50 Conceptual Questions on Photoelectric Effect with Answers

1. What is photoelectric effect?
   Photoelectric effect is the emission of electrons from a metal surface when light of suitable frequency falls on it.

2. What are photoelectrons?
   The electrons emitted from the metal surface due to incident light are called photoelectrons.

3. Who explained photoelectric effect?
   Albert Einstein explained photoelectric effect using the photon theory of light.

4. What is a photon?
   A photon is a packet of light energy.

5. What is the energy of one photon?
   Energy of one photon is E = hf.

6. What is threshold frequency?
   Threshold frequency is the minimum frequency of light required to emit electrons from a metal surface.

7. What happens if frequency is less than threshold frequency?
   No photoelectrons are emitted, whatever the intensity of light may be.

8. What is work function?
   Work function is the minimum energy required to remove an electron from the metal surface.

9. What is the relation between work function and threshold frequency?
   Work function = h × threshold frequency.

10. Does intensity affect the maximum kinetic energy of photoelectrons?
    No, maximum kinetic energy depends on frequency, not intensity.

11. What happens when intensity of light increases?
    The number of emitted photoelectrons increases, so photoelectric current increases.

12. What happens when frequency of light increases?
    Maximum kinetic energy of photoelectrons increases.

13. What is Einstein’s photoelectric equation?
    hf = work function + maximum kinetic energy.

14. What is stopping potential?
    Stopping potential is the minimum reverse potential required to stop the fastest photoelectrons.

15. What is the relation between stopping potential and kinetic energy?
    Maximum kinetic energy = e × stopping potential.

16. Does stopping potential depend on intensity?
    No, stopping potential depends only on frequency of incident light.

17. Does stopping potential depend on frequency?
    Yes, it increases when frequency increases.

18. Why photoelectric effect supports particle nature of light?
    Because energy is transferred in packets called photons.

19. Why wave theory failed to explain photoelectric effect?
    Wave theory could not explain threshold frequency and instantaneous emission.

20. Is photoelectric emission instantaneous?
    Yes, emission starts almost immediately when suitable light falls on the metal.

21. What happens if light intensity is high but frequency is low?
    No emission occurs if frequency is below threshold frequency.

22. What happens if light frequency is high but intensity is low?
    Photoelectrons are emitted, but their number is small.

23. Which property of light controls number of photoelectrons?
    Intensity controls the number of photoelectrons.

24. Which property of light controls kinetic energy of photoelectrons?
    Frequency controls the kinetic energy of photoelectrons.

25. What is saturation current?
    Saturation current is the maximum photoelectric current when all emitted electrons reach the collector.

26. How does saturation current change with intensity?
    Saturation current increases with intensity.

27. How does saturation current change with frequency?
    For same intensity, saturation current is almost independent of frequency.

28. What is cut-off potential?
    Cut-off potential is another name for stopping potential.

29. Can photoelectric effect occur with any light?
    No, light frequency must be greater than or equal to threshold frequency.

30. Why alkali metals are used in photoelectric cells?
    Because they have low work function.

31. What is the unit of work function?
    Work function is usually measured in electron volt, eV.

32. What is one electron volt?
    One electron volt is the energy gained by an electron when it moves through 1 volt potential difference.

33. What happens at threshold frequency?
    Electrons are just emitted with zero maximum kinetic energy.

34. What is the graph of maximum kinetic energy vs frequency?
    It is a straight line.

35. What is the slope of kinetic energy vs frequency graph?
    The slope is Planck’s constant h.

36. What is the intercept of kinetic energy vs frequency graph?
    The negative intercept represents work function.

37. What is the graph of stopping potential vs frequency?
    It is a straight line.

38. What is the slope of stopping potential vs frequency graph?
    The slope is h/e.

39. What happens when metal surface is changed?
    Threshold frequency and work function change.

40. Why different metals have different threshold frequencies?
    Because different metals have different work functions.

41. Can increasing intensity overcome work function?
    No, if photon energy is less than work function, emission will not occur.

42. What is the condition for photoelectric emission?
    hf must be greater than or equal to work function.

43. What happens to excess photon energy?
    It appears as kinetic energy of the emitted electron.

44. Can one photon eject more than one electron?
    Generally, one photon ejects one electron.

45. Why photoelectric current becomes constant at high positive potential?
    Because all emitted photoelectrons are collected.

46. What is retarding potential?
    Retarding potential is negative potential applied to oppose photoelectrons.

47. Why all photoelectrons do not have same kinetic energy?
    Because electrons inside the metal lose different amounts of energy before escaping.

48. What is maximum kinetic energy?
    It is the kinetic energy of electrons emitted from the surface without energy loss.

49. What is the practical use of photoelectric effect?
    It is used in photoelectric cells, light sensors, automatic doors and solar-related devices.

50. What is the main conclusion of photoelectric effect?
    Light behaves as particles called photons, and photon energy depends on frequency.