The photoelectric effect is a phenomenon in which electrons are emitted from a material's surface when it is exposed to electromagnetic radiation, such as light. This effect was first discovered by Heinrich Hertz in 1887 and was later explained by Albert Einstein in 1905, for which he was awarded the Nobel Prize in Physics in 1921.

To understand the photoelectric effect, consider a metal plate that is illuminated with light. When the frequency of the incident light is above a certain threshold frequency, electrons are emitted from the surface of the metal. However, if the frequency is below this threshold, no electrons are emitted, regardless of the light's intensity.

This observation contradicts the classical wave theory of light, which suggests that the energy of the emitted electrons should continuously increase with the light's intensity. Instead, the photoelectric effect demonstrates that the energy of emitted electrons depends on the frequency of the incident light rather than its intensity.

The photoelectric effect has significant implications in understanding the dual nature of light and led to the development of quantum mechanics. It paved the way for the concept of photons, which are particles of light that carry energy proportional to their frequency.