A Level Physics Practice Exam

What is essential to observe during the photoelectric effect?

1. Light must have a frequency below a certain threshold

2. Light must be coherent

3. The surface has to be opaque

4. The energy of light must be above a threshold value

The correct answer is: The energy of light must be above a threshold value

In the context of the photoelectric effect, it is crucial to understand that light must have energy sufficient to dislodge electrons from the surface of a material, typically a metal. This is where the concept of a threshold value comes into play. Each material has a specific threshold frequency (and corresponding energy) below which no electrons will be emitted, regardless of the intensity of the light. The energy of light is directly related to its frequency through the equation \(E = hf\), where \(E\) is energy, \(h\) is Planck's constant, and \(f\) is the frequency of the light. If the frequency of the incident light is above this threshold, then the energy is also above the required threshold value, and electrons can be emitted. Coherence of light relates to how uniformly a wave maintains a single phase relationship and is not necessary for the photoelectric effect to occur. The requirement that the surface has to be opaque is also incorrect, as even though metals are often used, the condition pertains more to the electron emission rather than the transparency of the material. Instead, the key factor is focusing on whether the energy of the light surpasses the threshold value required for electron emission to take place.