Question

Sketch a graph between frequency of incident radiations and stopping potential for a given photosensitive material. What information can be obtained from the value of the intercept on the potential axis ? A source of light of frequency greater than the threshold frequency is at a distance of 1 m from the cathode of a photocell. The stopping potential is found to be V. If the distance of the light source from the cathode is reduced, explain giving reasons, what change will you observe in the (i) photoelectric current (ii) stopping potential. 

Answer 1

From Einstein’s photoelectric equation

E_k = hv – W

eV₀ = hv – W

V₀ = h/e v - W/e

Comparing with y = mx + c

The intercept W/e will help to obtain the work function of the substance.

(i) Photoelectric Current : As the distance of the light source from the cathode is reduced, the intensity of light is increased. Thus, photoelectric current is increased because more photo electrons will get emitted. 

(ii) Stopping Potential : The stopping potential remains unaffected by reducing the distance of the light source from the cathode, as frequency is not changed on reducing the distance between source of light and cathode.