Final answer:
The photoelectric effect refers to the ejection of electrons from a metal surface when exposed to light of sufficient frequency, highlighting the particle nature of light as photons. It defied classical physics and was explained by Einstein's quantum theory, leading to his Nobel Prize recognition.
Step-by-step explanation:
The phenomenon in which electrons are emitted from a metal's surface when light of a certain frequency, or higher than a certain frequency, shines on the surface is called the photoelectric effect. To eject electrons, the light must have a frequency equal to or above the so-called threshold frequency. If the frequency of the incident light is below this threshold, no electrons are emitted regardless of light intensity. However, once the threshold is reached or exceeded, the number of electrons emitted increases with the intensity of the light, and their kinetic energy depends on the frequency of the light.
The mystery of the photoelectric effect challenged classical physics because it suggested that light had particle-like properties, contrary to the wave theory of light at the time. The effect was famously explained by Albert Einstein using the concept of photons, which earned him a Nobel Prize. He showed that light could be thought of as made of particles, each carrying energy proportional to its frequency, demonstrating the dual nature of light as both wave and particle.