Question

Monochromatic radiation of wavelength 640.2 nm from a neon lamp irradiates a 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 photocell. Predict the new stopping voltage.

Answer 1

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