What physical effect did Albert Einstein explain in his 1905 paper on light quanta?

In his 1905 paper on light quanta, Albert Einstein explained the photoelectric effect. This phenomenon occurs when electromagnetic radiation strikes a material and ejects electrons from its surface.

Classical wave theory could not fully explain why brighter light did not necessarily give emitted electrons more energy, or why light below a certain frequency produced no electrons at all. Einstein proposed that light energy arrives in discrete packets, later called photons, with each packet carrying energy proportional to frequency: E = hν.

This model explained the threshold frequency: an electron is emitted only when an individual light quantum has enough energy to overcome the material’s work function. Increasing the light’s intensity adds more photons and can increase the number of emitted electrons, but it does not raise their maximum energy unless the frequency also increases.

Einstein’s proposal initially met resistance because it challenged the established wave theory of light. Robert Millikan’s experiments, completed in the 1910s, confirmed the predicted relationship even though Millikan remained skeptical of the photon concept. Einstein received the 1921 Nobel Prize in Physics specifically for his work on the law of the photoelectric effect, not for special relativity.