Hertz, in 1887 discovered the phenomena of photoelectric effect for the first time. He was experimenting on the generation of electromagnetic waves by spark discharge and he noticed that sparks around the detector loop were intensified when ultraviolet radiation fell on the emitter plate
Radiation falling on the metal surface provided free electrons enough energy to neutralize the attractive force of positive ions and escape the metal surface to intensify the sparks across the metal loop.
Hallwach and Lenard during 1886-1902, researched the process of photoelectric emission. They experimented on negatively charged zinc plate and found that on absorbing ultraviolet light zinc plate became neutral, losing its net positive charge. On further absorption of ultraviolet radiation, neutral zinc plate became negatively charged. Hence proved that electrons are released from the metal surface when ultraviolet radiation is incident on it. They also observed that electrons are not emitted when the frequency of light incident is less than a specific least value, called the threshold frequency. Threshold frequency depends on the nature of material used as an emitter
Some metals like zinc, magnesium etc. emitted electrons only by absorbing ultraviolet light. But, alkali metals like sodium potassium etc. could also absorb visible radiation to emit electrons, and so they were called photosensitive materials.
So, the process in which falling of electromagnetic radiation on the metal surface results in the emission of electrons is called photoelectric effect. And the electrons released due to photoelectric effect are called photo-electrons.