
1 eV = 1.6 × 10⁻¹⁹ J
- Formula Used:
ν₀ = φ₀ / h
- Calculation:
φ₀ = 2.14 × 1.6 × 10⁻¹⁹ J
φ₀ = 3.424 × 10⁻¹⁹ J
ν₀ = 3.424 × 10⁻¹⁹ / 6.63 × 10⁻³⁴
- Final Result:
ν₀ = 5.16 × 10¹⁴ Hz
Q21. Find maximum kinetic energy when light of frequency 6 × 10¹⁴ Hz falls on caesium.
The maximum kinetic energy is 0.345 eV. Caesium work function is 2.14 eV.
- Given Data:
ν = 6 × 10¹⁴ Hz
φ₀ = 2.14 eV
h = 6.63 × 10⁻³⁴ J s
- Formula Used:
Kmax = hν - φ₀
- Calculation:
hν = 6.63 × 10⁻³⁴ × 6 × 10¹⁴
hν = 3.978 × 10⁻¹⁹ J
hν = 2.485 eV
Kmax = 2.485 - 2.14
- Final Result:
Kmax = 0.345 eV
Q22. Find stopping potential for maximum kinetic energy 0.345 eV.
The stopping potential is 0.345 V. In electron volt units, Kmax in eV equals V₀ in volts.
- Given Data:
Kmax = 0.345 eV
- Formula Used:
eV₀ = Kmax
- Calculation:
V₀ = 0.345 V
- Final Result:
V₀ = 0.345 V
Q23. The cut-off voltage is 1.5 V. Find maximum kinetic energy.
The maximum kinetic energy is 1.5 eV. Stopping potential directly gives kinetic energy in eV.
- Given Data:
V₀ = 1.5 V
- Formula Used:
Kmax = eV₀
- Calculation:
Kmax = 1.5 eV
- Final Result:
Kmax = 1.5 eV
Photon Class 12 Physics Important Questions
Photon class 12 physics questions connect particle nature of light with energy and momentum. Photon energy depends on frequency, not intensity.
Q24. What is a photon?
A photon is a quantum or packet of electromagnetic radiation. It carries energy and momentum.
A photon has no electric charge and travels with speed c in vacuum.
Q25. What is the energy of a photon?
The energy of a photon is E = hν. It can also be written as E = hc/λ.
Here, h is Planck’s constant and ν is frequency.
Q26. What is the momentum of a photon?
The momentum of a photon is p = h/λ. It can also be written as p = hν/c.
Photon momentum proves particle-like behaviour of light.
Q27. Does photon energy depend on intensity?
Photon energy does not depend on intensity. It depends only on frequency.
Higher intensity means more photons per second, each with the same energy.
Q28. Calculate photon energy for frequency 6 × 10¹⁴ Hz.
The photon energy is 3.98 × 10⁻¹⁹ J. Use E = hν.
- Given Data:
h = 6.63 × 10⁻³⁴ J s
ν = 6 × 10¹⁴ Hz
- Formula Used:
E = hν
- Calculation:
E = 6.63 × 10⁻³⁴ × 6 × 10¹⁴
E = 39.78 × 10⁻²⁰ J
- Final Result:
E = 3.98 × 10⁻¹⁹ J
Q29. A laser emits power 2 × 10⁻³ W. Photon energy is 3.98 × 10⁻¹⁹ J. Find photons emitted per second.
The laser emits about 5.0 × 10¹⁵ photons per second. Power equals energy emitted per second.
- Given Data:
P = 2 × 10⁻³ W
E = 3.98 × 10⁻¹⁹ J
- Formula Used:
N = P/E
- Calculation:
N = 2 × 10⁻³ / 3.98 × 10⁻¹⁹
N = 0.5025 × 10¹⁶
- Final Result:
N = 5.0 × 10¹⁵ photons per second
Wave Nature of Matter Class 12 Important Questions
Wave nature of matter class 12 questions test de Broglie’s hypothesis. Moving microscopic particles show measurable matter waves.
Q30. What are matter waves class 12?
Matter waves are waves associated with moving material particles. They are also called de Broglie waves.
Electrons, protons, and neutrons show matter-wave behaviour under suitable conditions.
Q31. What is de Broglie wavelength class 12?
de Broglie wavelength is the wavelength associated with a moving particle. It is given by λ = h/p.
For a particle of mass m and speed v, λ = h/mv.
Q32. Write de Broglie relation.
The de Broglie relation is λ = h/p. Here, p is momentum.
For non-relativistic particles, p = mv, so λ = h/mv.
Q33. Why do daily objects not show wave nature?
Daily objects do not show wave nature because their de Broglie wavelength is extremely small. Their mass is large.
A cricket ball has a wavelength far below measurable length scales.
Q34. Calculate de Broglie wavelength of an electron moving at 5.4 × 10⁶ m/s.
The de Broglie wavelength is 0.135 nm. This is comparable to X-ray wavelength.
- Given Data:
m = 9.11 × 10⁻³¹ kg
v = 5.4 × 10⁶ m/s
h = 6.63 × 10⁻³⁴ J s
- Formula Used:
λ = h/mv
- Calculation:
p = 9.11 × 10⁻³¹ × 5.4 × 10⁶
p = 4.92 × 10⁻²⁴ kg m/s
λ = 6.63 × 10⁻³⁴ / 4.92 × 10⁻²⁴
- Final Result:
λ = 1.35 × 10⁻¹⁰ m = 0.135 nm
Q35. Calculate de Broglie wavelength of a ball of mass 150 g moving at 30 m/s.
The de Broglie wavelength is 1.47 × 10⁻³⁴ m. This is too small to measure.
- Given Data:
m = 150 g = 0.150 kg
v = 30 m/s
h = 6.63 × 10⁻³⁴ J s
- Formula Used:
λ = h/mv
- Calculation:
p = 0.150 × 30
p = 4.50 kg m/s
λ = 6.63 × 10⁻³⁴ / 4.50
- Final Result:
λ = 1.47 × 10⁻³⁴ m
Dual Nature Class 12 PYQ Boards and Graph Questions
Dual nature class 12 pyq boards often test photoelectric graphs. Students must separate intensity effects from frequency effects.
Q36. Why did wave theory fail to explain photoelectric effect?
Wave theory failed because it could not explain threshold frequency and instant emission. It also predicted kinetic energy should increase with intensity.
Experiments showed maximum kinetic energy depends on frequency, not intensity.
Q37. What does photocurrent versus intensity graph show?
The graph shows photocurrent increases linearly with light intensity. Frequency must stay above threshold frequency.
This means more intense light emits more photoelectrons per second.
Q38. What does stopping potential versus frequency graph show?
The graph shows stopping potential varies linearly with frequency. Its slope equals h/e.
The intercept gives threshold frequency for the metal.
Q39. Why is photoelectric emission instantaneous?
Photoelectric emission is instantaneous because one photon gives energy to one electron in one event. The electron does not collect energy gradually.
NCERT states emission occurs in about 10⁻⁹ s or less.
Q40. Why is threshold frequency different for different metals?
Threshold frequency differs because work function differs for different metals. Higher work function needs higher photon frequency.
The relation is ν₀ = φ₀/h.
Dual Nature Class 12 Important Topics
Dual nature class 12 important topics include definitions, graphs, and numericals. The chapter rewards formula clarity and correct unit conversion.
Q41. Which topics are most important in dual nature important questions class 12?
The most important topics are photoelectric effect, Einstein’s equation, photons, and de Broglie waves. These topics cover theory and numericals.
Students should also revise work function, stopping potential, threshold frequency, and graph interpretation.
Q42. What are the most repeated class 12 physics dual nature important questions?
The most repeated questions ask definitions, formulae, and graph conclusions. Photoelectric effect numericals also repeat often.
CBSE 2026 can ask work function, maximum kinetic energy, stopping potential, and de Broglie wavelength.
Q43. Where do students lose marks in this chapter?
Students lose marks in eV to joule conversion and graph interpretation. They also confuse intensity with frequency.
Stopping potential depends on frequency, while saturation current depends on intensity.
Q44. What formulae should students revise first?
Students should revise E = hν, Kmax = hν - φ₀, eV₀ = Kmax, and λ = h/p. These formulae cover most numericals.
For wavelength questions, students should also use E = hc/λ.
Q45. How is Dual Nature linked to modern Physics?
Dual Nature proves that light and matter need quantum explanation. Classical wave theory cannot explain all observations.
Photoelectric effect supports photon theory, while de Broglie relation supports matter waves.
Class 12 Physics Chapter List