Why did Einstein receive the 1921 Nobel Prize in Physics for work on the photoelectric effect?

Albert Einstein was awarded the 1921 Nobel Prize in Physics for his 1905 explanation of the photoelectric effect, a phenomenon where light causes a material to emit electrons.12

Classical physics could not explain why the kinetic energy of these electrons depended on the light's frequency rather than its intensity, nor why emission occurred without a time delay. Einstein resolved these contradictions by proposing a radical corpuscular theory of light. He hypothesized that light consists of particles called photons, each carrying a quantum of energy ($E=hf$) proportional to its frequency.13

According to Einstein, an electron is ejected when it absorbs a single photon, with the photon's energy covering the work function (the energy required to escape the metal) and the remainder becoming the electron's kinetic energy. This model provided a foundational shift in understanding the nature of light and was later verified with high precision by Robert Millikan, confirming the validity of Planck’s constant.13