The photoelectric effect, first described by Heinrich Hertz in 1887, was later extensively investigated by Philipp Lenard and Robert Millikan, leading to fundamental experimental evidence that supported its existence and established its key characteristics.

Lenard carried out experiments in the late 19th century to study the properties of electrons emitted from metals under the influence of light. He observed that as the intensity of incident light increased, the kinetic energy of the emitted electrons also increased. This finding was in accordance with the idea that light energy is transferred to electrons as discrete packets called photons.

Millikan's experiments in the early 20th century further expanded the understanding of the photoelectric effect. He discovered that the kinetic energy of emitted electrons depends on the frequency of the incident light, rather than its intensity. Moreover, Millikan identified the existence of a threshold frequency below which no electrons were emitted, regardless of the light's intensity. This finding suggested that a minimum energy, corresponding to the threshold frequency, is required for electrons to be released from the material.

These experiments provided crucial evidence for the photoelectric effect, demonstrating that electrons are indeed emitted from a material when exposed to light, and that the energy of emitted electrons depends on the intensity and frequency of light. This experimental confirmation laid the foundation for Albert Einstein's later theoretical explanation of the phenomenon.