In first case when source of radiation is neon lamp.
λ = 640.2 nm = 640.2 × 10–9 m, V0 = 0.54 V
From Einstein’s photoelectric equation
eV0 = hc/λ-ϕ0 ......(1)
⇒ e x 0.54 = hc/640.2 x 10-9 -ϕ0 .....(2)
n second case when iron source is the source of radiation, i.e., λ = 427.2 nm = 427.2 × 10–9 m, let V0 ’ be the new stopping potential, then equation (1) gives