Final answer:
The photoelectric effect explains why a photoelectric diode requires a minimum "threshold" frequency of light: the photons must have enough energy to free electrons from the metal's surface.
Step-by-step explanation:
The observation that in a photoelectric diode, no current will flow unless light of a certain minimum "threshold" frequency is used can be explained by the photoelectric effect. The correct explanation is (a) The energy of the photons of light must match or exceed the energy needed for electrons to break free from the metal. This threshold frequency is a characteristic property of the metal. When light with a frequency above this threshold strikes a metal, it provides enough energy for electrons to overcome the binding energy and be ejected, resulting in an electrical current flow. However, if the light's frequency is below this threshold, no amount of intensity will cause electrons to be ejected, and hence no electrical current will be produced. The kinetic energy of ejected electrons increases linearly with the frequency of light above the threshold, but the effect does not occur at all for frequencies below this minimum value. The photoelectric effect disproved the classical wave theory's prediction that the intensity of light should affect the ejection of electrons, as Albert Einstein's explanation, incorporating quantum theory, showed that the energy of light is quantized and depends on its frequency, not its intensity.