Albert Einstein · Nobel-era portrait, circa 1923
From Planck's energy quanta to Einstein's light quanta
Max Planck introduced energy quantisation in 1900 to explain the spectrum of black-body radiation. In his model, energy exchange by the material's oscillators occurs in discrete amounts related to frequency. In 1905, Albert Einstein extended the quantum idea to light itself, proposing that light can behave as discrete packets of energy—later called photons—each with energy E = hν. The distinction is important: Planck introduced the energy-quantum idea; Einstein applied it to light.
The photoelectric effect is the emission of electrons from a material when incident light has sufficient frequency. Each electron absorbs energy from a single photon. Increasing light intensity at a frequency above the threshold generally increases the number of emitted electrons; it does not, by itself, increase their maximum kinetic energy. That maximum energy increases with the light frequency.
Here, Kmax is the maximum kinetic energy of the emitted electrons, h is Planck's constant, ν is the incident-light frequency, and φ is the material's work function—the minimum energy needed to release an electron. Emission occurs when hν ≥ φ; therefore, the threshold frequency is ν0 = φ/h.
Remember: Above the threshold frequency, greater intensity usually means more emitted electrons, while greater frequency means greater maximum kinetic energy. Below the threshold frequency, increasing intensity alone cannot cause photoelectric emission in the standard one-photon explanation.
Portrait source: Wikimedia Commons (public-domain image). For the historical award citation, see the Nobel Prize official summary.