Photoelectric effect
In 1887, Heinrich Hertz observed a strange behavior inside his spark-gap apparatus. He placed the device in a darkened box to see the sparks more clearly. A glass panel between the source and receiver absorbed ultraviolet radiation that helped electrons jump across the gap. When he removed the glass, the spark length increased significantly. This observation marked the first recorded instance of what we now call the photoelectric effect.
Wilhelm Hallwachs took this discovery further by connecting a zinc plate to an electroscope. He allowed ultraviolet light to fall on a freshly cleaned zinc plate. The plate became uncharged if it was initially negatively charged. It turned positively charged if it started out neutral. If it began as positive, it became even more positive. These observations led him to conclude that some negatively charged particles were emitted from the zinc plate when exposed to ultraviolet light.
Aleksandr Stoletov performed detailed analysis of the photoeffect between 1888 and 1891. He reported results in six separate publications. Stoletov invented a new experimental setup better suited for quantitative analysis. He discovered a direct proportionality between light intensity and induced photoelectric current. This relationship later became known as Stoletov's law.
Albert Einstein published his paper On a Heuristic Viewpoint Concerning the Production and Transformation of Light in 1905. This work appeared among his Annus Mirabilis papers. He proposed that light energy is carried in discrete quantized packets rather than continuous waves. Each packet carries energy equal to the frequency of light multiplied by a constant now called Planck's constant.
This hypothesis explained why electron energy did not depend on incident light intensity. Classical wave theory predicted electrons would gather up energy over time before emission. The quantum model showed instead that only photons above a threshold frequency could eject single electrons. A photon below this frequency possessed insufficient energy regardless of beam brightness.
Robert Millikan conducted highly accurate measurements of Planck's constant from the photoelectric effect starting in 1914. His data supported Einstein's model even though he found corpuscular theories quite unthinkable at the time. Einstein received the 1921 Nobel Prize in Physics for his discovery of the law of the photoelectric effect. Millikan won the Nobel Prize in 1923 for his work on elementary charge and the photoelectric effect.
Philipp Lenard observed that current flows through an evacuated glass tube enclosing two electrodes when ultraviolet radiation strikes one electrode. As soon as the radiation stops, the current also ceases. This initiated the concept of photoelectric emission. Lenard used a powerful electric arc lamp to investigate large changes in intensity.
Lenard's results remained qualitative due to experimental difficulties. Experiments needed fresh metal surfaces because pure metal oxidized within minutes even in partial vacuums. The current emitted by the surface depended directly on light intensity. Doubling the intensity doubled the number of electrons ejected from the surface.
Millikan designed precise laboratory setups involving vacuum tubes with emitting and collector electrodes. He applied negative voltages to stop the most energetic photoelectrons. When no current flowed through the tube, the retarding voltage reached its stopping potential value. This allowed him to calculate maximum kinetic energy using the equation eVo equals Kmax. His measurements confirmed Einstein's linear relationship between frequency and electron energy.
Electrons bound in atoms occupy distinct states with well-defined binding energies. When light quanta deliver more than this amount to an individual electron, it may escape into free space with excess kinetic energy. The distribution of kinetic energies reflects the binding energy distribution within the atomic or crystalline system.
In metals, electrons emitted from the highest occupied states possess the greatest kinetic energy. These electrons originate from what physicists call the Fermi level. The minimum energy required to remove an electron from a material surface is called the work function. It is sometimes denoted as phi or W.
The three-step model for ultraviolet excitation decomposes emission into inner transitions, propagation to the surface, and escape through the barrier. Electrons originating deeper in solids suffer collisions that alter their energy and momentum. Their mean-free path follows a universal curve dependent on electron energy. Some electrons lose energy equal to the work function while escaping into vacuum.
For decades scientists believed photoemission occurred instantaneously. Recent research has overturned this assumption using attosecond pulse generation techniques. Pierre Agostini, Ferenc Krausz, and Anne L'Huillier received the 2023 Nobel Prize in physics for these experimental methods.
Measurements taken in 2010 revealed electron emission takes approximately 20 attoseconds. Later studies involving tungsten indicated around 100 attoseconds are required to liberate an electron. Another research effort found values near 45 attoseconds. A broad consensus now emerges that photoemission involves finite time rather than being instantaneous.
These findings show photoemission associates with complex multielectron correlations. The process is not merely a single-electron event. Electric field orientation also plays a role. Radiation with specific electric field orientations can excite electrons leading to enhanced emission in the Terahertz range.
Photomultipliers are extremely light-sensitive vacuum tubes containing coated photocathodes inside their envelopes. These cathodes use materials like cesium, rubidium, and antimony selected for low work functions. Even very low levels of light cause them to readily release electrons.
A series of electrodes called dynodes accelerate these electrons at ever-higher potentials. Secondary emission substantially increases electron numbers to provide detectable output current. Photomultipliers remain common wherever low light detection is necessary. Video camera tubes used the effect early on. Philo Farnsworth's Image dissector transformed optical images into scanned electronic signals using charged screens.
Light from the Sun hitting lunar dust causes it to become positively charged via the photoelectric effect. Charged dust repels itself and lifts off the Moon surface through electrostatic levitation. This manifests as an atmosphere of dust visible as thin haze or dim glow after sunset. Surveyor program probes photographed this phenomenon in the 1960s. Chang'e 3 rover observed dust deposition on lunar rocks reaching heights of about 28 centimeters.
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Common questions
What did Heinrich Hertz observe in 1887 regarding the photoelectric effect?
Heinrich Hertz observed a strange behavior inside his spark-gap apparatus where removing a glass panel allowed ultraviolet radiation to increase spark length. This observation marked the first recorded instance of what we now call the photoelectric effect.
When did Aleksandr Stoletov publish results on the direct proportionality between light intensity and induced photoelectric current?
Aleksandr Stoletov performed detailed analysis of the photoeffect between 1888 and 1891 and reported results in six separate publications. He discovered a direct proportionality between light intensity and induced photoelectric current that later became known as Stoletov's law.
Why did Albert Einstein receive the 1921 Nobel Prize in Physics for the photoelectric effect?
Albert Einstein received the 1921 Nobel Prize in Physics for his discovery of the law of the photoelectric effect published in 1905. His work proposed that light energy is carried in discrete quantized packets rather than continuous waves.
How long does electron emission take according to measurements taken in 2010 involving tungsten?
Measurements taken in 2010 revealed electron emission takes approximately 20 attoseconds while later studies involving tungsten indicated around 100 attoseconds are required to liberate an electron. A broad consensus now emerges that photoemission involves finite time rather than being instantaneous.
What phenomenon occurs when light from the Sun hits lunar dust causing it to become positively charged?
Light from the Sun hitting lunar dust causes it to become positively charged via the photoelectric effect which allows charged dust to repel itself and lift off the Moon surface through electrostatic levitation. Surveyor program probes photographed this phenomenon in the 1960s and Chang'e 3 rover observed dust deposition on lunar rocks reaching heights of about 28 centimeters.