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

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The scenario asks us to consider the classical expectation for photoelectric emission when infrared light is involved and the intensity is increased.
Option a: 'The photoelectric current increases immediately' This would imply that as soon as the infrared light is applied, any electrons start being emitted without needing a higher intensity, which is not what the classical picture necessarily predicts when discussing how current depends on light intensity. In the classical view, the emission rate is tied to the energy being delivered to electrons, and simply applying light does not guarantee an instantaneous jump to a current level independent of intensity; as intensity grows, the current response would reflect that growth rather than an instant fixed rise.
Option b: 'The photoelectric current increases gradually' In the classical model, the electromagnetic wave delivers energy in a continuous fashion, and the rate at which electrons gain energy (and thus the emission rate) scales with the amplitude of the wave. As you raise the intensity, more energy is available to excite more electrons and to give them enough energy to escape, so the current should rise in step with increasing intensity. The change is gradual because intensity is continuously adjustable, and the emission responds to that change.
Option c: 'The photoelectric current decreases immediately' A decrease in current upon increasing intensity lacks physical motivation in the classical framework. If anything, increasing the driving energy would either produce more emission or have no effect if the material saturates, but it would not cause an immediate drop in current simply due to higher intensity.
Option d: 'The photoelectric current decreases gradually' This would imply a steady decline of current as intensity is raised, which contradicts the expectation that more energy delivery tends to promote emission rather than suppress it, at least in a straightforward classical interpretation.
Option e: 'There is no change to the photoelectric current' This would be true if intensity had no effect, which contradicts the basic idea that a stronger electromagnetic field delivers more energy and should influence emission rates, making some current change expected in the classical view.
Option f: 'Changes to the photoelectric current cannot be determined' This is overly vague; within the classical picture, there is a clear, monotonic relationship between intensity and current, so the behavior can be determined to be an increase in current as intensity increases.

Putting the pieces together, the most consistent classical prediction is that increasing the light intensity would lead to a gradual increase in the photoelectric current, reflecting more energy being available to drive electron emission as the driving force is amplified. The other options either imply an immediate fixed rise, a decrease, no change, or an indeterminacy, which do not align with the straightforward classical expectation that current scales with the applied energy density.

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

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

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