A Level Physics Practice Exam

What happens to the maximum kinetic energy of emitted electrons when the frequency of the incident light decreases?

1. It increases

2. It remains the same

3. It decreases

4. It fluctuates

The correct answer is: It decreases

In the context of the photoelectric effect, the maximum kinetic energy of the emitted electrons is directly related to the frequency of the incident light. According to Einstein’s photoelectric equation, the maximum kinetic energy (K.E.) of the emitted electrons can be expressed as: K.E. = hf - φ where h is Planck’s constant, f is the frequency of the incident light, and φ is the work function of the material, which is the minimum energy needed to eject an electron from the surface. When the frequency of the incident light decreases, the energy of the photons (hf) decreases as well, since the energy is directly proportional to frequency. If the frequency falls below a certain threshold frequency (the point at which hf is equal to the work function φ), no electrons will be emitted, and thus the maximum kinetic energy will also reduce. If the frequency continues to decrease after this threshold, the energy of the photons becomes less than the work function, leading to a scenario where no electrons are emitted at all. As a result, the maximum kinetic energy of any emitted electrons decreases because the energy imparted to the electrons becomes insufficient to overcome the work function. Thus, it logically follows that as the frequency of the incident light decreases, the