The threshold frequency ν₀ for a metal is related to its work function by

1. At threshold frequency, incident photons exactly free electrons with K_max = 0. 2. Einstein's equation: hν = φ₀ + K_max. 3. Setting ν = ν₀ and K_max = 0: hν₀ = φ₀. 4. Below ν₀, photons have too little energy to overcome φ₀ and no photoemission occurs. _Source: syllabus Physics, Ch 11 "Dual Nature of Radiation and Matter", §11.5_