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How does the photoelectric effect support the photon model of light?

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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.

The photoelectric effect provides critical evidence for the photon model of light by demonstrating that electromagnetic radiation behaves as discrete packets of energy rather than continuous waves.12

Classical wave theory failed to explain why the maximum kinetic energy of emitted electrons depends on the light's frequency rather than its intensity. According to Albert Einstein's 1905 hypothesis, light consists of quanta, or photons, each carrying energy $E = h ν$ (where $h$ is Planck’s constant and $ν$ is frequency).321

This model explains the phenomenon through the following observations:
* Threshold Frequency: Electrons are only ejected if a photon has enough energy to overcome the material's work function. Light below this minimum frequency cannot trigger emission, regardless of intensity.31
* Energy Dependence: The kinetic energy of an ejected electron is determined by the photon's frequency, expressed as $E_{k} = h ν - ϕ$ , where $ϕ$ is the work function.2
* Intensity: Increasing light intensity increases the number of photons, thereby increasing the number of emitted electrons, but it does not change the energy of individual electrons.32