Question

A photo electric cell using caesium as the sensitive element is illuminated with light of wavelength of 4.2 × 10^–7 m. The work function of caesium is 1.90 V. The reverse potential which will supress the electron emission is 

(1) 1.5 V 

(2) 1.9 V

(3) 1.05 V 

(4) 1 V

Answer 1

Correct option :  (3)

Explanation : 

From Einstein’s photoelectric equation we have eV_s = hv - ω₀ 

= (2.95–1.9) V = 1.05 V 

Alternately;  

λ = 4.2 × 10^–7 m = 420 nm

The energy of the photon in eV is E = 1240/420 eV = 2.5 eV