Einstein's photoelectric equationequation
Is light a stream of bullets, or waves? The photoelectric effect declared: bullets. Electrons are ejected from metal only if the light frequency exceeds a threshold, and the energy depends on the color, not the brightness. Einstein's equation E = hf — W precisely describes how much energy the electron retains after escape.
How it works
Used in solar cells (photons create electron-hole pairs), in light sensors, in photomultipliers for detecting weak light signals. It also explains the red edge of the photoelectric effect — the minimum frequency at which current still flows.
💡 Einstein received the 1921 Nobel Prize for explaining the photoelectric effect, not for the theory of relativity, which initially seemed too bold.
E_k = h f - W
E_k — maximum kinetic energy of the ejected electron (in joules or eV), h — Planck's constant (h ≈ 6.626×10⁻³⁴ J·s), f — frequency of the incident light, W — work function of the material (energy required for an electron to leave the surface, typically 2 to 5 eV for metals).
E = h f
E — energy of the photon, h — Planck's constant, f — frequency of the radiation.