Given below are two statements:
Statement I: If the frequency of the incident radiation is above the threshold frequency, photoelectric emission from a metal starts without any apparent time lag, even when the radiation is very dim.
Statement II: On the wave picture of light, an electron in a metal exposed to very dim light would also be emitted without any delay.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Statement I is correct but Statement II is incorrect
Statement I If the frequency of the incident radiation is above the threshold frequency, photoelectric emission from a metal starts without any apparent time lag, even when the radiation is very dim. — True. Emission is seen to begin within about \( 10^{-9}\,\mathrm{s} \) however weak the light is. In the photon picture, an electron comes out by absorbing one whole light quantum in a single act. So dim light gives fewer photoelectrons, but no delay.
Statement II On the wave picture of light, an electron in a metal exposed to very dim light would also be emitted without any delay. — False. In the wave picture the energy is absorbed continuously over the whole wavefront and shared by a large number of electrons. Each electron then gains energy only slowly. With dim light, it could take hours or more to collect enough energy to overcome the work function. So the wave picture predicts a long time lag, which is not what is observed.
NCERT (2026-27):
Effect of Frequency of Incident Radiation on Stopping Potential – “Note that in all the above experiments, it is found that, if frequency of the incident radiation exceeds the threshold frequency, the photoelectric emission starts instantaneously without any apparent time lag, even if the incident radiation is very dim. It is now known that emission starts in a time of the order of 10–9 s or less.”
Einstein’s Photoelectric Equation: Energy Quantum of Radiation – “… In Einstein’s picture, the basic elementary process involved in photoelectric effect is the absorption of a light quantum by an electron. This process is instantaneous.”
Photoelectric Effect and Wave Theory of Light – “Further, we should note that in the wave picture, the absorption of energy by electron takes place continuously over the entire wavefront of the radiation. Since a large number of electrons absorb energy, the energy absorbed per electron per unit time turns out to be small. Explicit calculations estimate that it can take hours or more for a single electron to pick up sufficient energy to overcome the work function and come out of the metal.”