Physics · Ch 7 — Dual Nature of Radiation and Matter
Hertz, Hallwachs and Lenard's observation
Hertz, Hallwachs and Lenard's observation
Heinrich Hertz, in 1887, was the first person to experimentally generate and detect electromagnetic waves, using a high-voltage induction coil to produce a spark discharge between two metal spheres (the transmitter) and a bent copper-wire loop (the receiver) to detect the resulting waves. While trying to make the faint detector spark easier to see, Hertz noticed, almost by accident, that the spark became noticeably more vigorous whenever the detector was exposed to ultraviolet light. He could not explain this at the time; it was only later understood that the UV light was knocking electrons out of the metal sphere by photoelectric emission, and this extra charge made the spark stronger. In a nice historical irony, the very same experiment that confirmed light is an electromagnetic wave also produced the first evidence for its particle nature.
In 1888, the German physicist Wilhelm Hallwachs confirmed that ultraviolet light itself was responsible for this behaviour, using a much simpler setup: a clean circular zinc plate mounted on an insulating stand and wired to a gold-leaf electroscope. When an uncharged zinc plate was irradiated with ultraviolet light from an arc lamp, it became positively charged and its gold leaves visibly opened, showing that negatively charged particles (electrons) had been driven off the plate, leaving it net positive. When a negatively charged plate was irradiated instead, its leaves quickly closed, since the UV light drove off the excess electrons and neutralised the charge. When a positively charged plate was irradiated, its leaves opened even further, since it kept losing electrons and grew still more positive. All three results independently established that ultraviolet light ejects negatively charged electrons from a zinc surface. …
What this figure shows. Three panels each show a circular zinc plate mounted on an insulating stand and connected to a gold-leaf electroscope, whose pair of thin gold leaves is drawn either hanging together (uncharged/neutral) or diverging apart (charged) depending on the state described. In panel (a), an initially uncharged zinc plate is irradiated with ultraviolet light from an arc lamp and its leaves are shown opening, meaning the plate has become positively charged as negative electrons are ejected from it. In panel (b), a negatively charged plate loses its charge quickly under UV irradiation and its leaves are shown closing, because the excess negative charge leaks away as photoelectrons are emitted. In panel (c), a positively charged plate is shown becoming even more positive under UV irradiation, with its leaves opening further still, since the plate had no excess negative charge to lose but continued ejecting photoelectrons and grew more positive. Together, all three panels are Hallwachs' direct proof that ultraviolet light ejects n …
What this figure shows. Two metallic plates, labelled C (negative) and A (positive), are shown facing each other inside an evacuated quartz bulb, connected in a circuit that includes a galvanometer G and a battery B, with ultraviolet radiation drawn falling on plate C from outside the bulb. When the UV light strikes the negative plate C, a stream of electrons is shown crossing the vacuum toward the positive plate A, completing the circuit and producing a measurable current on the galvanometer; the figure also captures Lenard's complementary finding that irradiating the positive plate A instead produces no current at all, since electrons ejected there are simply pulled back onto the same plate rather than crossing the gap. This apparatus is what let Lenard establish, quantitatively, that it is specifical …