A Level Physics Practice Exam

What happens to the electrons in a photocell when yellow light of wavelength 570 nm is incident on its cathode?

1. Electrons are emitted with high kinetic energy

2. Electrons are emitted with almost zero kinetic energy

3. No electrons are emitted

4. Electrons are absorbed

The correct answer is: Electrons are emitted with almost zero kinetic energy

In a photocell, the emission of electrons from the cathode occurs when light of sufficient energy is incident upon it. The energy of the photons can be calculated using the equation \(E = \frac{hc}{\lambda}\), where \(h\) is Planck's constant, \(c\) is the speed of light, and \(\lambda\) is the wavelength of the incident light. For yellow light of wavelength 570 nm, we can determine its energy. Converting the wavelength into meters gives us \(570 \times 10^{-9}\) m. Plugging this value into the equation yields a photon energy that falls below the work function energy required to liberate electrons from the cathode material in a typical photocell. If the energy of the incident photons is less than the work function, no electrons will have enough energy to overcome the potential barrier and be emitted, leading to a scenario where the electrons are emitted with almost zero kinetic energy. This happens because while the photons may interact with the surface, they do not provide sufficient energy to free the electrons properly, resulting in negligible emission or retention of their motion. Since photons of yellow light do not provide enough energy to liberate the electrons effectively, the scenario implies that