Monochromatic radiation of wavelength 640.2 nm (1nm = 10–9 m) from a neon lamp irradiates photosensitive material made of caesium on tungsten. The stopping voltage is measured to be 0.54 V. The source is replaced by an iron source and its 427.2 nm line irradiates the same photo-cell. Predict the new stopping voltage

λ=640.2nmV∘=0.54Vλ=640.2nmV∘=0.54VLet ϕ∘ϕ∘ be work function and vv be frequency of emitted light.So, eV∘=hv−ϕ∘eV∘=hv−ϕ∘⇒ϕ∘=hcλ−eV∘⇒ϕ∘=hcλ−eV∘=6.6×10−34×3×108(640.2×10−9)–1.6×10−19×0.54=6.6×10−34×3×108(640.2×10−9)–1.6×10−19×0.54=3.093×10−19–0.864×10−19=3.093×10−19–0.864×10−19=2.229×10−19J1.6×10−19=2.229×10−19J1.6×10−19=1.39eV=1.39eVλ′=427.2nmλ′=427.2nmLet V′∘V∘′ be new stopping potential.eVo′=hcλ′−ϕ∘eVo′=hcλ′−ϕ∘=6.6×10−34×3×108(427.2×10−9)–2.229×10−19=6.6×10−34×3×108(427.2×10−9)–2.229×10−19=4.63×10−19–2.229×10−19=4.63×10−19–2.229×10−19=2.401×10−19J1.6×10−19=2.401×10−19J1.6×10−19=1.5eV