Photoelectric effect
Light hits a metal surface and ejects electrons — but only if each photon carries enough energy. Increasing brightness sends more photons but cannot eject a single electron below the threshold frequency. Einstein explained this in 1905, proving light comes in quanta.
Ek = hf − φ • h = 6.626 × 10−34 J·s • 1 eV = 1.602 × 10−19 J
The photoelectric effect
When light shines on a metal surface, electrons are emitted — but only if the light frequency exceeds a threshold that depends on the metal. Classical wave theory predicted that brighter light should always eventually eject electrons, and that brighter light should produce faster electrons. Neither prediction was observed.
Einstein's explanation (1905)
Einstein proposed that light consists of discrete packets of energy — photons — each carrying energy E = hf, where h is Planck's constant and f is the frequency. An electron can only escape the metal if it absorbs a single photon with enough energy to overcome the work function φ of the metal. The remaining energy becomes the electron's kinetic energy: Ek = hf − φ.
Key predictions
Below the threshold frequency (f < φ/h), no electrons are emitted regardless of intensity. Above the threshold, the maximum kinetic energy depends only on frequency, not intensity. Higher intensity means more photons per second, so more electrons per second, but each electron's energy is unchanged. These predictions were confirmed by Robert Millikan in 1916, and Einstein received the 1921 Nobel Prize for this work.
Stopping voltage
The stopping voltage V0 is the potential difference needed to halt the fastest photoelectrons: eV0 = hf − φ. Plotting V0 versus frequency gives a straight line with slope h/e, providing a direct measurement of Planck's constant.