Work Function and Threshold Frequency - UNSOLVED PRACTICE SET
Chapter: Dual Nature of Radiation and Matter | Topic: Work Function and Threshold Frequency
WORK FUNCTION AND THRESHOLD FREQUENCY - UNSOLVED PRACTICE SET
Topic: Work Function and Threshold Frequency
Multiple Choice Questions
Q1. The work function of a metal is defined as:
- The minimum energy required to eject an electron from the metal surface
- The maximum kinetic energy of emitted photoelectrons
- The energy of the incident photon
- The threshold frequency of the metal
Q2. The threshold frequency is related to the work function by:
- ฮฝโ = hฯโ
- ฮฝโ = ฯโ/h
- ฮฝโ = h/ฯโ
- ฮฝโ = ฯโ ร h
Q3. If the work function of a metal is 4.5 eV, its threshold wavelength is approximately:
- 276 nm
- 450 nm
- 550 nm
- 900 nm
Q4. The work function depends on:
- The intensity of incident light
- The frequency of incident light
- The nature of the photosensitive material
- The area of the metal surface
Q5. Two metals A and B have work functions ฯ_A and ฯ_B such that ฯ_A > ฯ_B. Then:
- The threshold frequency of A is greater than that of B
- The threshold frequency of A is less than that of B
- Both have the same threshold frequency
- The threshold wavelength of A is greater than that of B
Q6. The threshold wavelength of a metal is 600 nm. Light of wavelength 500 nm is incident on it. Then:
- No photoelectric emission occurs
- Photoelectric emission occurs with zero kinetic energy
- Photoelectric emission occurs with non-zero kinetic energy
- The work function becomes zero
Short Answer Questions
Q7. Define work function and threshold frequency. How are they related?
Q8. Why does the work function vary from metal to metal? What property of the metal determines its work function?
Q9. The work function of caesium is 2.14 eV and that of platinum is 5.65 eV. Which metal is more suitable for a photocell sensitive to visible light? Give reason
Q10. Calculate the threshold frequency for a metal with work function 3.3 eV.
Q11. Why do alkali metals like sodium and potassium have low work functions compared to transition metals like iron and copper?
Q12. What happens to the threshold frequency of a metal if its surface is coated with a thin layer of another metal?
Long Answer Questions
Q13. Define work function and threshold frequency. Derive the relationship between them. Explain why different metals have different work functions.
Q14. Discuss the factors that affect the work function of a metal. How does the work function influence the choice of materials for photoelectric devices?
Q15. Explain the significance of the threshold frequency in the photoelectric effect. Why is it impossible to observe photoelectric emission below the threshold frequency, regardless of the intensity of incident light?
Numerical & Application-based Problems
Q16. The work functions of four metals are given below:
Sodium: 2.3 eV
Potassium: 2.25 eV
Calcium: 2.9 eV
Copper: 4.7 eV
(a) Calculate the threshold frequency for each metal.
(b) Calculate the threshold wavelength for each metal.
(c) Which of these metals will show photoelectric emission when illuminated with light of wavelength 400 nm?
(d) For the metals that do emit photoelectrons, calculate the maximum kinetic energy of the photoelectrons.
Q17. A metal has a threshold wavelength of 450 nm.
(a) Calculate the work function of the metal in eV.
(b) Calculate the threshold frequency.
(c) Light of wavelength 300 nm is incident on this metal. Calculate the maximum kinetic energy of the emitted photoelectrons.
(d) Calculate the stopping potential for this case.
Q18. In your school's physics lab, a student is comparing the photoelectric properties of different metals for a project on solar energy.
(a) She measures the threshold wavelength of zinc to be 288 nm. Calculate its work function and determine whether zinc can be used in a solar cell designed to work with visible light (400โ700 nm).
(b) The student finds that caesium (work function = 2.14 eV) emits photoelectrons when illuminated with red light (ฮป = 650 nm), but zinc does not. Explain this observation and calculate the maximum kinetic energy of photoelectrons from caesium.
(c) The student proposes using a material with a very low work function (1.0 eV) for a highly sensitive photodetector. Calculate its threshold wavelength and discuss whether such a material would be practical for use in daylight conditions.
(d) A classmate suggests that by heating a metal, we can reduce its work function and make it sensitive to lower frequency light. Evaluate this suggestion based on your understanding of the photoelectric effect.