A Level Physics Practice Exam

Why are electrons not emitted if the frequency of the radiation is below a certain threshold?

1. The energy of the photons is too high

2. The work function exceeds the energy of the photons

3. The intensity of the radiation is insufficient

4. The photons lack sufficient momentum

The correct answer is: The work function exceeds the energy of the photons

Electrons are not emitted when the frequency of the incident radiation is below a certain threshold due to the relationship between photon energy and the work function of the material. The work function is the minimum amount of energy needed to remove an electron from the surface of a material. Photon energy is determined by the equation \(E = hf\), where \(h\) is Planck's constant and \(f\) is the frequency of the radiation. If the frequency of the radiation is below the threshold frequency, the energy of the photons is insufficient to overcome the work function of the material. As a result, even if photons are incident on the material, there aren't enough energy photons to free electrons, leading to no emission. In summary, when the frequency is too low, the energy associated with each photon does not meet the minimum requirement to release an electron from the material. This principle is part of the photoelectric effect, which highlights the importance of both the frequency of radiation and the specifics of the material involved.