Q.Photoelectric emission is observed to begin, for light above the threshold frequency, without any measurable time delay, even when the incident light is very weak. Explain why this observation could not be reconciled with the classical wave theory of light.

The classical prediction. In the wave picture, the energy an electron can absorb from an incident wave accumulates gradually and continuously over time, at a rate set by the wave's intensity. For a sufficiently WEAK (low-intensity) beam, a straightforward estimate of how long it should take an electron to accumulate the energy $\phi_0$ needed to escape gives a time delay of the order of MINUTES or more -- large enough that it ought to be easily observable with ordinary laboratory timing.

The experimental fact. Photoelectric emission is instead found to begin essentially INSTANTANEOUSLY -- with no measurable time lag at all -- as soon as light above the threshold frequency is switched on, at ANY intensity above zero, including intensities so low that the classical estimate above predicts a delay of many minutes. …

State the classical time-lag prediction with its physical basis (gradual accumulation), contrast it with the observed instantaneous emission, and …

• Giving only a qualitative statement ('the wave theory predicts a delay') without explaining WHY it predicts one (gradual, intensity-limited energy accumulation). …