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How did Albert Einstein explain light as photons?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — electromagnetic radiation Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

In 1905, Albert Einstein proposed that electromagnetic radiation is not merely a continuous wave but is composed of discrete energy packets, later termed photons. He reached this conclusion by applying thermodynamic arguments to radiation, suggesting that light itself is granular and that each quantum carries an energy $E = h ν$ (or $h f$ ), where $h$ is Planck’s constant and $ν$ (or $f$ ) is the frequency of the light.1234

Einstein used this model to explain the photoelectric effect, which classical wave theory could not account for. He proposed that when light strikes a metal surface, an atom absorbs a single photon. If the photon's energy exceeds the work function—the minimum energy required for an electron to escape the material—the electron is ejected.345

This explained why the kinetic energy of emitted electrons depends on the light's frequency rather than its intensity, and why there is no time lag in emission. His prediction was later verified by Robert Millikan, confirming the particle-like nature of light.3514