Question

For the photoelectric effect in sodium, the figure shows the plot of cut-off voltage versus frequency of incident radiation. The threshold frequency is __________

(a) 6.5 X 10^14 Hz

(b) 4.5 X 10^14 Hz

(c) 3 X 10^14 Hz

(d) 5 X 10^14 Hz

Answer 1

The correct answer is (c) 3 X 10^14 Hz

To explain I would say: The threshold frequency corresponds to the frequency for which the cut-off voltage is zero. Hence, the threshold frequency for sodium, as observed in the graph, is 3 X 10^14 Hz.