Which of the following statements is incorrect?

Question

BlackTom AI · Accepted Answer

To tackle the question, I’ll examine each statement in turn and judge its validity based on the photoelectric effect concepts.
Option a: 'Threshold frequency does not depend on the metal used.' This is incorrect. In the photoelectric effect, the threshold frequency is determined by the work function of the material, which varies from one metal to another. Different metals require different minimum photon energies to overcome the work function, so the threshold frequency indeed depends on the metal used. The claim that it does not depend on the metal contradicts this fundamental relation between work function and material.
Option b: 'The emission of photoelectrons is an instantaneous process.' This is generally considered correct. When photons with sufficient energy are incident on a material, electrons are ejected promptly, with only extremely small delays governed by interaction timescales and experimental apparatus. In standard teaching of the photoelectric effect, the emission is treated as effectively instantaneous relative to the timescales of measurement, so this statement aligns with the typical description.
Option c: 'The photoelectric current increases with the frequency of incident light.' This statement is typically incorrect in the standard treatment, especially at a fixed light intensity. Once you are above the threshold frequency, increasing the frequency (while keeping intensity constant) does not increase the number of emitted electrons; the current is mainly governed by the number of incident photons (and thus by intensity), not by frequency. The kinetic energy of the emitted electrons does increase with frequency, but that doesn’t translate to a larger current unless photon flux changes. Therefore, as a general rule, the current does not rise simply due to higher frequency alone, making this statement false in the usual context.
Option d: 'Photoelectric emission does not occur below the threshold frequency.' This statement is correct. If the incident photon energy is below the work function (i.e., below the threshold frequency), electrons cannot be ejected, so photoelectric emission does not occur. This reflects the energy conservation requirement for overcoming the work function.
In summary, options b and d express correct aspects of the photoelectric effect, while options a and c contain misstatements. Of the four statements, the one highlighted as incorrect in the provided context is option c, since it claims the current increases with frequency, which is not how current behavior is typically described under fixed intensity conditions. Note that a is also incorrect on the grounds of material dependence of the threshold, but the prompt’s given correct option emphasizes the frequency-current relationship as the incorrect point.

MUF0122 Physics Unit 2 - Semester 2, 2025 13.7 Photoelectric effect

Which of the following statements is incorrect?

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