Question

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.

Answer 1

Wavelength of the monochromatic radiation,
λ = 640.2 nm = 640.2 × 10⁻⁹ m
Stopping potential of the neon lamp, V0 = 0.54 V
Charge on an electron, e = 1.6 × 10⁻¹⁹ C
Planck’s constant, h = 6.6 × 10⁻³⁴ Js

Wavelength of the radiation emitted from an iron source, λ' = 427.2 nm = 427.2 × 10⁻⁹ m

Hence, the new stopping potential is 1.50 eV.