Photoelectric Effect - UNSOLVED PRACTICE SET

Q7. What is the photoelectric effect? Name the particles emitted during this phenomenon.

Q8. Define threshold frequency and work function. How are they related?

Q9. Why does increasing the intensity of light increase the number of photoelectrons but not their kinetic energy?

Q10. What happens when light of frequency less than the threshold frequency falls on a metal surface, even if the intensity is very high?

Q11. The automatic doors at your local shopping mall open when you approach them. This happens because an infrared beam is interrupted, triggering a sensor. Explain how this is related to the photoelectric effect. Why does the sensor use infrared rather than visible light?

Q12. Write Einstein's photoelectric equation and explain each term.

Q13. Explain the photoelectric effect in detail. Describe the experimental setup and observations. How did Einstein's explanation of the photoelectric effect support the particle nature of light? Write Einstein's photoelectric equation and explain each term. 

Q14. Discuss the main features of the photoelectric effect that cannot be explained by the wave theory of light but are perfectly explained by the quantum theory. Explain:

(a) The existence of threshold frequency

(b) The instantaneous nature of electron emission

(c) The independence of kinetic energy on intensity

(d) The dependence of kinetic energy on frequency

Why was Einstein awarded the Nobel Prize for this work rather than for relativity?

Q15. A physics teacher demonstrates the photoelectric effect in class using a photocell and a variable light source.

(a) When blue light (high frequency) is shone on the photocell, electrons are emitted and the ammeter shows a current. When red light (low frequency) is used, no current flows. Explain why, using the concept of threshold frequency.

(b) When the intensity of blue light is doubled, the current doubles but the maximum kinetic energy of electrons remains the same. Explain both observations.

(c) The teacher replaces the metal with one having a higher work function. What changes would you expect in the experiment?

(d) Solar panels on the rooftops of Indian homes work on a similar principle. Explain how sunlight generates electricity in a solar cell, and why certain materials are better than others for this purpose.

Q16. The work function of a metal is 3.5 × 10⁻¹⁹ J.

(a) Calculate the threshold frequency for this metal.

(b) If light of frequency 8.0 × 10¹⁴ Hz falls on this metal, calculate the kinetic energy of the emitted photoelectrons.

(c) If the intensity of this light is doubled, what happens to the kinetic energy and the number of photoelectrons?

(Planck's constant h = 6.626 × 10⁻³⁴ J·s)

Q17. When light of wavelength 300 nm falls on a metal surface, photoelectrons are emitted with a kinetic energy of 1.5 × 10⁻¹⁹ J.

(a) Calculate the energy of the incident photons.

(b) Calculate the work function of the metal.

(c) Calculate the threshold wavelength for this metal.

(d) Would this metal show the photoelectric effect with visible light (λ = 400-700 nm)?

(Planck's constant h = 6.626 × 10⁻³⁴ J·s, c = 3 × 10⁸ m/s)

Q18. A photocell uses caesium metal with a work function of 2.14 eV.

(a) Convert the work function to joules. (1 eV = 1.6 × 10⁻¹⁹ J)

(b) Calculate the threshold frequency and threshold wavelength for caesium.

(c) If light of wavelength 400 nm is incident on the caesium surface, calculate the maximum kinetic energy of the emitted electrons in eV.

(d) Why is caesium commonly used in photoelectric devices?

(Planck's constant h = 6.626 × 10⁻³⁴ J·s, c = 3 × 10⁸ m/s)