Q.State the key experimental findings of Hallwachs and Lenard on the photoelectric effect, and explain briefly why these findings could not be accounted for by the classical (wave) picture of light.

Hallwachs' finding. A negatively charged, clean zinc plate connected to an electroscope loses its charge rapidly under ultraviolet illumination, but a positively charged plate does not -- showing the plate is emitting NEGATIVE charge under UV light (Section 11.3).

Lenard's findings. Using a proper evacuated-tube apparatus, Lenard established: photocurrent is proportional to intensity (fixed frequency); the maximum kinetic energy of the photoelectrons depends only on frequency, not intensity; a threshold frequency exists below which no emission occurs at any intensity; and emission begins with no measurable time lag. …

State Hallwachs' qualitative finding first, then Lenard's quantitative laws, then briefly flag which of those laws are …

• Attributing Lenard's quantitative laws to Hallwachs, or vice versa -- Hallwachs' contribution was the qualitative charge/discharge observation; Lenard's was the quantitative apparatus and the numerical laws. …