Physics · Ch 11 — Dual Nature of Radiation and Matter
Hertz's Observations
Hertz's Observations
The Discovery of the Photoelectric Effect
The story of the photoelectric effect begins not with a deliberate search for electrons, but with an experiment on electromagnetic waves. In 1887, Heinrich Hertz was conducting his famous experiments to prove the existence of electromagnetic waves predicted by Maxwell's theory. His apparatus consisted of a spark gap transmitter and a receiver — a loop of wire with a small gap where sparks would jump when electromagnetic waves were detected.
Hertz noticed something puzzling. When he directed ultraviolet light from an arc lamp onto the receiver's spark gap, the sparks became noticeably stronger and easier to produce. The light was somehow helping the spark jump across the gap. This was the first observation of what we now call the photoelectric effect, though Hertz did not understand what caused it.
What was happening at the microscopic level? The ultraviolet light was striking the metal surface of the detector. Near the surface, free electrons in the metal were absorbing energy from the incoming light. When an electron absorbed enough energy, it could overcome the attractive force of the positive metal ions holding it in the material. Once free, the electron escaped from the metal surface into the surrounding space. These freed electrons made it easier for the spark to form across the detector gap.
Hertz's discovery was accidental — he was not looking for electron emission, but for confirmation of electromagnetic wave theory. The photoelectric effect was a byproduct of that larger investigation.
Hallwachs' and Lenard's Systematic Investigation
Between 1886 and 1902, Wilhelm Hallwachs and Philipp Lenard conducted detailed experiments to understand this phenomenon. Their work transformed a curious observation into a well-characterized physical effect.
The Basic Experimental Setup
Lenard used an evacuated glass tube containing two metal electrodes. One electrode, called the emitter plate (labelled C in the textbook), was exposed to ultraviolet radiation. The other electrode, the collector plate (labelled A), was kept at a positive potential relative to the emitter.
When ultraviolet light fell on the emitter plate, a current flowed in the external circuit. When the light was stopped, the current stopped immediately. The explanation was clear: ultraviolet light caused electrons to be ejected from the emitter plate. These electrons, being negatively charged, were attracted toward the positively charged collector plate. They travelled through the evacuated tube, completing the circuit and producing a measurable current.
Hallwachs' Key Experiments with a Charged Zinc Plate
Hallwachs performed a series of elegant experiments using a zinc plate connected to an electroscope. His results were:
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Negatively charged zinc plate: When illuminated by ultraviolet light, the plate lost its negative charge. The electroscope showed the charge decreasing to zero.
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Uncharged zinc plate: When irradiated with ultraviolet light, the plate became positively charged.
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Positively charged zinc plate: When illuminated by ultraviolet light, the positive charge actually increased.
The conclusion was inescapable: negatively charged particles were being emitted from the zinc plate under the action of ultraviolet light. The plate lost negative charge (electrons) and therefore became more positive, or if already positive, became even more positive.
A common mistake is to think the light itself carries charge. It does not. The light provides energy to electrons in the metal, enabling them to escape. The charge separation is a consequence of electrons leaving, not of the light being charged.
The Role of the Electron Discovery
In 1897, J.J. Thomson discovered the electron. This immediately clarified the photoelectric effect: the negatively charged particles being emitted were electrons. These emitted electrons were named photoelectrons, and the overall phenomenon was called the photoelectric effect.
The Threshold Frequency Discovery
Hallwachs and Lenard made a crucial observation: for a given metal, no electrons were emitted at all if the frequency of the incident light was below a certain minimum value. This minimum frequency is called the threshold frequency.
The threshold frequency depends on the material of the emitter plate. Different metals have different threshold frequencies.
The existence of a threshold frequency is the single most important experimental fact about the photoelectric effect. It cannot be explained by classical wave theory, which would predict that any frequency of light, given enough intensity, should eventually eject electrons. The threshold frequency was the clue that led Einstein to propose the photon model of light.
Material Sensitivity to Different Wavelengths
The experiments revealed a clear pattern based on the type of metal: …