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Work function φ is the minimum energy to free a surface electron: φ = $hf_{0}$ = hc/λ_{0}. Metals ordered by increasing φ: Cs (2.14 eV) < K (2.30) < Na (2.75) < Ca (3.20) < Cu (4.65) < Ag (4.70) < Pt (5.65). Alkali metals (Cs, K, Na) respond to visible light; transition metals require UV. The threshold wavelength λ_{0} = 1240/φ nm determines the cutoff: only photons with λ < λ_{0} can eject electrons. JEE questions often ask: given a light source wavelength, which metals will show photoelectric emission? Compare λ_source with λ_{0} for each metal. If λ_source < λ_{0}, emission occurs. The work function depends on the metal, crystal orientation, and surface conditions but is independent of temperature (to first order).