Consider the following statements about photoelectric emission from a metal surface:
(A) All the photoelectrons emitted from the metal have the same kinetic energy.
(B) As the retarding potential on the collector is made more negative, the photocurrent drops to zero at a sharply defined value of the potential.
(C) More tightly bound electrons come out with kinetic energies less than the maximum value.
(D) The threshold frequency is the same for all metals.
Which of the above statements are incorrect?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) and (D) only
Every photon of the light brings the same energy \( h\nu \). An electron that is held more tightly uses up more of this energy in escaping. The work function, and so the threshold frequency, depends on the metal.
(A) All the photoelectrons emitted from the metal have the same kinetic energy. — Incorrect statement. The photoelectrons have a range of kinetic energies, from zero up to \( K_{\max} \).
(B) As the retarding potential on the collector is made more negative, the photocurrent drops to zero at a sharply defined value of the potential. — Correct statement. Fewer electrons reach the collector as the retarding potential grows. The current becomes zero at the stopping potential \( V_0 \), when even the fastest electrons are turned back.
(C) More tightly bound electrons come out with kinetic energies less than the maximum value. — Correct statement. These electrons lose more energy in coming out of the metal, so they leave with less than \( K_{\max} \).
(D) The threshold frequency is the same for all metals. — Incorrect statement. The threshold frequency is \( \nu_0=\phi_0/h \), and the work function \( \phi_0 \) changes from metal to metal.
NCERT (2026-27):
Effect of potential on photoelectric current – “… All the photoelectrons emitted from the metal do not have the same energy. …”
Effect of potential on photoelectric current – “… The photocurrent is found to decrease rapidly until it drops to zero at a certain sharply defined, critical value of the negative potential V0 on the plate A. …”
Einstein’s Photoelectric Equation: Energy Quantum of Radiation – “… More tightly bound electrons will emerge with kinetic energies less than the maximum value. …”
Effect of frequency of incident radiation on stopping potential – “This minimum, cut-off frequency ν0, is called the threshold frequency. It is different for different metals.”