The threshold energy for production of a particle is the minimum K.E., a pair of travelling particles must have when they collide. Absorbed energy =KE+ threshold energy hv′=KE+hv0′ hv0′=hv′−KE 6.26×10−34×v0 =6.26×10−34×4×1014−6.63×10−20J ∴ v0=2.999×1013s−1=3×1014s−1 Hence, threshold frequency (v0′) of metal is 3×1014s−1.