Hallwachs' and Lenard's Observations

Hallwachs' experiment. Wilhelm Hallwachs, in 1888, devised a much simpler and more direct way to study Hertz's puzzling observation. He connected a clean, freshly-polished zinc plate to a gold-leaf electroscope (an instrument that shows the presence and, roughly, the amount of electric charge by how far its thin gold leaves diverge) and charged the plate NEGATIVELY. When ultraviolet light was allowed to fall on the negatively charged zinc plate, the electroscope's leaves were observed to collapse rapidly -- the plate was losing its negative charge. When the SAME plate was instead given a POSITIVE charge and illuminated in exactly the same way, no such loss of charge occurred; the electroscope's leaves stayed put. Hallwachs correctly inferred that ultraviolet light was causing the zinc surface to emit NEGATIVE charge (later identified as electrons): a negatively charged plate loses negative charge and so discharges, while a positively charged plate, having emitted the same negative charge, simply attracts it straight back, showing no net effect.

Lenard's quantitative study. Philipp Lenard, in 1902, built the first proper quantitative apparatus for this effect -- essentially the arrangement described in Section 11.4 -- allowing him to vary the intensity and the frequency of the incident light independently and to measure both the resulting photoelectric current and, using a retarding (opposing) voltage, the maximum kinetic energy the emitted electrons carried. His results uncovered a set of facts that would turn out to be genuinely difficult to explain:

• Photoelectric emission begins and stops essentially INSTANTANEOUSLY as the light is switched on and off, with no measurable time lag, even for very weak illumination.
• For light of a FIXED frequency (above a certain minimum), the photoelectric current increases in direct proportion to the INTENSITY of the incident light.
• The MAXIMUM kinetic energy of the emitted electrons does NOT depend on the intensity of the light at all -- a brighter light of the same colour (frequency) ejects MORE electrons per second, but does not eject them any faster.
• The maximum kinetic energy of the emitted electrons DOES depend on the FREQUENCY of the incident light, increasing as the frequency is raised.
• For each metal, there exists a definite minimum (threshold) frequency of light below which NO photoelectric emission occurs at all, however intense the light is made. …