A beam of light falls on a metal surface such that photo-electrons are generated. If the power of the light source starts to decrease linearly with time t, then the variation of the photocurrent I and the magnitude of the stopping potential |V| with time is best represented by:

Solution & Explanation

Photoelectric principles to apply: • Photocurrent depends on the NUMBER of photoelectrons per second, i.e. on the intensity (number of photons per second). • Stopping potential depends on the maximum kinetic energy of the photoelectrons, which is set by the photon ENERGY (frequency), via eV = hf - φ. Step 1 — What changes as power falls. Power P = (photons per second) × (energy per photon). The wavelength/frequency of the light is unchanged, so energy per photon hf is constant; only the number of photons per second decreases. Since P decreases LINEARLY with t, the number of photons per second — and hence the photocurrent I — decreases linearly with t. Step 2 — What stays fixed. The photon energy hf is unchanged, so the maximum kinetic energy hf-φ is unchanged, so the stopping potential magnitude |V| stays CONSTANT (it does not depend on intensity). Conclusion: I decreases linearly with t while |V| remains constant — option A. Trap: changing intensity affects only how many electrons are emitted (current), never their maximum energy (stopping potential). Options that make |V| change are wrong.