7.2 Photoelectric Effect

Short Answer Questions

1. What is the photoelectric effect?
   Answer
   The photoelectric effect is the phenomenon where electrons are emitted from the surface of a metal when it is illuminated by light of a sufficiently high frequency.
2. What are the electrons emitted in the photoelectric effect called?
   Answer
   Photoelectrons.
3. What is the threshold frequency?
   Answer
   The threshold frequency (f₀) is the minimum frequency of light required to emit photoelectrons from a metal surface.
4. What happens if the frequency of light is below the threshold frequency?
   Answer
   No photoelectrons will be emitted, regardless of the intensity of the light.
5. What is the formula for the threshold frequency?
   Answer
   f₀ = W/h, where W is the work function and h is Planck’s constant.
6. What is Einstein’s photoelectric equation?
   Answer
   hf = W + TK_max, where hf is the energy of the photon, W is the work function, and TK_max is the maximum kinetic energy of the emitted photoelectron.
7. What is the work function?
   Answer
   The work function (W) is the minimum energy required to eject an electron from the surface of a metal.
8. What does the kinetic energy of photoelectrons depend on?
   Answer
   The frequency of the incident light.
9. What is Planck’s constant?
   Answer
   Planck’s constant (h) is approximately 6.63 × 10⁻³⁴ J s.
10. What is the formula for photon energy?
    Answer
    E = hf, where E is energy, h is Planck’s constant, and f is the frequency of light.
11. What happens to the kinetic energy of photoelectrons when the frequency of light increases?
    Answer
    The kinetic energy of the photoelectrons increases.
12. What is the maximum kinetic energy formula for a photoelectron?
    Answer
    TK_max = hf – W.
13. What is the relationship between the threshold frequency and the threshold wavelength?
    Answer
    λ₀ = c/f₀, where c is the speed of light and f₀ is the threshold frequency.
14. What is the effect of increasing light intensity on the number of photoelectrons?
    Answer
    Increasing light intensity increases the number of photoelectrons emitted but does not change their kinetic energy.
15. Why does classical wave theory fail to explain the photoelectric effect?
    Answer
    Classical wave theory cannot explain the existence of a threshold frequency, the instantaneous emission of electrons, and the dependence of kinetic energy on light frequency.
16. What did Einstein propose about light to explain the photoelectric effect?
    Answer
    Einstein proposed that light consists of discrete energy packets called photons.
17. What happens when a photon strikes the surface of a metal?
    Answer
    The photon transfers all its energy to a single electron, causing it to be ejected if the energy is greater than the work function.
18. What is the mathematical representation of the maximum kinetic energy of a photoelectron?
    Answer
    TK_max = 1/2 mv², where m is the mass of the electron and v is its velocity.
19. What is quantisation of energy?
    Answer
    Quantisation of energy means that energy is absorbed or emitted in discrete packets called photons.
20. What is the speed of light (c) in a vacuum?
    Answer
    3.00 × 10⁸ m/s.

Explanation Questions

1. Explain the concept of the photoelectric effect.
   Answer
   The photoelectric effect is the emission of electrons from a metal surface when it is illuminated by light with a frequency higher than the threshold frequency. It demonstrates the particle nature of light.
2. What is the significance of the threshold frequency in the photoelectric effect?
   Answer
   The threshold frequency is the minimum frequency required to emit photoelectrons from a metal. Below this frequency, no photoelectrons are emitted, regardless of light intensity.
3. Describe Einstein’s photoelectric equation and its components.
   Answer
   Einstein’s equation, hf = W + TK_max, relates the photon energy (hf) to the work function (W) and the maximum kinetic energy (TK_max) of the emitted photoelectrons.
4. Why does increasing the intensity of light not affect the kinetic energy of photoelectrons?
   Answer
   The kinetic energy of photoelectrons depends on the frequency of the light, not its intensity. Intensity only affects the number of photoelectrons emitted.
5. How did the photoelectric effect challenge classical physics?
   Answer
   Classical physics predicted that the kinetic energy of photoelectrons should depend on light intensity and that any frequency of light should emit electrons if the intensity is high enough. The photoelectric effect showed otherwise.
6. Explain the concept of instantaneous emission in the photoelectric effect.
   Answer
   Photoelectrons are emitted from the metal surface almost immediately when light of sufficient frequency is incident, with no time delay, contradicting classical wave theory.
7. What is the work function, and how is it related to the threshold frequency?
   Answer
   The work function (W) is the minimum energy required to eject an electron from a metal surface. It is related to the threshold frequency by W = hf₀.
8. Describe the role of photons in the photoelectric effect.
   Answer
   Photons are discrete packets of light energy that transfer their energy to electrons in a one-to-one interaction, causing photoemission if the energy is greater than the work function.
9. How does increasing the frequency of light affect the maximum kinetic energy of photoelectrons?
   Answer
   Increasing the frequency of light increases the maximum kinetic energy of photoelectrons, as described by TK_max = hf – W.
10. What evidence does the photoelectric effect provide for the particle nature of light?
    Answer
    The photoelectric effect shows that light interacts with matter in discrete packets (photons), supporting the particle theory of light.