• summary_type: concept
• word_count: 180

The photoelectric effect demonstrates the particle nature of light. Key observations: (1) Below the threshold frequency f_{0} = φ/h, no electrons are emitted regardless of intensity. (2) Above f_{0}, emission is instantaneous (< 10^{-9} s). (3) KE_max = hf - φ depends only on frequency, not intensity. (4) Photocurrent is proportional to intensity. (5) Stopping potential $V_{0}$ = (hf - φ)/e is intensity-independent. Einstein's equation KE_max = hf - φ = e$V_{0}$ explains all these observations. The $V_{0}$ vs f graph is a straight line with slope h/e (universal, same for all metals), x-intercept f_{0}, and y-intercept -φ/e. For two metals, the lines are parallel. The work function φ varies from ~2 eV (Cs, K) to ~5 eV (Pt, W). Threshold wavelength λ_{0} = 1240/φ(eV) nm. Classical wave theory fails to explain observations (1)-(3), establishing the quantum nature of light. Millikan's experiments (1916) precisely confirmed the linear $V_{0}$-f relationship.