Infrared light shined on a metal initially emits no electrons. What is the quantum prediction if the intensity of the light is increased?

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The question asks about the quantum prediction when infrared light initially below the threshold frequency shines on a metal and the intensity is increased.
Option a: 'The photoelectric current increases immediately.' This would imply that increasing intensity at sub-threshold frequency leads to emission, which contradicts the quantum condition that emission requires the photon energy to exceed the work function (i.e., be above threshold). Since the energy per photon is still below threshold, no electrons are emitted, so no immediate current rise occurs.
Option b: 'The photoelectric current increases gradually.' A gradual increase would suggest a continuous emission as intensity grows, but with sub-threshold frequency no electrons are emitted regardless of intensity according to quantum theory, so there is no gradual rise in current to begin with.
Option c: 'The photoelectric current decreases immediately.' There is no mechanism for a sub-threshold frequency to produce a decreasing current upon increasing intensity; the baseline current is zero and cannot spontaneously drop below zero, making this option physically inconsistent.
Option d: 'The photoelectric current decreases gradually.' Similar to option c, there is no rationale for a decreasing current since no emission occurs at sub-threshold energy, so no gradual change should happen as intensity increases.
Option e: 'There is no change in the photoelectric current.' This is consistent with the quantum prediction: if the photon energy is below the work function threshold, electrons are not emitted regardless of how much you increase intensity, so the current remains zero.
Option f: 'Any change to the photoelectric current cannot be determined.' The scenario provides a clear criterion (photon energy below threshold) and a well-established result: emission does not occur; hence the change is deterministically zero, not indeterminate.
In summary, with infrared light below the threshold frequency, boosting intensity does not cross the threshold energy barrier required to liberate electrons, so no photoelectric current is produced and the current remains unchanged. This aligns with the fundamental quantum requirement that emission requires photon energy exceeding the work function, not merely higher light intensity.

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

Infrared light shined on a metal initially emits no electrons. What is the quantum prediction if the intensity of the light is increased?

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