Photoelectric Effect

When light of sufficient frequency strikes a metal surface, electrons are ejected — the photoelectric effect, discovered by Hertz (1887) and explained by Einstein (1905).

Einstein's photon hypothesis: light comes in quanta of energy E = hf. Each photon interacts with ONE electron.

Einstein's equation: K_max = hf − φ, where φ is the work function (minimum energy to release an electron from the metal surface).

Below the threshold frequency f₀ = φ/h, NO electrons are emitted, regardless of intensity. Above, electrons emerge instantaneously.

Maximum KE depends on the frequency, NOT intensity. Intensity affects only the NUMBER of photoelectrons per second.

Stopping potential V_s is the retarding voltage that just stops the most energetic photoelectrons: eV_s = K_max = hf − φ.

Quantum nature: classical wave theory cannot explain (i) threshold frequency, (ii) zero time-delay, (iii) K_max independent of intensity.

Result: light has particle-like (photon) behaviour even though it's a wave — laid the foundation for quantum mechanics.