Sketch a graph between the frequency of incident radiation and the 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 placed at a distance of 1 m from the cathode of a photo-cell. The stopping potential is found to be . If the distance of the light source from the cathode is reduced, explain, giving reasons, what change you will observe in the:
(i) photoelectric current,
(ii) stopping potential.
(i) photoelectric current,
(ii) stopping potential.
Answer: Intercept gives ; current increases, stopping potential unchanged.
Solution
The graph of stopping potential against frequency is a straight line with slope . From , the intercept on the potential axis equals , from which the work function of the material is found.
Reducing the distance increases the intensity (more photons per unit area per second), but the frequency is unchanged:
(i) Photoelectric current increases, because more electrons are ejected per second.
(ii) Stopping potential is unchanged, because it depends only on the frequency, which has not changed.
Reducing the distance increases the intensity (more photons per unit area per second), but the frequency is unchanged:
(i) Photoelectric current increases, because more electrons are ejected per second.
(ii) Stopping potential is unchanged, because it depends only on the frequency, which has not changed.
CBSE Class 12 Board · 2006 · All India · Subjective