Britannica Sources used in this answer

Why does the photoelectric effect depend on light frequency rather than only light intensity?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

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 depends on light frequency rather than intensity because light interacts with matter as discrete packets of energy called photons, rather than as a continuous wave.21

According to Albert Einstein, the energy of an individual photon is determined by the formula $E = h ν$ , where $h$ is Planck’s constant and $ν$ is the frequency of the light. To eject an electron from a material, a photon must transfer enough energy to overcome the photoelectric work function—the minimum energy required for an electron to escape the surface.21

If the frequency of the light is too low, the energy of each individual photon is insufficient to liberate an electron, regardless of how many photons (intensity) strike the surface. Conversely, increasing the intensity of the light simply increases the number of photons hitting the material, which results in a greater number of ejected electrons, but it does not change the kinetic energy of those individual electrons. The maximum kinetic energy of an ejected electron is instead determined by the difference between the photon's energy and the work function: $ϵ_{m a x} = h ν - ψ$ .132