Q.The classical wave theory of light predicts that photoelectric emission should occur at any frequency provided the light is intense enough, and that there should be a measurable time lag before emission begins at low intensity. Explain why both these predictions fail when tested experimentally.

Classical prediction 1: any frequency should eventually work. Since the classical wave picture lets an electron accumulate energy gradually and continuously from an incoming wave, it predicts that even a LOW-frequency wave should, given enough time or high enough intensity, eventually deliver enough total energy to free an electron -- there being no reason, in this picture, for a strict, unconditional threshold frequency.

Why it fails. Experimentally, a threshold frequency $\nu_0$ exists that CANNOT be crossed by raising intensity or waiting longer -- below $\nu_0$, emission never happens, at any intensity, for any duration. In the photon picture (Section 11.7), this is because a single photon's energy $h\nu$ is fixed once and for all by its frequency; a sub-threshold photon can never be 'topped up' by another photon's energy, so no accumulation over time is possible.

Classical prediction 2: a time lag at low intensity. The classical picture also predicts that, at low intensity, an electron should take a calculable, and easily measurable (of order minutes), amount of time to slowly gather up the energy $\phi_0$ needed to escape, since the incoming wave delivers energy only gradually. …

State each classical prediction in turn (no threshold; measurable time lag), then explain, using the discrete-photon pict …

• Explaining only ONE of the two predictions asked about (threshold or time lag) rather than both.
• Giving the CORRECT (photon) explanation without first stating what the WRONG (classical) prediction actually was -- the q …